Free-Electron Laser Beam Instrumentation and Diagnostics

Transkript

1 Free-Electron Laser Beam Instrumentation and Diagnostics Volker Schlott Paul Scherrer Institut, Villigen, Switzerland Abstract Beam instrumentation and diagnostics for FEL user facilities have to provide single-shot and shot-to-shot information about all relevant beam parameters such as beam charge, transmission, transverse beam position and profile as well as beam energy and energy spread, bunch compression, bunch length and arrival time. The diagnostics systems have to be designed such that they support accelerator set-up and commissioning as well as user operation. Non-invasive monitors are used in beam-based feedbacks that stabilize the photon yield for user experiments. This article provides a layout of beam diagnostics for an FEL user facility and describes the technical realization of the most important beam instrumentation devices, including examples of the latest achievements. Keywords Beam loss monitor Beam position monitors (BPM) Cavity type pick up Stripline pick-up Electronics and signal processing Coherent radiation Digital signal processors (DSP) Injector diagnostics LINAC diagnostics Longitudinal diagnostics Bunch arrival time monitor Bunch form factor Compression monitor Faraday cup Transverse deflecting structure Wall current monitor Integrating current transformer Screen monitor Part II: Diagnostics for Free Electron Lasers The second part of this chapter covers beam instrumentation and diagnostics for (X-ray) free electron lasers (FEL). It will first introduce the roles and main functionalities of the different instrumentation devices within the FEL accelerator complex in view of the beam dynamics and user requirements, which aim for the achievement of reproducible lasing conditions in different operation modes. After this more general overview of the beam diagnostics layouts, more detailed descriptions of the working principles and technical implementations of the most important FEL LINAC diagnosticsdevices willbegiveninthefollowingsections, wherea brief recall of physical processes of signal generation will lead to the description of technical realizations, including examples of the latest achievements for the various diagnostics devices. volker.schlott@psi.ch Page 1 of 40

2 General Beam Diagnostics Layouts and Functionalities for FEL Facilities X-ray free electron lasers are single-pass machines, which require high resolution, single-shot and shot-by-shot information on all relevant electron (and possibly gun laser as well as photon) beam parameters from every instrumentation device in an FEL facility. For normal conducting FEL accelerators with bunch repetition rates between 10 and 120 Hz, single- shot and shot-by-shot measurements can be obtained with standard (well-known) detectors and state-of-the-art electronics. Instrumentation for superconducting accelerators, however, may require much faster detectors and data processing units, which allow the measurement of several thousands of electron buckets within a bunch train at bunch distances of a few hundred nanoseconds to a few microseconds. In order to accommodate for the increasing interest of scientists in ultrashort (<10fs) photon pulses, low-charge operation modes of FEL accelerators have been established, pushing the FEL diagnostics devices to their sensitivity limits. The reliable and online measurement of such short electron bunches (and photon pulses) represents one of the remaining R&D tasks in the field of FEL instrumentation. When comparing SRLS and FEL diagnostics, the transverse beam stability especially along the undulators as synchrotron radiation source points can be considered of equal importance, while the measurement and stabilization of the longitudinal beam parameters such as bunch length, compression, and beam arrivaltimeposeadditionalchallengesto FELbeam instrumentation. In an FEL facility, the monitoring of beam position, beam charge and transmission, as well as beam loss is provided by distributed monitors along the different parts of the accelerator (injector, LINACs, transfer, and undulator lines), whereas the beam properties, which determine the lasing process, such as projected and sliced transverse emittances, the longitudinal bunch distribution (bunch length or peak current), as well as energy and energy spread, are usually measured in dedicated diagnostics sections. Since the electron bunches need to be highly compressed (from a few ps to a few fs) to obtain the required peak currents of several ka for driving the self-amplifying spontaneous emission (SASE) process, such diagnostics sections are usually placed behind the magnetic bunch compressor chicanes as integral parts of the standard machine optics (Fig. 1). Table 1 provides an overview of the main FEL diagnostics devices, their resolution requirements, as well as their dynamic ranges and typical modes of operation. Motivated by the setup and commissioning strategy of an FEL facility, the following paragraphs intend to describe a set of measurement procedures and the related diagnostics devices, revealing therequired informationaboutthemostrelevantfel beam parameters. Moredetailed descriptions on specific monitor designs and their functionalities are given in the subsequent passages on FEL beam instrumentation devices. Injector Diagnostics: Monitoring FEL Beam Generation and Conditioning By following Liouville s theorem, which postulates that the phase-space volume occupied by a system of particles is preserved (in our case an electron bunch in a particle accelerator), it becomes obvious that the performance of a SASE FEL depends strongly on the brilliance of the electron source. Photo-injectors, which are typically consisting of a (UV) laser- driven photocathode, located in a high field RF gun, are capable of providing electron beams with sufficiently high brightness. The close relation of the electron beam properties to the photocathode laser parameters motivates the integration of the gun laser diagnostics in the overall accelerator control system. Stable and reproducible photo-injector operation depends on multiple laser parameters such as Page 2 of 40

3 Gun Laser Heater BC 1 (Spectrometer + TDS) BC 2 Collimator Dump (Spectrometer + TDS) charge arrival time energy & spread (slice) energy & spread energy & spread energy & spread (slice) energy matching emittance (slice) emittance emittance arrival time energy & spread compression & bunch length compression compression emittance (slice) bunch lenght matching matching matching electron & x-ray bunch length BC 1 BC 2 Deflector Laser Heater Gun Booster 1 Booster 2 Linac 1 Linac 2 Linac 3 Deflector X-ray photons Harmonic Linearizer Collimation Undulators z = 16 m 63 m 210 m 498 m E = MeV, I = 20 A 355 MeV, 150 A 2.0 GeV; 2.7 ka GeV, 2.7 ka σ z σ z = 124 μm (413 z z = 871 μm (2.9 ps) fs) σ = 6 μm (21 fs) σ = 6.2 μm (21 fs) σ δ = 0.15 % ε N.slice = 0.23 μm ε N.proj. = 0.27 μm σ δ = % ε N.slice = 0.29 μm ε N.proj. = 0.51 μm Photon Diagnostics m ~ 4-12 kev intensity wavelength bandwidth position pulse length arrival time... Fig. 1 Schematic layout of an XFEL facility (here: SwissFEL ARAMIS) with the most important beam parameters to be measured at critical locations along the accelerator Page 3 of 40

4 Table 1 Main FEL diagnostics devices and typical measurement parameters Diagnostic device Parameters Resolution Ranges Modes LINAC & TL BPM (cavity, stripline, button) Undulator BPM (cavity) Screen monitor (OTR or scintillator) Position 5 50 m <40mm Shot-by-shot Rel. bunch charge < pc 1 nc noninvasive Position <1 m 500 m Shot-by-shot Rel. bunch charge < pc 1 nc noninvasive Transverse profile <10 m Single-shot invasive 5 20 mm position <50 m Single-shot noninvasive Synchrotron radiation monitor Turbo-ICT Abs. bunch < pc 1 nc Single shot noninvasive charge ICT Abs. bunch nc 20 pc charge Single shot noninvasive Dark current nc WCM Abs. bunch charge 20 pc Single shot noninvasive nc Coax. FC dark current Few pc time resolved EO monitor (ZnTe, GaP Bunch length, 100 fs ps Single shot noninvasive crystals) Streak camera Transverse deflector BAM Longitudinal bunch distribution Bunch arrival time Bunch arrival time >200 fs ps Single shot invasive (OTR) 20 fs (BC-1) 2 fs (BC-2) 20 fs (BC-1) 5 fs (BC-2) <100 ps Single shot invasive Few 100 ps Single shot noninvasive rel. to reference pulse energy, wavelength, transverse and longitudinal pulse shaping, as well as pointing stability and laser pulse arrival time on the photocathode. Equally important for low drifts of gun laser parameters are the environmental conditions (mainly temperature, humidity) in the gun laser hutch and along the laser transfer line to the photocathode. All these parameters should be monitored online and provided shot by shot in order to permit correlations with electron beam measurements. The electron beam diagnostics in an FEL injector should allow for a full characterization of the six-dimensional phase-space volume with bunch-by-bunch information on peak current (bunch charge and longitudinal bunch distribution), transverse emittances, as well as energy and energy spread. While most of the FEL projects have optimized their photo-injector brightness in gun test facilities with dedicated diagnostics sections, the injector instrumentation in an FEL user facility needs to be integrated in the low-energy beam optics to allow for setup and optimization of the desired operation mode and to provide online monitoring of the main electron beam parameters. The RF photo gun is typically characterized by means of a Schottky scan, which measures the bunch charge as a function of the gun laser pulse launch phase, which is the relative laser pulse arrival time at the photocathode in relation to the RF acceleration field in the gun. The regular working point of the RF photo gun is established for the minimum energy spread, which is close to the maximum momentum of the electron bunches. The recording of the bunch charge as a function of the gun laser energy yields the quantum efficiency of the photocathode, which depends mainly on the cathode material and the gun laser wavelength. While for copper cathodes Page 4 of 40

5 and ultraviolet (UV) laser pulses at 266 nm quantum efficiencies in the order of 10 5 to 10 4 can be achieved, Cs 2 Te cathodes yield much higher quantum efficiencies of a few percent at laser wavelengths around 250 nm but suffer from comparably short lifetimes of only a few months. Integrating current transformers (ICT), wall current monitors (WCM), or Faraday cups can be installed as charge measurement devices in the low- energy beam transport line between the electron source and the booster accelerator. They can also be used to measure dark current, which is emitted due to the high field strength in an RF gun and which should be filtered (collimated) from the FEL bunch before acceleration. A low-energy spectrometer line allows the setting of the gun parameters for the desired beam energy and lowest energy spread. The spectrometer setup is typically consisting of a dipole magnet to create dispersion, a drift path, and a screen monitor to visualize the beam centroid (energy) and the transverse beam distribution (energy spread). Alternatively, a scraper in the dispersive plane and an ICT or a Faraday cup could also be installed in the low-energy spectrometer line. For low-energy (few MeV) and thus space charge-dominated beams, the transverse emittances can be measured with hole-array ( pepper-pot ) masks or by means of horizontal and vertical slit scans. Tungsten is a preferred mask material to block the lowenergy electron beam. Hole diameters or slit widths in the order of a few tens to hundred microns are typically used to provide the desired resolution. The position and spot size of the emerging beamlets are visualize on a scintillating screen (e.g., YAG:Ce or LuAG:Ce crystals), which is located at a proper drift length behind the slit or hole-array mask. In this way, a position-resolved measurement of the beamlet divergence and its transverse momentum can be performed, which allows a reconstruction of the transverse phase-space distribution. Injector test facilities may even desire the flexibility of a moveable transverse emittance measurement device (emittance meter) to perform parameter scans for optimum gun performance and LINAC matching (Ferrario et al. 2007). In FEL user facilities, the pepper-pot or slit measurement options can be kept in the low-energy beam transport line for re-optimization of the injector settings in case of photocathode or gun replacements. Details of this low-energy emittance measurement method are given in Anderson et al. (2002). During initial accelerator commissioning and after regular shutdowns, the FEL injector setup as well as the establishment of different operation modes is most efficiently done at higher energies of a few hundred MeV, where the electron beam is not space charge dominated anymore. The moments of the phase-space distributions can thus be derived from a betatronic beam transport between a reference point and a transverse beam profile monitor used for beam size measurements. Both, quadrupole scans (Minty 2004) and the passing of the beam through an alternating FODO lattice (Minty 2004), represent equivalent techniques to determine projected emittances and Twiss parameters for beam matching (e.g., into the FEL LINAC). The visualization of the transverse beam distributions is typically achieved by means of screen monitors, which are equipped with scintillators or optical transition radiators (Al foils or aluminized Si wafers). The minimal systematic errors from the beam profile determination can be obtained, when the beam sizes at the screen are kept within predefined limits, which are most suitable for the measurement. This can be achieved by using three (instead of only two) quadrupole magnets for adjusting the required phase advance, while compensating at the same time for constant beam size. The combination of such an emittance measurement optics with a transverse (typically vertical) RF deflecting structure (TDS) provides information about the sliced emittance (typically in the horizontal plane) and when passing the beam to an energy spectrometer, about the sliced energy spread. These timeresolved beam properties as well as the longitudinal bunch distribution (bunch length and profile) represent the most relevant quantities to be determined in an FEL facility, and an RF TDS behind Page 5 of 40

6 each compression stage can be considered as the most essential instrumentation devices. S-band RF TDS with deflecting fields in the order of a few MV at a few hundred MeV beam energy permit temporal resolutions of some tens of femtoseconds, which is sufficient to obtain time-resolved phase-space information behind the injector, where the bunch lengths are typically ranging from 10 ps (FWHM) for uncompressed beams to a few hundred femtoseconds when passing the first (magnetic) bunch compressor. Flat-top shapes of the photocathode laser pulses in the transverse and longitudinal planes are requested to obtain uniform properties along the electron bunch and good matching between sliced and projected emittances. In this respect, the measurement and online monitoring of the longitudinal charge distribution of the uncompressed bunch before the first bunch compression helps to find and monitor the desired settings for the laser pulse length and its waveform. This timeresolved information on the sub-ps scale may allow corrections of the photocathode laser system by adjustment of the pulse-forming processes like pulse stacking or chirped pulse amplification. Invasive instrumentation devices like a TDS or a streak camera, which requires visible radiation from an OTR screen or a Cherenkov radiator (e.g., Aerogel), can of course be used to measure the longitudinal electron bunch distribution during FEL injector setup and commissioning. During user operation, however, a noninvasive monitoring of the bunch length and its longitudinal distribution is preferred and can be provided by an electrooptical (EO) monitor, which correlates the THz electrical field emitted by the relativistic electron bunches with a short (or high spectral bandwidth) laser pulse in an electrooptically active crystal like ZnTe or GaP. Single-shot methods like temporal and spectral decoding as well as scanning along the bunch by means of EO sampling have been applied in numerous accelerators providing sub-100 fs time resolution, which is even sufficient to measure the electron bunch length after the first compression stage. In an FEL injector, information on the electron bunch arrival time can be used to reveal the relative timing jitter of the photocathode laser pulse to the RF phase of the gun and the injector accelerating structures. Such beam arrival time fluctuationsmay lead to unstableconditionsfor the first bunch compression. An electrooptical type of bunch arrival time monitor (BAM) can measure the arrival time jitter with <10 fs precision by correlating the electron beam-induced signals from an ultrahigh bandwidth (40 GHz) pickup with the optical pulses from a highly stable reference laser. By using the BAM signals, beam-based feedback loops can be implemented to actively stabilize the phase errors of the accelerator structures upstream of the bunch compressors (BC- 1 and BC-2) (Löhl et al. 2010). BAM data from downstream of the undulator sections is also of interest for the FEL beam line users, since the electron beam arrival time is measured in relation to the reference laser system, which synchronizes all other subsystems in an FEL facility e.g., also the experimental pump lasers to a precision of a few femtoseconds. In case of pump-probe experiments, BAM arrival time information can thus be used to correct for timing jitter and to post-process experimental data for improved time resolution. Bunch Compressor Diagnostics and Measurements of Compressed Bunches The generation of x-ray photon pulses with GW power levels by the SASE process requires high peak current electron bunches. They are typically obtained by multistage bunch compression, where the initially picosecond-long bunches from the electron source are compressed to a few femtoseconds, leading to the desired peak currents of several ka. Similar diagnostics systems are required for setting up and monitoring the electron beam parameters of both compression stages (BC-1 and BC-2) in an FEL facility: BPMs are used to measure the energy and energy stability Page 6 of 40

7 of the electron beam, transverse profile monitors (screens and/or visible synchrotron radiation monitors) allow the observation of the energy distribution, and compression monitors can obtain bunch length information. Bunch compression is achieved when a chirped electron bunch, which has been given a longitudinal energy dependence by off-crest acceleration, travels on an energydependent path through a four-dipole magnetic chicane. While the electrons with the nominal energy stay on the central orbit, the low-energy electrons travel a longer path than the high-energy electrons, resulting in a corresponding shortening of the bunch. The energy stability and the energy chirp of the electron bunches are best observed at the location of maximum dispersion in the center of the bunch compressor. Noninvasive monitoring of the beam center of mass (energy) and its transverse distribution (energy spread) can be achieved by imaging visible synchrotron radiation from the third dipole of the bunch compressor chicane onto a camera system with a good spatial resolution (few tens of m) and a sufficient field of view (typically about 200 mm) to capture the entire, horizontally flat beam. Destructive visualization of the same beam parameters (e.g., for bunch compressor setup) can be obtained with a screen monitor, located between the two central dipoles of the bunch compressor. A pair of cavity-type, stripline, or button-type BPMs in both arms of the bunch compressor, where standard beam pipes can still be used, is capable of monitoring the beam position, which is directly related to the energy stability of the beam. Shot-by-shot position resolutions of a few m at some tens of mm dispersion provide energy resolutions of a few times 10 4, enabling beambased energy feedbacks of the RF field in the injector accelerating structures upstream of the bunch compressor. The same information can also be achieved with a special kind of BPM in the center of the bunch compressor, which needs to account for the large horizontal width of the flat ( pancake -like) vacuum chamber. However, the unfavorable shape of the BPM chamber and the large horizontal beam size at this location complicate the achievement of the desired position (energy) resolution. Information about the effectiveness and stability of the bunch compression process can be obtained by measuring the intensity of coherent synchrotron (CSR), edge (CER), transition (CTR), or diffraction radiation (CDR), which is emitted by relativistic electrons at wavelengths comparable to their bunch lengths. For a few hundred femtosecond-long bunches behind the first bunch compressor, the coherent part of the radiation spectrum falls in the THz to mm-wave range, while the ultrashort bunches ( <20fs) behind the second bunch compression stage radiate coherently in the far to near infrared (FIR to NIR) or even in the visible part of the spectrum. Compression monitors are either installed at the 4th dipole of a bunch compressor chicane (using CSR or CER) or directly behind the compression stage (using CTR or CDR). They can be equipped with a single (e.g., pyroelectric) detector, which is monitoring the integratedcoherent radiation intensityor with a multichannel detection scheme, providing spectrally resolved information. Cross-calibration with the phase settings of the off-crest accelerating structures in front of the bunch compressor provides relative bunching information, while absolute bunch length readings can be obtained by comparing the intensity distribution of the coherent radiation spectrum with TDS measurements behind the compression stage. The signals from compression monitors are frequently used in beam-based feedbacks, which are acting on the phase settingsofthe off-crest accelerating structures in front of the compression stage. The desired time resolution of a few femtoseconds for bunch lengths and sliced beam parameter measurements of the fully compressed, high-energy electron bunches ( <20fs) behind the second bunch compression stage (typically at 3 GeV) or even behind the undulator sections (at final FEL energies of 6 20 GeV) requires C-band or X-band RF TDS with deflecting fields in Page 7 of 40

8 the order of some tens of MV. Time resolutions as low as 1 fs (rms) have already been obtained at the Linac Coherent Light Source (LCLS) at the Stanford Linear Accelerator Laboratory (SLAC), showing the lasing part of the electron beam, which is equivalent to the photon pulse profile. Such online and in case of an RF TDS behind the undulator non-disturbing electron bunch/photon pulse length information allows a reliable and consistent setup of the short-pulse FEL operation modes and permits beam physics studies for improved understanding of the FEL process. Specific Complications for Measurements of Highly Compressed Bunches In case of highlycompressed and ultrashort electron bunches, the measurement of transverse beam profiles with scintillating screens or OTR experiences a specific complication, which is related to the phenomenon of coherent optical transition radiation (COTR). This effect was unexpectedly observed for the first time in the LCLS injector and downstream of their bunch compressors (Loos et al. 2008) and could so far been validated by most other FEL facilities (Wesch et al. 2011). While for ultrashort (few femtoseconds) bunches, it is obvious that the coherent emission, which is related to the bunch length and its shape, occurs in the near infrared and even at optical wavelengths, it was found that parts of a randomly energy-modulated electron bunch as emitted, e.g., by a photocathode radiate coherently in the optical spectrum after passage through an energy-dispersive beam path (e.g., a bunch compressor or any kind of deflection by a bending magnet), which converts the energy modulation in a corresponding current density modulation. The largely enhanced intensity from the coherently emitting part of the bunch fluctuates from shot to shot and dominates the beam image, preventing transverse profile measurements under these conditions. While the installation of a Laser Heater, which increases the uncorrelated energy spread of the electron bunches, mitigates the effect, several remedies in the design of screen monitors have also been found during the past years to save the quite important two-dimensional profile measurements with screens. Some of them will be described in more detail in one of the subsequent passages of the instrumentation chapter. Alternatively to the redesign of screen monitors, wire scanners have been used (e.g., at LCLS) to determine transverse beam profiles for highly compressed beams. Due to the thin wire diameters (10 m) and their rather small cross section with the beam, they have the general advantage to be only partially destructive, thus making them usable as an online profile and/or emittance diagnostics, which can even be operated in the background during FEL operation. At high beam energies and extremely small beam sizes, they are also not suffering from saturation effects, which limit in general the usability of screen monitors. However, wire scanners are no single-shot devices, so that the raw profile data taken through a scan need to be corrected for beam motions and charge fluctuations, which can be determined by a nearby beam position monitor. Wire scanners are also of limited usability for sliced beam parameter measurements, since they provide only one-dimensional information. Thus, it is advisable to find a well-balanced combination of two-dimensional screen (or OTR) monitors and one-dimensional wire scanners for covering transverse profile measurements throughout an FEL facility. Beam Position, Charge, Transmission, and Beam Loss Monitoring The most stringent performance requirements for an FEL BPM system in terms of single-shot position resolution are motivated by the achievement of an optimal electron photon beam overlap Page 8 of 40

9 along the undulator sections and the provision of highest pointing stability (<0.1 ) of the X-ray beam towards the experimental end stations. The desired single-shot BPM position noise is <1 m over a limited position range of 500 m. One BPM per undulator segment provides sufficient sampling of beam positions along the undulator sections to determine the optimum electron beam trajectory by means of a beam-based alignment procedure (Emma et al. 1999). The, in this way, established golden orbit needs to be kept constant during an FEL user run (predefined operation mode), which can last from single shifts (8 h) to weeks. Hence, extremely low drifts of the BPM electronics in the order of 1 m per week is required to support stable and reproducible user operation. Depending on the general alignment tolerances of an FEL facility, the beam position in the injector and along the main LINACs needs to be controlled in the order of 5 50 m. In order to obtain a sufficient sampling of the beam trajectory along the accelerator, BPMs are placed in the vicinity of quadrupole magnets. Dispersive locations like energy spectrometers and bunch compressors pose more stringent resolution requirements on the BPM systems. Here, position readings of 1 m transcribe to relative energy changes in the order of 10 4 (assuming a dispersion of a few tens of mm at the location of the BPM). While cavity-type BPMs are the primary choice for the highest-resolution applications in undulator sections and at these dispersive locations (bunch compressors, spectrometers, and collimators), button-type and stripline BPMs are frequently used throughout the linear accelerator and the remaining transfer lines. The sum signal from all four buttons or striplines of a BPM as well as the beam-induced intensity from a monopole cavity in case of cavity-type BPMs is directly proportional to the bunch charge. In this sense, every BPM in an FEL facility does not only serve as a beam position sensor but provides at the same time beam charge information. The high sensitivity in the order of 10 3 of the total bunch charge is achieved by narrowband filtering of the BPM signal, which reduces the thermal noise contribution. The charge-related BPM signals still need to be calibrated with an integrating current transformer (ICT), which is capable of determining the absolute bunch charge via a built-in online calibration unit. Although the sensitivity of ICTs is only in the order of 1 % of their measurement ranges and thus lower than the charge resolution of BPMs, the combination of one ICT behind every dispersive section in the FEL accelerator, where beam loss can occur, and the usually quite large number of BPMs, which are distributed over the entire machine, provides a complete high-resolution bunch charge and transmission mapping. Malfunctioning or failure of accelerator components as well as accidental misdirection of the electron beam can lead to localized or distributed beam losses. Furthermore, dark current generated in the RF gun or in the high gradient accelerating structures might also be lost due to mismatched beam transport optics at undefined locations along the entire accelerator. In both cases, undetected beam loss can lead to exceeding radiation dose levels inside and outside the shielding walls and to unwanted irradiation or even damage of accelerator components in the bunker. Two types of beam loss monitors are typically used to provide a complete mapping of the accelerator. Distributed beam loss monitors consisting of large core optical fibers of about 100 m lengths are run alongside the accelerator for identifying beam loss locations. A shower of secondary particles caused by the beam loss generates a scintillating light pulse in the fiber, which is detected by a photomultiplier tube at one end of the fiber. The determination of the transit time of the light pulse through the fiber provides the location of the loss with a spatial resolution of about 1 m. Compensation of irradiation-induced transmission changes in the fiber can be achieved with a pulsed LED and is necessary, if such a distributed loss monitor is used as a threshold detector in a machine protection system. Undulator segments as well as other permanent magnet arrays (e.g., phase shifters) are Page 9 of 40

10 most sensitive to irradiation and suffer from radiation dose-induced demagnetization. Extensive beam loss monitoring and absolute dose rate measurements are thus mandatory to protect these expensive and sensitive beam line elements. Dose rate monitors (Fröhlich et al. 2013), which integrate over the lifetime of an undulator, are used for this purpose. Design and Performance of FEL Diagnostics Devices The first part of the chapter on FEL beam instrumentation and diagnostics has been describing the diagnostics layout for FEL user facilities, its overall integration in the accelerator, and its role to support commissioning and setup of the accelerator for the different FEL operation modes. The following paragraphs will provide more detailed information on the design concepts of the most important monitor types, their technical implementations, and examples of latest results, which have been achieved at state-of-the-art FEL facilities. Due to the limited space, this overview is far from complete, and detailed derivations of formulas and particular implementations of not yet fully established diagnostics devices (e.g., electro-optical bunch length monitors) have to be taken from the referenced literature. Beam Position Monitors: General Introduction As noninvasive diagnostics devices, BPMs are the main workhorses in FELs, providing beam center of mass position information along the accelerator, transfer lines, and undulator sections. BPM systems consist of a pickup, which is part of the accelerator (ultrahigh) vacuum system, generating a position-sensitive electrical signal, an analog (RF) front end for signal conditioning, and digital data acquisition electronics for processing of beam positions, selection of operation modes, and the transfer of the position information to the control system or automated feedback loops. The decision on a specific pickup type is based on several criteria, including the required sensitivity, its operating frequencies and the desired signal bandwidth, mechanical and environmental conditions, as well as manufacturability and even costs. Depending on the signal strength from the pickup and its attenuation through the RF cable, which passes the signal from the accelerator (radiation environment) to the technical gallery (outside the shielding walls), the first (analog) signal conditioning stage can either be located close to the BPM pickup in the accelerator tunnel or in a well-temperature-stabilized and EMC-shielded electronics rack in the technical gallery. The main building blocks of FEL (or LINAC) BPM systems and the most prominent configuration with electronics outside of the accelerator tunnel are shown in Fig. 2 with button- or stripline- (upper part of Fig. 2) and cavity-type (lower part of Fig. 2) pickups. Beam Position Monitors: High-Resolution, Single-Shot LINAC and FEL Pickups The achievement of m level position resolution in single-pass accelerators (e.g., FELs or SRLS injectors) demands higher signal levels than typically obtained from button-type pickups. In case of multiple bunches within an RF pulse with relatively short bunch distances (few tens to hundred Page 10 of 40

11 Fig. 2 Schematic illustration of main BPM system building blocks for LINACs, transfer lines (FELs or SRLS injectors), and storage rings (SRLS) nanoseconds), low latency and thus high bandwidth are required at the same time. Matched and resonant stripline BPM pickups, which are often used in LINACs and beam transfer lines with relatively large beam pipe diameters (16 40 mm), have proven to provide <10 m position resolution for bunches charges between 10 pc and 1 nc (Xu et al. 2013; Keiletal.2010). The primary choices for sub- m single-bunch accuracies as usually required in the FEL undulator sections are low and/or high Q cavity BPMs, operating in the GHz range. Stripline BPMs consist of four electrodes, which form a transmission line with the beam pipe, allowing the determination of the beam position by the same difference-over-sum algorithm as for button BPMs. In a matched stripline pickup, a relativistic electron bunch induces an image current at the beginning of the transmission line, where one half of the current pulse passes through the out-coupling port while the other half is traveling with the beam along the stripline. When the beam exits the stripline, an inverted image current pulse is generated. One half of it cancels with the incoming current pulse at the downstream port and the other half of the inverted pulse continues to co-propagate towards the upstream port of the stripline, where it is coupled out to produce two pulses of opposite sign, separated by twice the propagation delay of the stripline. In the ideal case that the bunch distances match twice the stripline length L, the inverted reflected signal cancels with the incoming signal from the following bunch. For a Gaussian bunch distribution I beam.t/ D I 0 exp t 2 =2t 2 with a bunch duration t, the voltage V A at one of the stripline ports is given by V A.t/ D Z W 2 2 I 0 e t 2 =2 2 t e.t 2L=c/2 =2 2 t : (1) Here, Z W is the stripline characteristic wave impedance (typically equal to the load impedance at the out-coupling port), is the coverage angle of the stripline within a cylindrical vacuum tube, and I 0 is the bunch current. Resonant striplines with increased signal levels (compared to matched striplines) have been designed as FEL LINAC and transfer line BPM pickups for supporting low-charge operation. In this configuration (see lower part of Fig. 3), the œ=4 stripline resonators are excited by the beam at the open-circuit end, and the signal is extracted at out-coupling ports, which are shifted from the short-circuit end towards the middle of the striplines, where the magnetic field of the modes is highest. This shift causes a mismatch between stripline and coupler impedance, which reduces the bandwidth of the pickup signal and increases the signal level by the same amount. The Page 11 of 40

12 Fig. 3 Schematic sketch of matched stripline (top) and resonant stripline (bottom) pickups with typical waveforms corresponding improvement of the position resolution ıx; y is inversely proportional to the loaded quality factor Q L. The coupling between the four striplines leads to different modes: a monopole mode with all voltages on the strips in phase, being proportional to the bunch charge; two dipole modes (one mode per plane) with the voltages from opposite electrodes 180 ı out of phase, being proportional to the beam positions; and a quadrupole mode. The design presented in Citterio et al. (2009) was optimized for the same monopole and dipole frequency and highest sensitivity at low bunch charges. Using correlation measurements with several LINAC BPMs, the LCLS- matched stripline BPM system has obtained an excellent rms position resolution of 5 m at bunch charges of 1 nc and 220 pc (Medvedko et al. 2009). At low charges (20 pc), however, the BPM signal becomes weaker, and the position noise has been reported to increase to 25 m (Loos et al. 2010),whichinsome locations is comparable to the beam size and thus insufficient for beam orbit control. The resonant stripline BPM design for the SwissFEL Injector Test Facility has achieved a single-bunch rms position resolution of 7 m for the standard SwissFEL low charge mode at 10 pc. The same system has obtained a relative charge resolution of 1.5 % for bunch charges as low as 2 pc, with a charge resolution limit of 10 fc (Keil et al. 2010). High precision, sub- m shot-by-shot beam position measurements as required along the undulator sections of an FEL can only be achieved with cavity BPM pickups, which provide the highest signal output levels of all pickup types discussed so far. Several cavity BPM systems have already been designed and successfully operated in FEL facilities to cope with a large variety of beam parameters, like different bunch charge ranges, bunch repetition rates, and beam pipe diameters (Lill et al. 2007; Maesaka et al. 2011; Stadler et al. 2012; Keiletal.2013; Young et al. 2013). A short, relativistic electron bunch, passing near the center of a cavity pickup, excites electromagnetic fields of a wide spectral bandwidth. The amplitudesand frequencies of the excited eigenmodes in the cavity depend on its particular configuration. The two fundamental resonances are most interesting for beam position measurements, since the symmetric TM 010 monopole mode is proportional to the bunch charge and the antisymmetric TM 110 dipole modes provide signals, which are (linearly) proportional to horizontal and vertical beam displacements times the bunch Page 12 of 40

13 Fig. 4 Scheme of a waveguide-loaded cavity BPM (upper or top left) with field distributions for the TM 010 monopole and TM 110 dipole modes (upper or top right) and a sketch of the corresponding frequency spectrum (bottom) charge. A schematic view of a waveguide-loaded, cylindrical cavity BPM pickup and its first two eigenmodes is shown in Fig. 4. Contrary to button or stripline pickups, where the beam-induced voltages from all four electrodes are equal for a centered beam, the signal from the positionsensitive dipole mode of a cavity BPM vanishes for an on-axis bunch. This circumstance and the increased signal strength due to the resonant characteristic of the pickup make cavity BPMs most attractive for high-resolution position measurements, especially when the beams are not experiencing large orbit deviations as along the undulator sections of an FEL. The beam position from a cavity BPM is determined from the out-coupled voltage of the TM 110 dipole mode (offset times charge), normalized with the charge-related TM 010 monopole mode signal, which is usually provided by a reference cavity. With the combination of position and reference cavities, it is also possible to determine the direction of the beam displacement by considering the relative phases between the monopole and dipole modes. The electrical fields, which are induced by a beam passing close to the center of a cavity, can be derived from the d Alembert equation. Its solutions lead to Bessel functions J m of the order m D 0 and 1 as the first two eigenmodes, which are used for beam position measurements. The energy W n transferred into these modes oscillates in the resonator until it is dissipated in the cavity walls or coupled out through external waveguides towards the BPM electronics. In this way, the resonance frequencies n, which are related to the eigenmodes of the cavity, have a finite decay time or spectral width (in the frequency domain), which is defining the internal and external quality factors Q 0 ;Q ext Q n 0 D! n W n P n diss and Q n ext D! n W n : (2) Pout n Here, Pdiss n is the dissipated power for the nth cavity mode and P out n is the power coupled out of this mode. The stored energy decays proportional to the so-called loaded quality factor QL 1 D Q0 1 C Qext 1, and the decay time is given by Page 13 of 40

14 n D 2Q L! n : (3) By following the analytical treatments of the theory of cavity BPMs as given in the references (Liapine 2004; Walston et al. 2007; Lipka 2009), the output voltage V out for an off-centered, Gaussian-like bunch of length z and charge q into an output line with impedance Z can be expressed as V out D p ZP out D s Z! nw n Q ext D! n 2 s Z Q ext Rsh Q 0 q x x 0 exp!2 z 2 ; (4) 2c 2 whereas the line voltage V x.t/ parallel to the cavity axis is given by V x.t/ D V out e t sin.! t/ : (5) Here, R sh is the shunt impedance, which is a measure of the interaction (energy transfer) of the electron beam with a cavity eigenmode, and x is the beam displacement from the cavity BPM axis x 0. Beam Position Monitors: Electronics and Signal Processing The input signal levels to the BPM electronics (usually the analog RF front end), the processing frequency, and its spectral composition can be obtained from the combination of the beam excitation spectrum and the pickup response, which is schematically illustration of BPM pickup responses is in Fig. 5 for typical FEL BPM pickup types. The choice of BPM electronics design needs to consider the main operational demands of the particular accelerator (SRLS or FEL) on beam position measurements such as resolution, sensitivity, drift, dynamic range, bandwidth and processing time as well as possible budgetary constraints. The preferred choice for normal- and superconducting FELs, where high resolution single-shot and shot-to-shot position measurements are required, is the combination of high sensitivity resonant pickups like cavities and/or striplines with high bandwidth (few MHz) electronics. For cavity BPMs, which are exclusively covered in this section, the processing frequency is usually not determined by the bunch repetition rate but by the resonant frequency of the pickup. While cavity BPMs with high resonant frequencies (6 12 GHz) promise excellent position resolution, the very tight mechanical tolerances, substantial signal attenuation at high frequencies, as well as the limited availability of well-characterized electronic components may favor the choice of lower frequency (<6 GHz) designs, resulting in the similar performance levels at lower overall BPM system costs. Quite elaborate overviews of the most common approaches of BPM signal processing schemes for different accelerator types (i.e., SRLS and FELs) have been described and reviewed in Vismara (1999), Forck et al. (2008), and Decker (2013). However, as stated already in the chapter on SRLS diagnostics, the rapid advancement in digital signal processing within the past decade with the availability of high-resolution (14 16 bit) and high-sampling rate (150 MSample/s 1 GSample/s) analog-to-digital converters (ADC), high processing power DSPs (digital signal processors), and FPGA (field programmable gate arrays) provides the possibility of unifying the signal processing schemes and simplifies the state-of-the-art SRLS and FEL BPM electronics by using modular Page 14 of 40

15 Fig. 5 Relation of signal spectra for eligible SRLS and FEL BPM pickups (solid lines) with typical excitation spectra for electron bunches (gray background/dashed lines) Fig. 6 Simplified building blocks of FEL cavity BPM electronics (from Keil et al. 2012). Note that the digital signal processing part (green) isthesameforsrlsbpms(seealsofig.4 of the chapter on SRLS diagnostics) and FEL BPMs, and just the RFFE and the signal processing firmware is adapted to the specific BPM and accelerator type building blocks as shown in Fig. 6 for FEL BPM systems and as illustrated for SRLS BPM systems in Fig. 4 of the chapter on SRLS diagnostics. Following this conceptual approach, the digital signal processing parts use generic hardware with accelerator-specific software and firmware for data processing and transfer to the control system or feedback networks, while the analog RF front end (RFFE) electronics is customized to the specific input signals from the BPM pickup. In this respect, the design approach for high sensitivity cavity BPM electronics, which are mainly used throughout FEL undulator sections but may also be a solution for FEL accelerators with low bunch charge (e.g., 10 pc) operation modes, can be summarized in the following way as illustrated in Fig. 7: Low signal levels may require a 180 ı hybrid close to the pickup (in the accelerator tunnel) to double the amplitude of the (position-sensitive) dipole mode signal and to reduce the influence of the (beam charge-related) signal from the monopole mode. Phase shifters before the hybrid account for differences in cable lengths and compensate for temperature-induced phase drifts. Page 15 of 40

16 Fig. 7 Block diagram of cavity BPM electronics as used in FEL accelerators (only the Y channel is shown in detail) At high processing frequencies (>6 GHz), the quite strong signal attenuation in the RF cables requires a down-conversion stage to an intermediate frequency (IF) close to the BPM pickup in the accelerator tunnel. The down-converter operates at a phase-locked local oscillator (LO) frequency, which is usually derived from the accelerator reference signal. Additional band-pass filtering of the RF input signals at the fundamental dipole mode rejects higher-order resonances and avoids saturation of electronics components. A remotely switchable gain stage allows the selection of the input signal amplification depending on the actual bunch charge. After further band-pass filtering and amplification, the IF signals can be transferred to the digital signal processing electronics, which is located outside of the accelerator tunnel. Lower-frequency cavity BPM signals (typically 6 GHz) can be transferred without substantial losses to the RFFE electronics in the technical gallery outside of the accelerator shielding walls. Apart from avoiding potential radiation damage of the RFFE, this also facilitates the BPM electronics maintenance during FEL user operation. Similar to the RFFE electronics in the accelerator tunnel, band-pass filtering, remotely switchable gain adjustment, and quadrature down-conversion, followed by band-pass filtering and amplification at the IF stage, allow subsequent digitization of the I and Q signals by a phase adjustable ADC, which is synchronously clocked to the accelerator reference signal. In case of lowq cavity BPMs, a so-called homodynedown-conversion to baseband is preferred, where the BPM input and LO signals are at the same frequency and the position information is taken from the amplitude modulation of the RFFE output signal. High Q cavity BPM signals are usually down-converted according to the heterodyne method using a LO at a different frequency than the input signal from the beam. The IF output provides amplitude and phase information, which are both digitized and further (digitally) down-converted to baseband. The beam positions are obtained by digital normalization of the RFFE output signal amplitudes with the bunch charge-dependent reference cavity (monopole mode) signal followed by a scaling with predefined or beam-based calibration data (Fig. 7). At FEL facilities like LCLS, stripline BPM systems have been installed and operated along the LINAC and beam transfer lines, providing single-shot position resolutions of 5 m at beam charges above 200 pc and a long-term stability of 5 m per hour (Medvedko et al. 2008). At lower bunch charges (10 50 pc), which are typically applied for XFEL short-pulse operation modes, cavity BPM systems may also be the best choice along the LINAC and transfer lines to obtain single-shot position resolutions below 10 m. The required resolution of well below 1 m along Page 16 of 40

17 2 12 Beam Position [μm] BPM-17 x-position Number of BPMs y-resolution histogram BPM-17 y-position Day of Feb Resolution (nm) 0.6 BPM Resolution [mm] SACLA undulator line x-positions y-positions Z [m] Fig. 8 Long-term position stability for a LCLS X-band cavity BPM over days. Blue error bars represent the rms spread of position, red bars are the expected error of the mean (Smith et al. 2009) (top left or upper left). Histogram of the vertical position resolution achieved with the LCLS cavity BPMs in the undulator section (Smith et al. 2009) (top right or upper right). SACLA C-band cavity BPM position resolutions along the undulator line (Maesaka et al. 2011)(bottom) the XFEL undulator sections to provide sufficient overlap between the electron beam and the X- ray pulses for effectively driving the SASE gain mechanism has been achieved with various cavity BPM systems at beam pipe diameters of 10 mm. Examples of the performance of the LCLS X-band cavity BPMs and the SACLA C-band cavity BPMs are shown in Fig. 8. Transverse Profile Monitors: General Introduction In FELs, SR monitors are only utilized behind bending magnets in bunch compressors, collimator sections, and beam distribution lines to observe the beam energy distribution. Such noninvasive diagnostic devices can unfortunately not be used for two-dimensional profile measurements in linear accelerators, where scintillating or optical transition radiation (OTR) screens need to be inserted in the beam path to visualize the transverse beam distributions. One-dimensional projections of the transverse beam profiles may also be obtained with wire scanners, which are only partially destructive to the beam when thin wires of 5 20 m diameters are used. However, only averaged beam profiles can be obtained, by detecting the loss signal from the scattered particles at a location downstream of the wire scanner. While online monitoring of beam profiles even during FEL user operation might be feasible, wire scanners will not be treated in detail in this chapter, but further information can be taken from Hayano (2000), Castro et al. (2005), and Krejcik et al. (2012). The following section focuses on two-dimensional (optical) profile monitors, which are capable of single shot (and shot-by-shot) measurements, thus highlighting recently developed screen monitor designs for FELs, which suppress the imaging of coherent OTR (COTR) from high brilliant and highly compressed electron bunches. Page 17 of 40

18 Screen Monitors for Transverse Profile Measurements in FELs While scintillator or optical transition radiation (OTR) screens are also used in SRLS injectors (pre-injector LINACs and booster synchrotrons), as well as in the SRLS storage ring injection region as beam profile monitors, in this context these monitors are only treated in connection to FELs or other linear accelerator-based facilities, where they are the standard diagnostics for two-dimensional transverse beam profile measurements. Apart from beam finding during the early commissioning phase, the 2-D profile information from screen monitors are frequently used to control the matching into different parts of the accelerator and in dedicated diagnostics sections for the determination of the transverse phase space (projected and sliced emittances). At dispersive locations like spectrometer arms, bunch compressors, and collimator sections, screen monitors can also provide information about the beam energy distribution. Figure 9 provides schematic layouts of OTR and scintillating screen monitors as well as angular emissions of OTR and a selection of the most prominent scintillating materials used in particle accelerators. Transition radiation (TR) is generated when relativistic charged particles cross the boundary between two media with different dielectric properties (e.g., the beam pipe vacuum and a metallic foil). It is emitted in the backward direction respectively along the specular reflection axis of the medium s surface, when the induced mirror charges in the medium are suddenly canceled by the entering charged particle beam. When the charged particles exit the medium, TR is also emitted in the direction of the particle beam. TR is radially polarized, and its spectrum is a continuum ranging from the plasma oscillation wavelength of metals (œ p 10 nm) to mm-waves. For highly relativistic particles ( 1), the maximum OTR intensity is emitted at an angle of 1=.OTR Fig. 9 Schematic OTR screen monitor setup and OTR angular emission for different electron beam energies (top). Schematic scintillator screen monitor setup and different selection of scintillator materials for high- energy, highbrightness electron beams (bottom) Page 18 of 40

19 is instantaneously generated (on the scale of the particle beam duration), and its intensity is linear with the incident number of particles (charge), which makes it highly attractive for beam profile measurements. Unlike OTR, which is a surface emission process of light, scintillation in an inorganic crystal is a quite complex relaxation process of electronic excitations, which is activated when a relativistic charged particle beam passes the bulk material. The spectral composition and the decay time of the generated light depend on the activator material, which is a doped impurity in the scintillator crystal. Decay times for scintillator crystals, which are used for high-energy electron beam detection, are typically in the order of 100 ns, and the scintillation light emission spectra are well matched to the sensitivity of CCD or CMOS cameras (around a wavelength of 550 nm). In addition to the high light conversion efficiency, inorganic scintillator crystals are radiation hard and have good mechanical and thermal properties. However, as the overall light yield depends on the scintillator thickness, where photons are created all along the beam path through the bulk material, multiple scattering inside the scintillator crystal may cause an increase of the measured particle beam size. Likewise, the scintillating light, which is emitted in all directions (4 ), may be reflected multiple times by the inner surface of the scintillator causing an additional blurring of the beam image. Dependencies of spatial resolution of different scintillators on material thickness and light observation angle have been examined in Kube et al. (2012). For observation geometries with a tilt angle between screen and optical axis, the perspective distortions over the region of interest have to be corrected by considering the Scheimpflug imaging principle (Merklinger 1996), resulting in a tilt of the camera chip (see, e.g., upper image of Fig. 10). Screen monitors usually consist of ultrahigh vacuum (UHV) components, which interact with the beam, and a high-resolution imaging system outside of the accelerator vacuum system. A pneumatic or motorized UHV linear feedthrough permits the insertion of a scintillator or an optical transition radiator in the beam. The generated (visible) light is coupled out through a high optical quality UHV viewport (usually made of Kodial glass, (crystalline) quartz or sapphire) and sent via an imaging system towards a CCD or CMOS camera to record the beam profile. The imaging system needs to provide the desired region of interest (RoI), which for FEL facilities is typically in the order of mm, and the required spatial resolution, which for most measurement locations along an FEL accelerator is below 10 m. Since screen insertion typically causes beam loss, the camera should be sufficiently shielded against the associated background radiation and/or located outside of the beam plane. The resulting working distances between screen and camera can thus be up to 0.5 m, so that high- performance telephoto lenses or customized lens designs (Yanagida et al. 2008) are required to provide the desired spatial resolutions. Examples for layouts of FEL screen monitor stations and imaging optics are shown in Fig. 10; results from spatial resolution measurements with test targets and with beam have been presented in the references indicated in the figure caption. The most commonly used scintillator material for screen monitors in FELs is YAG:Ce (Cr-doped yttrium aluminum garnet) crystals of a few tens to hundred microns thickness, while OTR screens have been built in various designs including high optical quality metal foils or metalized (e.g., 100 nm Al) silicon wafers. OTR was for many years the first choice for screen monitors in LINACs; the brilliant and highly compressed FEL beams however generate coherent OTR (COTR) from hot spots in the beam, which are caused by micro-bunching effects such that cameras saturate and beam profile information is completely lost. This COTR emission, which is many orders of magnitude more intense than incoherent OTR, was first observed during LCLS commissioning (Loos et al. 2008) and later validated at other FEL facilities like FLASH (Wesch et al. 2009, 2011; Page 19 of 40

20 Fig. 10 Screen monitor layouts for the European XFEL (Wiebers et al. 2013) (top) and optics bench at SACLA (Yanagida et al. 2008)(bottom) Behrens et al. 2012) and SACLA (Matsubara et al. 2012). Since COTR is not only generated by metallic OTR screens but occurs also at the surface of scintillating screens, alternative screen monitor designs for suppression of COTR have been developed in order to keep access to twodimensional beam profile information, which is especially important for time-resolved (sliced) beam parameter measurements in FEL facilities (for more information, see section on longitudinal FEL diagnostics). Several methods for COTR suppression have been proposed and investigated: The spectral separation of COTR and scintillation light was considered in cases where COTR is predominantly emitted at longer wavelengths and towards the near infrared. The use of short wavelength (UV) optical high-pass or band-pass filters in combination with UV-sensitive cameras was investigated, but the actual COTR suppression was found to be insufficient for beam profile measurements. The temporal separation of COTR and scintillation light has been considered, since the instantaneous COTR emission from ultrashort (ps- to fs-scale) bunches has already disappeared, when scintillating light is still emitted due to the much longer decay times of the excited states in the scintillator material. The use of gated, intensified CCD cameras (ICCD) with a few ns shutter times provides very efficient COTR suppression, and beam profiles with pure scintillation light can be recorded. Unfortunately, ICCDs are rather expensive and may not survive sufficiently long in the hostile radiation environment of FEL accelerators. Page 20 of 40

21 The spatial separation of COTR from scintillating light turns out to be the most effective and at the same time the most economical solution for COTR suppression. It uses the circumstance that COTR is emitted in a narrow cone with opening angle 1= in the direction of specular reflection (for backward OTR), while scintillating light is emitted in all directions (4 /. Two alternative approaches are followed: (a) At SACLA, the forward emitted COTR light cone from the scintillator crystal surface is blocked by introducing a central stop or by using a perforated mirror in the optical path. COTR is either masked out or not reflected towards the camera, while the scintillating light is used for imaging of the beam. This method allows SACLA to keep their existing screen monitor design, providing fairly good imaging results, which are only slightly affected by COTR stray light. (b) The proper choice of the observation angle can be identified by considering Snell s law of refraction (for the selected scintillator crystal material) and the Scheimplug imaging principle. The resulting screen monitor geometries, which have been implemented for the European XFEL (Wiebers et al. 2013) and the SwissFEL projects (Ischebeck et al. 2014), provide sufficient suppression of backward reflected COTR from the scintillator surface and excellent spatial resolution in the order of 10 m for the imaging of scintillating light. Examples for temporal and spatial suppression of COTR are given in Fig. 11. Fig. 11 Temporal COTR suppression at FLASH using a LUAG:Ce scintillator crystal and an ICCD camera (top: images without and with delay time). Spatial COTR suppression at SACLA using a YAG:Ce scintillator crystal and a spatial mask in the optical path (bottom: images without and with mask) Page 21 of 40

22 Longitudinal Diagnostics: General Introduction The following section about longitudinal diagnostics highlights FEL instrumentation devices, which are capable of measuring the bunch length on femtosecond (fs) time scales. For the ultrashort FEL bunches, both time domain and spectral domain techniques have been developed to reveal the longitudinal charge distribution. Although (single-shot) streak cameras have demonstrated time resolutions of a few hundred fs (examples are shown in Fig. 12 of the chapter on SRLS diagnostics), they are predominantly used in FEL facilities to monitor the photo-injector laser pulses and electron bunches before and after the first compression stage. As streak cameras are the main longitudinal diagnostics in SRLS, they have been described in detail in the chapter on SRLS diagnostics and will not be treated here. Direct streaking of the high-energy electron bunches with transverse RF deflecting structures (TDS) is the most frequently used method to measure and optimize the bunch length and the time-resolved ( sliced ) beam parameters, such as sliced emittance and sliced energy spread. Although TDS measurements provide unprecedented time resolutions of a few fs, they are destructive to the beam, so that for normal conducting LINACs (with a single bunch per RF pulse and up to a few hundred Hz repetition rate) they can only be used for setup and commissioning. In case of superconducting LINACs, however, it is possible to monitor the bunch length and sliced beam parameters in the background by executing TDS measurements in the so-called pulse stealing mode, where a single bunch within a bunch train (during the several hundred s long RF pulse) is streaked and analyzed while all other bunches are used to provide SASE light to the users. Online monitoring of the bunching process is provided by bunch compression monitors (BCM) which measure the spectral intensity distribution of coherent radiation, generated by the ultrashortbunches. Therelativetimedifference between bunches in the FEL accelerator and the actively stabilized laser pulses from the reference distribution system (with a typical stability of <10 fs rms jitter and <20 fs pp drift per day) can be measured by electrooptical bunch arrival time monitors (BAM) with <10 fs time resolution. Both BCM and BAM measurements are noninvasive so that their signals can be fed in longitudinal feedback loops to stabilize the bunching process in an FEL accelerator. Measurements beyond the time resolution limit of TDS may be accomplished in the spectral domain by analyzing coherent THz/FIR radiation, which is emitted by ultrashort (sub-fs) bunches. Experiments with such monitors, which are presently under consideration for the ultrashort-pulse (attosecond) operation modes of (future) FEL facilities, as well as electrooptical bunch length measurements, which have not (yet) been established as baseline diagnostics devices at FELs and SRLS, will not be covered in detail but will be referenced at the end of this section. Fig. 12 TDS principle. A short electron bunch is vertically sheared in the TDS transverse deflection field, so that the longitudinal bunch (charge) distribution can be observed on a screen. The 90 ı phase advance between TDS and screen is adjusted with a set of quadrupole magnets (beam transport optics) Page 22 of 40

23 Transverse RF Deflecting Structures Transverse RF deflecting structures (TDS) have been developed for the 1960s high-energy physics experiments as iris-loaded RF waveguide structures for separation of charged secondary particles with different masses and equal momentum (Altenmueller et al. 1964; Loew and Altenmueller 1965). TDS operation in the original particle separator mode requires a relative phase setting between the RF deflecting field and the electron bunch of 90 ı or 270 ı, so that two adjacent bunches experience a maximum deflection in opposite directions. A maximum angular divergence between the head and the tail of an electron bunch can be achieved by operating a TDS at the zero-crossing point (or at 180 ı ) between the phase of the RF deflecting field and the beam. The resulting transverse (often vertical) shearing of the bunch transfers the longitudinalplane intothe transverse direction such that the longitudinal beam profile can be observed on a screen monitor downstream of the deflector. This TDS operation mode is quite similar to the time domain profile measurement of light pulses by streak cameras. It has been first proposed by LCLS (Emma et al. 2000) and since then implemented at every FEL facility as the most important baseline (but destructive) singleshot longitudinal electron beam diagnostics. Figure 12 gives a schematic illustration of the TDS operating principle for bunch length measurements. The angular (here vertical) kick y 0 experienced by the electron bunch can be obtained by integrating the deflecting force F? from the RF electrical field over the path length of the interaction (length of TDS). For ultrashort relativistic electron bunches, it can be approximated to y 0 TDS D 1 E Z F? ds e V? E z! RF c.cos ' RF C sin ' RF / ; (6) where E is the energy of the electron bunch in ev units and s its position along the TDS, e is the electron charge, V? is the deflecting voltage, z is the longitudinal particle coordinate within the bunch relative to its center of mass,! RF D 2 RF the frequency of the RF field, c the speed of light, and ' RF the phase between the RF deflecting field and the centroid of the electron bunch. The deflecting voltages of traveling wave (TW) TDS can be expressed in practical units as MV V?.TW-TDS/ D 1:6 m p LŒm p PŒMW : (7) MW For standing wave (SW) TDS (Alesini et al. 2006), it results in V?.SW-TDS/ D p 2 ŒM W R S ŒM : (8) Here, L is the length of a TW TDS and P is the RF power in the structure. R S is the shunt impedance of a SW TDS. The actual transverse (here: vertical) deflection of the bunch is related to the cos ' RF term, while the sin ' RF term refers to the bunch centroid motion, which is negligible for ' RF D 0. Operating the TDS at zero-crossing results in the maximum shearing (angular divergence) of the beam. When taking the beam optics (transport matrix) between the TDS and the view screen into account, the vertical excursion of a deflected, pencil-like beam ( zero transverse emittance) with initial bunch length z on the screen results to y-screen D e V? E! RF c z cos ' RF pˇtds ˇscreen sin TDS-screen (9) Page 23 of 40

24 The matching optics maximizes the beta-function in the TDS for most effective beam deflection, while the beam transport optics introduces a phase advance between TDS and view screen (ideally 90 ı or 270 ı ) and produces a beam waist on the screen in order to increase the effective (time) resolution of the measurement. Ideally, the beam waist on the screen should match the resolution of the optical imaging system of the electron beam. However,too strong focusing of a high-brightness (low-emittance) beam onto the observation point may cause at low- to medium-energy diagnostics sections beam blowup due to space charge effects. If the beam transport optics is replaced by a drift of length L between TDS and view screen, Eq. (9) simplifies to y-screen D e V? E! RF c z cos ' RF L TDS-screen (10) Calibration of the time axis can be achieved by variation of the RF phase RF around the zerocrossing of the deflecting force using the linear relation between vertical distances y on the view screen and longitudinal distances z within the deflected bunch. The time calibration can be obtained according to the relations z c t and t D ' RF =! RF : (11) Figure 13 illustrates schematically the time calibration of a TDS-based measurement system (top) and shows real calibration data from the LCLS X-band TDS (Behrens et al. 2014), which were taken at beam energies of 15.1 GeV behind the LCLS undulator. The measured scaling factor for the deflected beam on the view screen versus X-band RF phase results to 2.48 mm/degree. With 50 m optical resolution of the imaging system, rms time slices as short as 4.3 fs (at 15.1 GeV) and even 0.8 fs (at 4.7 GeV) can be resolved along the bunch. One TDS behind each FEL compression stage allows the full characterization of the FEL beam in terms of longitudinal charge distribution, time-resolved (sliced) emittance, and (sliced) energy spread by combining the one-dimensional (e.g., vertical) TDS beam deflection with a quadrupole scan or a beam energy spectrometer in the opposite plane. The placement of a TDS behind the undulator permitted in combination with a beam energy spectrometer for the first time the direct visualization of the lasing part of the electron beam. The unprecedented femtosecond temporal resolution provides online information about the FEL (X-ray) pulse length to the users, which allows post-experimental sorting of their collected data points (Fig. 14). Spectral Analysis of Coherent Radiation Incoherent radiation from relativistic particle bunches is emitted at wavelengths, which are shorter than the bunch length of the particle beam. At radiation wavelengths, which are comparable or longer than the bunch length for femtosecond pulses typically in the FIR and THz regime a fixed phase relation between the individual emitters is established, resulting in a temporal coherence of the radiation. This effect applies for all types of radiation generated by relativistic particle bunches, namely, synchrotron radiation (Hofmann 2004), transition and diffraction radiation (Ter-Mikaelian 1972), Smith-Purcell radiation (Kube et al. 2003), as well as Cherenkov radiation (Gatti et al. 2007). Figure 15 illustrates the incoherent and coherent emission of radiation from relativistic particle bunches. Page 24 of 40

25 Fig. 13 Calibration scheme for TDS measurement (top) and calibration data from the LCLS X-band TDS (bottom) (Behrens et al. 2014) At wavelengths much shorter than the bunch length, the phase relation of the radiation field from each individual particle in the bunch is lost, and the sum signal of the amplitudes averages out. The total radiation intensity of such an incoherent emission process is proportional to the number of radiators ( N). For ultrashort relativistic particle bunches with bunch lengths in the order of some tens to some hundreds of micrometer, the individual electrons start to emit in phase resulting in a constructive interference of the individual field amplitudes and a corresponding coherent enhancement of the radiated energy flux. In this case, the total radiation intensity becomes proportional to the square of the number of particles in the bunch ( N 2 /. The degree of coherence of the radiation emitted by the relativistic particle bunches is expressed by the so-called bunch form factor F./, which relates to the Fourier transform of the longitudinal charge distribution S.z/. The total radiated (coherent and incoherent) is given by the following expression: Z I total./ D I 1./.N C N.NC1/ F.// with F./ D ˇ e i 2 c z S.z/ dz ˇ Here, I 1./ is the radiated power from a single particle (electron), N is the number of particles in the bunch, is the frequency of the radiation, and z is the longitudinal coordinate along the bunch. 2 (12) Page 25 of 40

26 a Energy deviation (MeV) c Energy deviation (MeV) Time (fs) 10 0 Time (fs) Current (ka) Time (fs) 10 b d Power (GW) ± 0.3 fs 10 0 Time (fs) Datapoints Interpolation 11.3 ± 1.4 fs 10 Datapoints Interpolation Deconvolution 10 Fig. 14 Single-shot time-resolved images of the LCLS electron beam energy distribution behind the undulator at 20 pc bunch charge and 4.5 GeV beam energy when not lasing (top) and when lasing (bottom). The electron bunch length is 11.3 fs (FWHM), and the FEL pulse length, which can be related to the lower-energy particles transferring energy to the X-ray pulse, has been determined to be 2.6 fs (FWHM) (Images are taken from Behrens et al. 2014) Fig. 15 Incoherent emission of radiation from relativistic particle bunches (top). Coherent enhancement of radiation at wavelengths longer than the bunch length (bottom) Figure 16 shows the CSR spectral intensity for different bunch length of Gaussian beam profiles (left side) and the longitudinal bunch form factors (right side) for different bunch profiles (middle). The recording of the coherent intensity yields information about the compression of the electron bunch, while the measurement of the spectral composition by application of Fourier spectroscopy in the THz regime can be related to the actual longitudinal bunch configuration. The proper design of a spectrally resolving compression or bunch length monitor for sub-ps beams requires the consideration of several peculiarities of coherent radiation, which is emitted in the sub-mm, THz, and far-infrared (FIR) regime and the actual transfer function of the experimental setup needs to be carefully determined in order to obtain a bunch length-related signal. Page 26 of 40

27 Fig. 16 CSR power versus wavelength for Gaussian bunch profiles of different length (top). Different longitudinal bunch configurations (lower left) yield to different spectral compositions of longitudinal form factors (lower right) (Images are taken from Wang 1997 and Menzel 2005) The opening angles of long wavelengths coherent radiation are much wider than the typical 1= approximations for radiation emitted by relativistic particle beams. Gauss optics needs to be applied for the design of the THz beam transport systems, and all quasi-optical components (e.g., vacuum windows, first surface mirrors, beam splitters, wire grids polarizers, etc...) need to be sufficiently dimensioned for avoiding diffraction losses (Goldsmith 1997). Wavelength-dependent properties of quasi-optical components as well as water absorption in the THz to FIR spectral range may dominate the effective transfer functions of the optical systems and strongly distort the characteristics of the measured radiation spectrum. The spectral bandwidth, optical sensitivity, and response time of the detector systems need to match the experimental conditions. Noninvasive bunching or compression monitors are important longitudinal diagnostics to tune and control the bunching process in an FEL accelerator by measuring the spectral intensity of coherent synchrotron or edge radiation emitted by the ultrashort electron bunches. The quadratic dependence of the coherent radiation intensity from the number of particles in a bunch provides a very sensitive signal, which is directly related to the electron bunch length. Figure 17 shows a schematic setup of a compression monitor using coherent synchrotron (CSR) or edge radiation (CER). A plane metallic out-coupling mirror, which can be positioned close to the electron beam path, directs the THz radiation through an UHV window towards a THz detector. Depending on the bunch length and the related wavelength range of interest, crystal quartz, Si, or diamond might be chosen as the UHV window material. The required sensitivity and bandwidth as well as the Page 27 of 40

28 Fig. 17 Schematic setup of a noninvasive compression monitor using CSR or CER (top). Relation of compression monitor signals to bunch length (bottom), taken from Loos et al. (2010) and RF phase settings (bottom), taken from Behrens et al. (2010) wavelength range of interest determine the choice of the THz detectors, which in most of the cases are pyro-detectors, Schottky diodes, or liquid He and liquid nitrogen cooled bolometers. The use of THz high-pass (so-called thick grid) filters, consisting of hexagonally arranged holes in a thick conducting plate, may improve the monitor sensitivity within the desired spectral range. Cross-calibration with the electron bunch length, which can be directly measured by a downstream TDS, or the phase settings of the upstream RF accelerator structures, allows the integration of a compression monitor signal in a longitudinal feedback, which stabilizes the bunching process and may thus improve the stability of the FEL output power. The longitudinal profile of ultrashort particle bunches can be determined by using Fourier spectroscopic methods. Autocorrelation measurements of coherent radiation intensities have been performed at several accelerator facilities by using Michelson or Martin-Puplett type of interferometers providing the Fourier transform of the coherent radiation spectrum. These intensity-based interferometers can determine the absolute value of the bunch form factor F./, while the complex phase, which is required to unambiguously reconstruct the longitudinal bunch profile, is lost during the measurement. Although it is possible to recover the missing phase information by applying the Kramers-Kronig relation (Lai and Sievers 1997), these scanning types of measurements yield only the average bunch length and have thus not been implemented as a standard longitudinal diagnostics in FEL facilities. At FLASH, however, single-shot spectral measurements of coherent radiation intensities performed with a so-called polychromator (a multichannel grating spectrometer) show excellent Page 28 of 40

29 agreement with TDS measurements, so that the future application of spectral bunch length monitors seems a promising diagnostics for ultrashort (<10 fs) bunches, where prism-spectrometers, mid to near-infrared optics, and sensitive detectors can be used. First successful bunch length measurements have already been reported with this method at LCLS in Maxwell et al. (2013). Electrooptical Bunch Arrival Time Monitors The generation of high peak current electron bunches by successive bunch compression and the increasing interest and use of ultrashort photon pulses at the FEL user end stations require precise timing of all critical subsystems in an FEL facility as well as the exact knowledge of the electron and photon beam arrival times. Ultra-stable synchronization of remote accelerator and experimental components can be achieved by distributing an optical time reference from a master laser oscillator (typically a mode-locked laser) via length-stabilized optical fibers to remote accelerator and experimental subsystems like gun as well as seeding and experimental pump-probe lasers, accelerator RF stations, and beam diagnostics systems such as bunch arrival time monitors. Point-to-point timing jitters of <10 fs rms in the frequency band between 10 Hz and 10 MHz as well as <20 fs pp drift over 24 h have been demonstrated with cw and pulsed optical synchronization systems in an accelerator environment. The measurement of the electron bunch arrival time can be obtained by direct correlation of the highly stable laser pulses from the pulsed optical reference distribution system with a transient signal from a high bandwidth beam pickup. Figure 18 shows the operation principle of such a bunch arrival time monitor (BAM), which is described in more detail in Löhl et al. (2010). A commercially available Mach-Zehnder-type electrooptical modulator is the key component of a BAM. It modulates the amplitude of the reference laser pulses according to the applied fast transient RF voltage signal from the high bandwidth beam pickup, which is sensitive to the transverse Coulomb fields of the electron bunches. The acquisition is performed at the zerocrossing of the pickup signal. Any deviation due to bunch arrival time jitter will cause an offset to the reference laser pulse, which will lead to its amplitude modulation. By the use of a delay line with a high-accuracy absolute position encoder, the amplitude modulation can be calibrated with time. A set of electrooptical BAMs have been implemented at the FLASH facility showing strongly correlated variations of the electron beam arrival time in the order of 200 fs (rms). The BAM resolution has been estimated to 6 fs over short time periods and still to <15 fs over the duration of user experiments (up to 4.5 h). In case of the superconducting FLASH accelerator, which contains up to several hundred bunches along its 800 s long RF pulse, an arrival time feedback acting on the RF field amplitude of the first accelerator module could be successfully implemented leading to a strong reduction of the rms bunch arrival time jitter from about 200 fs to <25 fs depending on the setting of the BAM measurement and feedback parameters (Fig. 19). Single-shot, noninvasive measurements of electron bunch length and beam arrival time have also been successfully performed in LINACs and just recently in an SRLS (ANKA storage ring) by electrooptical detection of a Coulomb field-induced beam signal (THz field) in birefringent crystals like ZnTe and GaP (Steffen et al. 2009; Hiller et al. 2014; Müller et al. 2012). Temporal resolutions in the order of 100 fs have been achieved by applying the techniques of temporal and spectral decoding, where the longitudinal electron beam profile is modulated on the spectrum of a laser pulse. Although such EO monitors have shown great potential as noninvasive, online bunch length diagnostics in FEL injectors (before and after the first bunch compression stage), they have Page 29 of 40

30 Fig. 18 Operation principle of electrooptical BAM not been implemented yet as standard longitudinal diagnostics devices and will thus not be treated in more detail within this chapter. Beam Charge, Current, and Loss Monitors The Lorentz contracted electric fields of relativistic charged particle beams (here: electron bunches) induce an image current on the inner surface of the surrounding metallic beam pipe (accelerator UHV system), which is opposite and equal to the absolute value of the free, moving charge inside the tube. Most of the nondestructive beam charge and beam current monitors utilize this relation, which is based on Gauss s law, by intercepting the wall current at a certain point in the beam pipe. A coaxial Faraday cup on the other hand is a piece of high Z, conducting material (e.g., Cu, Ta), which intercepts the electron beam at the end of an accelerator section or along the beam path, when designed as a retractable device. The absorbed charges can be directly measured with a DC amperemeter or an oscilloscope. For accurate charge measurements, all particles need to be stopped in the block of material and the design needs to account for backscattered particles as well as for electrons and positrons, which may escape after multiple scattering in the device. The overall collection efficiency can be simulated by Monte-Carlo codes such as EGS4 (Nelson and Rogers 1985) or GEANT (Hirayama). Application of a positive potential to the absorber block or the surrounding of the Faraday cup by a negatively biased cage may reduce these error sources, Page 30 of 40

31 Fig. 19 Arrival time measurement for 1500 consecutive electron bunches with 0.8 nc bunch charge at FLASH (upper top) and arrival time difference signal between BAM1 and BAM2 (lower top). The effect of a bunch arrival time feedback along a FLASH bunch train results in a strongly improved beam arrival time stability of 25 fs (bottom) (measurements have been taken from Löhl et al. 2010) but its application in SRLS injectors and FEL accelerators remains usually limited to low- and medium-energy electron beams of a few MeV to a few tens of MeV. Careful impedance matching to a 50 transmission line provides high bandwidth up to many GHz, allowing the measurement of the particle beam s time structure. Figure 20 shows the outline of a coaxial Faraday cup, the design as a retractable device as being used behind the electron gun of the Swiss Light Source (SLS) pre-injector LINAC, and a low-energy bunch charge measurement. A wall current monitor (WCM) is a noninvasive device, which provides similar high bandwidth bunch charge information as a coaxial Faraday cup. The beam-induced image (wall) current develops a voltage across several resistors, which are placed over an insulating gap (either a ceramic material or a vacuum gap). The resistors are connected in parallel (e.g., 16 resistors of 50 each resulting in a gap resistance of 3 / and placed symmetrically around the outside of the beam pipe in order to avoid a beam position dependence of the gap voltage. The WCM signal Page 31 of 40

32 33.7 (31) 13 (46.7) Elektronenstrahlschweissen inner and outer conductor N-connector CF40 flange Bunch Charge [nc] Bunch Duration [ns] Fig. 20 CAD drawing of a coaxial Faraday cup (top or upper left), retractable coaxial Faraday cup for SLS pre-injector LINAC (top or upper right), and bunch charge measurement (bottom) is usually sent via a 50 low loss coaxial cable to a wide bandwidth oscilloscope in the technical gallery. The WCM high- frequency response depends on the resistors and the gap capacitance, which is given by the permittivity of the insulating material and its length. Thus, small-sized (SMD) chip resistors, which still behave as resistors at high (GHz) frequencies, are preferred, and a simple vacuum cut in the beam pipe instead of a ceramic gap may be implemented to provide the highest possible bandwidth. The low frequency cut-off can be decreased by using high permittivity ferrite material in the WCM screening box, which shield against ambient/environmental (RF) noise. The WCM principle and a high bandwidth design for CTF-3 (CLIC Test Facility) as well as a WCM signal from the CTF beam are shown in Fig. 21. Integrating current transformers (ICT) are used to monitor the bunch charge and thus the transmission efficiency in LINACs and transfer lines. The ICT consists of two coupled transformers, which are combined in a tape-wound core of high permeability metal alloy, which is installed around a ceramic gap in the beam pipe. The ICT core is equipped with a metallic (e.g., copper) shield, which shunts unwanted beam pipe currents around the core and prevents radiation of beam noise. The image current of a short, coasting electron bunch induces a charge in a series of low-inductance capacitors, which are evenly distributed around the toroid. Discharging of the Page 32 of 40

33 I wall Ferrite ceramic gap R V out GAP CONNECTOR FEED THROUGH I beam φ114 mm 2 mm φ40 mm I wall Ferrite V out R vacuum gap FERRITES I beam mm Number Averages = 4 time: 50ps/div FWHM: 97ps scale: 25mV/div 03 Dec :13 Fig. 21 WCM operating principle (top or upper left). The implementation with a ceramic gap is represented at the top, a vacuum cut as a gap in the beam pipe is shown on the bottom (the actual device is cylindrical symmetric, the sketches show only a single signal out-coupling). CTF-3 WCM design (top or upper right) and a signal from the CTF beam showing the fast WCM response time (bottom) (The central and the right images have been taken from Odier 2003) capacitors via the readout transformer is accomplished at a much slower time constant (tens of nanoseconds) than the initial signal from the beam pulse, which is in the order of picoseconds. In this way, the high-frequency components from the beam are drastically reduced, so that the output signal becomes insensitive to bunch length and beam position changes. In addition, the eddy current losses in the cores are almost negligible, and the collected charge in the load resistor corresponds to the bunch charge divided by the turn ratio of the two coupled transformers. The ICT output pulse is usually integrated over a time period of microseconds by a bunch charge monitor (BCM-IHR) electronics circuit, which can be read out in the accelerator control system by using a slow, high-resolution analog-to-digital converter. The whole system is pre-calibrated and provides good absolute accuracy for bunch charges ranging from a few nc to a few tens of pc. An onboard calibration generator provides the possibility of an online assurance of the measurement reliability. ICTs and BCM-IHR electronics are commercially available and can be provided in UHV compatible in-flange and in-air versions with a variety of different diameters (Bergoz) (Fig. 22). The short-pulse/low-charge operation modes of X-ray FEL facilities require an improved sensitivity of the bunch charge measurement to about a 1 % level at 10 pc. In addition, better immunity against dark current generated by high gradient photo-injectors and accelerator structures have been anticipated. These extended requirements triggered the development of an improved ICT version the so-called Turbo-ICT which maximizes the beam-induced signal, while minimizing the noise from various sources like dark current, electronics, RF, and other electromagnetic interference (Fig. 23). A combination of multiple cores (2, 4, or 8 in a single in-flange package) with combined output signals and a new, low loss alloy, which allows the transfer of the beam-induced charge at higher Page 33 of 40

34 SMA connector metallic shield ICT ceramic gap E 10 ICT output signal [V] E+00 5E 10 1.E 09 2.E 09 2.E 09 3.E 09 3.E 09 4.E Time [ns] Beam Charge [C] Fig. 22 Schematic drawing of an ICT (upper left) and its installation in one of the ALS (Advanced Light Source) beam transfer lines (upper right). ICT and BCM-IHR output signals from the ALS (bottom) (Images have been taken from Bergoz) bandwidth through the Turbo-ICT, provides increased output amplitudes at much shorter pulse durations (3 ns for the Turbo-ICT instead of 70 ns for the ICT). Since the noise for the signal processing electronics grows only proportional to the square root of the bandwidth increase, the signal-to-noise ratio is improved accordingly. Narrow-band filtering of the ICT output signals at a carrier frequency of 180 MHz and further signal processing with a logarithmic amplifier are done in the BCM-RF electronics. Both Turbo-ICT and BCM-RF electronics are also commercially available, and details about the development can be taken from Artinian et al. (2012). Beam loss and dose rate monitors are usually integral parts of accelerator and personal protection systems of SRLS and FEL facilities. They provide a detailed understanding of loss mechanisms, which may occur regularly and even well controlled at predefined locations (e.g., collimators, aperture limitations, etc.) or which may happen incidentally by misalignment, malfunctioning, and faulty operation of components. In addition to the obvious improvement of accelerator operation through reduction of beam loss (e.g., lifetime increase of an SRLS facility), unwanted activation and even damage of sensitive equipment in the accelerator tunnel can also be avoided with welldesigned loss monitor, dose rate, and machine protection systems. Loss monitor and dose rate information should be available at various data rates (from milliseconds for machine protection to years for monitoring accumulated dose rates). In any case, they should be regularly archived for follow-up of accelerator operating conditions and postmortem event analysis. Online information on loss locations with automatic alarm generation can be quite helpful for operators in the control room. The energy loss of an incident charged particle scattering on atomic electrons can be described by the Bethe-Bloch equation: Page 34 of 40