• A.H. Lumpkin, H.T. Edwards, A.S. Johnson, J. Ruan, J.K. Santucci, Y.-E. Sun, R. Thurman-Keup
  Fermilab, Batavia

It is recognized that beam manipulations such as a flat beam transformation followed by an emittance exchange (EEX) could support a high gain free-electron laser (FEL) push for shorter wavelengths. An ongoing program on demonstrating the exchange of transverse horizontal and longitudinal emittances at the Fermilab A0 photoinjector (A0PI) has benefited recently from the upgrade of several of the key diagnostics stations. The use of an array of 50-micron wide slits to sample the phase spaces to measure divergences of less than 100 microradians resulted in 20 times smaller images with positions distributed over several mm. Improvements in the screen resolution term and reduction of the system depth-of-focus impact by using YAG:Ce single crystals normal to the beam direction will be described. On the longitudinal side, the requirements to measure small energy spreads (<10 keV) in the spectrometer and bunch lengths less than 500 fs dictated specifications. Upgrades to the Hamamatsu C5680 streak camera and the addition of the Martin-Puplett interferometer addressed the short bunch lengths. An example of the EEX tables will be presented.

• S. Zhang, S.V. Benson, J.G. Gubeli, G. Neil, F.G. Wilson
  JLAB, Newport News, Virginia

A sophisticated research device such as an advanced photo-cathode injector for a high energy accelerator-based X-ray light source requires drive lasers with a flat-top shape both in time and space in order to generate high-quality short electron beam bunches. There are a number of different ways to spatially shape laser beams, but the practical methods for temporal shaping, in particular in the picosecond or femtosecond regime, are quite limited. One simple way to shape laser pulses is pulse stacking by birefringent crystals. This method has been adopted for several applications. While the method itself has the great advantage of simplicity, the overall performance depends on many factors. In this paper, we will present both analysis and a recent experimental study about important pulse shaping characteristics that, to our knowledge, have not been adequately explored before. Evaluation on the pros and cons of the method and how to improve the overall performance will be discussed.

• M. Otevrel, G. Asova, J.W. Bähr, M. Hänel, Ye. Ivanisenko, M. Krasilnikov, M. Mahgoub, D.A. Malyutin, A. Oppelt, S. Rimjaem, F. Stephan, M. Tanha, G. Vashchenko, X.H. Wang
  DESY Zeuthen, Zeuthen
• A. Brinkmann, K. Flöttmann, D. Reschke
  DESY, Hamburg
• M.A. Khojoyan
  YerPhI, Yerevan
• J. Saisut
  Chiang Mai University, Chiang Mai

A new photocathode electron gun is about to be characterized at PITZ*. It is an L-band normal conducting 1.6 copper cell cavity with improved cooling system. It has the same design as the previously installed gun, characterized at PITZ during the run period 2008/9**. Due to the particle-free surface cleaning method utilizing dry ice, a significant reduction of the dark current was achieved in case of the previously tested cavity. This effect is also expected for the new gun. To improve the accuracy of the RF power measurement and control, a new in-vacuum directional coupler was installed between the T-combiner combining the two 5 MW arms of the RF source and the input coaxial coupler. The new in-vacuum coupler will provide much more accurate information about the RF power in the gun and will allow applying appropriate control feedback. Consequently improved stability of the gun operation is expected. Tuning and conditioning results of this new gun cavity will be presented as well as the results of the measurements of the gradient and the gun phase measurements using this new coupler.

* Photoinjector Test Facility at Zeuthen
** S. Rimjaem et al., EPAC 2008, Genoa, Italy.

• M.A. Khojoyan, G. Asova, J.W. Bähr, H.-J. Grabosch, L. Hakobyan, M. Hänel, Ye. Ivanisenko, M. Krasilnikov, M. Mahgoub, M. Otevrel, B. Petrosyan, S. Rimjaem, A. Shapovalov, R. Spesyvtsev, L. Staykov, F. Stephan, G. Vashchenko
  DESY Zeuthen, Zeuthen
• G. Klemz
  MBI, Berlin
• S. Lederer
  DESY, Hamburg
• B.D. O'Shea
  UCLA, Los Angeles, California
• D. Richter
  HZB, Berlin
• J. Rönsch-Schulenburg
  Uni HH, Hamburg

The Photo Injector Test facility at DESY, Zeuthen site (PITZ), develops and optimizes electron sources for Free Electron Lasers (FEL’s) such as FLASH and European XFEL. The electrons are generated by the photo effect using a cesium telluride (Cs2Te) cathode and are accelerated in an 1.6-cell L-band RF-gun cavity with about 60MV/m maximum accelerating field at the cathode. The upgraded laser system at PITZ produces flat-top and Gaussian laser pulses of different time durations. Emittance measurements have been done for short Gaussian laser temporal profile ~2ps FWHM and for 6.6 MeV electron beam energy. The transverse projected emittance was measured for various transverse laser spot sizes at the cathode and different low bunch charges to find an optimum condition for thermal emittance measurements. ASTRA simulations were performed for various measurement conditions to estimate the space charge contribution to the emittance. The comparison of emittance measurement results and simulations is presented and discussed in this contribution.

• M. Hänel, M. Krasilnikov, F. Stephan
  DESY Zeuthen, Zeuthen

The Photoinjector Test Stand at DESY, Zeuthen site (PITZ) was established to develop and optimize electron bunch sources for linac-based free electron lasers like FLASH or the future European XFEL. The successful operation of such FELs requires electron bunches of very low normalized transverse emittance of the order of 1 mm mrad at a charge of 1 nC. One key issue for obtaining low-emittance electron bunches is the possibility to influence the electron bunch properties by varying the photocathode laser pulse characteristics. This contribution focuses on the discussion of deviations from the optimum transverse shape of a circular flat-top. Different types of modulations are added to the flat-top and the resulting change in transverse emittance will be discussed based on beam dynamics simulations.

• S. Rimjaem, G. Asova, J.W. Bähr, H.-J. Grabosch, L. Hakobyan, M. Hänel, Ye. Ivanisenko, M.A. Khojoyan, G. Klemz, M. Krasilnikov, M. Mahgoub, M. Otevrel, B. Petrosyan, A. Shapovalov, R. Spesyvtsev, L. Staykov, F. Stephan
  DESY Zeuthen, Zeuthen
• S. Lederer
  DESY, Hamburg
• M.A. Nozdrin
  JINR, Dubna, Moscow Region
• B.D. O'Shea
  UCLA, Los Angeles, California
• D. Richter
  HZB, Berlin
• J. Rönsch-Schulenburg
  Uni HH, Hamburg

Transverse projected emittance optimization is one of the main research activities at the Photo Injector Test facility at DESY, Zeuthen site (PITZ). The emittance measurement program in the 2009 run period concentrated on projected emittance measurements using a single sit scan technique. The photocathode laser profile has been optimized yielding small emittance. The flat-top temporal profile has been used in the standard projected emittance measurements. The small emittance values down to less than 1 mm-mrad have been measured for the nominal 1 nC bunch charge. Emittance optimizations for lower bunch charges have also been conducted using the same measurement setup and procedure as for the case of 1 nC. Numerical simulations have been carried out to compare the results with the measurements. Measurement and simulation results of the transverse emittance for the bunch charges of 0.1, 0.25, 0.5 and 1 nC will be reported and discussed in this contribution.

• J. Saisut, G. Asova, M. Krasilnikov, S. Rimjaem, F. Stephan
  DESY Zeuthen, Zeuthen
• J. Saisut
  ThEP Center, Commission on Higher Education, Bangkok
• C. Thongbai
  Chiang Mai University, Chiang Mai

The Photo-Injector Test Facility at DESY, Zeuthen site (PITZ) aims to optimize high brightness electron sources for linac-based FELs. Since the performance of an FEL strongly depends on the transverse electron beam emittance, the electron source is studied in details at PITZ by measuring the emittance with the help of the Emittance Measurement Systems (EMSYs). The EMSY employs the slit scan technique which is optimized for 1nC bunch charge and, therefore, it might not be an optimal choice for low charge bunches. To extend the ability of the facility for transverse phase space measurements, a module for phase-space tomography diagnostics and its matching section are installed in 2010. The basic components of the module are four screens separated by FODO cells. It is designed for operation with high charge and low energy beams*. This work studies the performance of the tomography module when it is operated with low charge beams. The influence of different beam parameters is evaluated according to the requirement to match the envelope to the optics of the FODO lattice. Simulation results and phase space reconstructions are presented.

G. Asova et al., ‘Design considerations for phase space tomography diagnostics at the PITZ facility', proceedings of DIPAC 2007, Mestre, Italy.

• H. Maesaka, S.I. Inoue, Y. Otake
  RIKEN/SPring-8, Hyogo
• S. Matsubara, Y. Tajiri
  JASRI/SPring-8, Hyogo-ken

To measure the femtosecond bunch length (10 - {10}00 fs) of the XFEL facility at SPring-8, we developed a coherent synchrotron radiation (CSR) monitor and a streak camera system. A pyro-electric detector was employed to measure the CSR intensity, since the CSR frequency region is THz or far infra-red. The CSR source is a dipole magnet of a chicane section. For the streak camera, we used Hamamatsu FESCA200, which has 200 fs resolution. The temporal structure of the optical transition radiation (OTR) from a metal mirror is observed by this camera. By using these monitors, the bunch length dependence was measured as a function of the rf phase of an S-band accelerator upstream of the bunch compressor at the SCSS test accelerator. A strong correlation between the CSR intensity and the S-band phase was observed. The CSR intensity was small at a debunching phase and the intensity increased as the rf phase was shifted to the bunching direction. Finally, it decreased in the over-bunching region. The bunch length data from the streak camera also had the same tendency. Thus, the bunch compression characteristics were appropriately measured and were consistent with our simulation results.

• E.J. Montgomery, D.W. Feldman, P.G. O'Shea
  UMD, College Park, Maryland
• J.R. Harris, J.C. Jimenez
  NPS, Monterey, California
• K. L. Jensen
  NRL, Washington, DC

Photocathodes are a promising electron source for future high average current FELs, with ps response, kA/cm² peak and A/cm² average current, but will require delicate cesium-based coatings to achieve requisite quantum efficiency (QE). The UMD dispenser photocathode replenishes cesium from a subsurface reservoir, extending lifetime [1]. Recesiation has been shown to reverse oxidizer-induced QE loss [2]. Optimization of pore size and spacing will enable uniform recesiation without emitting excess cesium into the cavity. We here quantify for the first time cesium emission from active dispenser photocathodes and summarize status of experimental and modeling efforts.

[1] N.A. Moody et.al., Appl. Phys. Lett. 90, 114108 (2007).
[2] E.J. Montgomery et al., AIP Conf. Proc. {10}86, 599 (2009).

• C. Vicario, R. Ganter, C.P. Hauri, S. Hunziker, F. Le Pimpec, C. Ruchert, T. Schietinger, A. Trisorio
  PSI, Villigen PSI

For high brigthness photocathode RF gun, proper laser pulses should be used to generate the photocurrent. Transverse uniformity and longitudinal laser flat top profile are predicted to improve the electron beam brigthness. Moreover the laser stability and its sub-ps synchronicity respect to accelerating field are essential for stable and reliable operation. Finally the intrinsic emittance, which is the ultimate limit for the beam emittance, could be tuned by varying the laser photon energy. For this purpose, we developed a mJ frequency tripled Ti:sapphire laser, tunable within 260-283 nm range. Dependence of the intrinsic emittence and of the quantum efficiency with the photon energy has been measured and compared to theory for various metallic photocathode. In this paper the R&D activities aiming at the photocathode laser for the future SwissFEL project are reported.

• B. Keil, A. Citterio, M.M. Dehler, R. Ditter, V. Schlott, L. Schulz, D.M. Treyer
  PSI, Villigen

This paper introduces the architecture and first beam commissioning results of the standard BPM system for the SwissFEL test injector, a 250MeV linac that is progressively being commissioned in order to perform R&D for the "SwissFEL" 5.8GeV hard-X-ray FEL facility proposed at PSI. Since the SwissFEL has a nominal bunch charge range of 10-200pC, the test injector is equipped with 500MHz resonant stripline BPMs that are optimized for high dynamic range and sensitivity, to support machine operation well below 10pC. Beam tests with a 5 GSa/s direct sampling electronics designed at PSI showed a single-bunch resolution of <20um RMS at 2pC and typically 7um RMS for charges >10pC. The BPMs also measure bunch charge, insensitively to dark current, with <30fC RMS resolution at 2pC.

• N. Milas, C.H. Gough
  PSI, Villigen

The SwissFEL facility will produce coherent, ultra-bright, and ultra-short photon pulses covering a wavelength range from 0.1 nm to 7 nm, requiring an emittance between 0.18 to 0.43 mm mrad. In order to provide electrons to the soft X-ray beam line of the SwissFEL a switchyard is necessary, which will divert the electron beam, with an energy of 3.4 GeV, after the first set of accelerating structures. This switchyard has to be design in such a way to guarantee that beam properties like low emittance, high peak charge and small bunch length will not be spoiled. In this paper we present the schematics and discuss ideas and constraints on the kicker, misalignments and charge fluctuation for the SwissFEL switchyard.

• B. Beutner, S. Reiche
  PSI, Villigen

The SwissFEL facility will produce coherent, ultra-bright, and ultra-short photon pulses covering a wavelength range from 0.1 nm to 7 nm, requiring an emittance between 0.18 to 0.43 mm mrad. It consists of an S-band rf-gun and booster and a C-band main linac, which accelerates the beam up to 5.8 GeV. Two compression chicanes will provide the required peak current of 2.7 kA. An important issue is the stability of the photon pulses leaving the undulator toward the user stations. Arrival time and peak current stability are crucial factors for the scientific return of the user experiments. Machine stability, especially the rf jitter, will directly affect these important figures. Shot-to-shot jitter is of main interest here since long term drifts can be compensated by slow feedback systems. We present a study on stability including rf tolerances for a new optimised layout of the SwissFEL.

• G.L. Orlandi, R. Ischebeck, V. Schlott
  PSI, Villigen
• B. Steffen
  DESY, Hamburg

The spectral and angular distribution of the radiation intensity by a single and individually radiating electron is in principle different from what expected from a high density electron beam. For a given wavelength, the beam particle density modifies via a charge form factor the angular and spectral distributions characterizing the radiation emission by a single electron. In particular, under high energy and high particle density conditions, the Transition Radiation (TR) energy spectrum by an electron beam is expected to be affected by the electron-transverse-density that, at very short wavelength – even in the visible, in principle - can influence the number of photons emitted at a given wavelength and their angular distribution (brightness increase with density). The investigation of such a phenomenon is relevant to beam diagnostics and to understand the bunch collective effects influencing TR emission. The status of the experimental investigation of the beam-transverse-size effects in the Optical Transition Radiation (OTR) at SLS and, in perspective, at the SwissFEL will be presented and the main formal aspects of the model predicting them will be described.

• H. Tomizawa, H. Dewa, H. Hanaki, S. Matsubara, A. Mizuno, T. Taniuchi, K. Yanagida
  JASRI/SPring-8, Hyogo-ken
• T. Ishikawa, N. Kumagai
  RIKEN/SPring-8, Hyogo
• K. Lee
  University of Tokyo, Tokyo
• A. Maekawa, M. Uesaka
  UTNL, Ibaraki

We developed a single-shot and non-destructive 3D bunch charge distribution (BCD) monitor based on Electro-Optical (EO) sampling with a manner of spectral decoding for XFEL/SPring-8. For the transverse detection, eight EO-crystals (Pockels effect) surround the beam axis azimuthally, and a linear-chirped probe laser pulse with a hollow shape passes through the EO-crystal. We plan to use an amorphous material which has only an even-order field dependence (Kerr effect) in donut shape without assembling eight conventional EO-crystals. The polarization axis of the probe laser should be radially distributed as well as the Coulomb field of the electron bunches. Since the signal intensity encoded at each crystal depends on the strength of the Coulomb field at each point, we can detect the transverse BCD. In the longitudinal detection, we use a prove laser with a broadband square spectrum (> 400 nm @ 800 nm) so that the temporal resolution is < 30 fs, if the pulse width of probe laser is 500 fs. In order to achieve 30-fs temporal resolution, we use an organic EO material, DAST crystal, which is transparent up to 30 THz. We report the first experimental results of our 3D-BCD monitor.

• R. Xiang, A. Arnold, P. Michel, P. Murcek, J. Teichert
  HZDR, Dresden

The thermal emittance of the photocathode is an interesting physical property for the photoinjector, because it decides the minimum emittance the photoinjector can finally achieve. In this paper we will report the latest results of the thermal emittance of the Cs2Te photocathode in FZD Superconducting RF gun. The measurement is performed with solenoid scan method with very low bunch charge and relative large laser spot on cathode, in order to reduce the space charge effect as much as possible, and meanwhile to eliminate the wake fields and the effect from beam halos.

• J. Teichert, A. Arnold, H. Büttig, D. Janssen, M. Justus, U. Lehnert, P. Michel, P. Murcek, Ch. Schneider, R. Schurig, R. Xiang
  HZDR, Dresden
• T. Kamps, J. Rudolph, M. Schenk, F. Staufenbiel
  HZB, Berlin
• G. Klemz, I. Will
  MBI, Berlin

The superconducting RF photo-injector (SRF gun) at FZD is the first operating electron injector of its kind. The gun with a 3½-cell cavity and a frequency of 1.3 GHz produces an electron beam of 3 MeV with a maximum bunch charge of about 400 pC. Also the design values for the acceleration gradient could not be reached with the cavity which is in use at present the SRF gun will improve the beam quality for ELBE users. End of 2009 the beamline was installed which connects the SRF gun with the ELBE accelerator. We will report on the first test and on the progress in applying the SRF gun for user operation.

• S.P. Niles, W.B. Colson, K.L. Ferguson, J.R. Harris, J.W. Lewellen, B. Rusnak, R. Swent
  NPS, Monterey, California
• C.H. Boulware, T.L. Grimm, J.L. Hollister
  Niowave, Inc., Lansing, Michigan
• P.R. Cunningham, M.S. Curtin, D.C. Miccolis, D.J. Sox
  Boeing Company, Seattle, Washington State
• T.I. Smith
  Stanford University, Stanford, California

The Naval Postgraduate School Beam Physics Laboratory, Niowave, Inc., and The Boeing Company have completed construction of a superconducting 500 MHz quarter-wave gun and photocathode drive laser system. This prototype gun went from conception to initial operation in just under one calendar year. Such rapid progress is due in part to the decision to develop the gun as a prototype, deliberately omitting some features, such as tuners and cathode loadlocks, desired for a linac beam source. This will enable validation of the basic concept for the gun, including high-charge bunch dynamics, as rapidly as possible, with lessons learned applied to the next generation gun. This paper presents results from initial testing of the gun, technical challenges of the prototype design, and improvements that would enhance capabilities in future versions of this novel design.

• Y.A. Kot
  DESY, Hamburg

The overall bunch compression in FLASH is limited on the one hand by the rf tolerances and on the other hand by linearization of the particle distribution in the longitudinal phase space. While the last one has been significantly improved after the installation of the third harmonic system during the upgrade 2009-2010, the constraint given by rf tolerances cannot be mitigated significantly. To avoid this limitation one has to operate with shorter bunches already at the injector. Since the bunch length is dominated there by the longitudinal space charge one has to go to lower bunch charges. Working points for the operation of the FLASH injector with 20-500pC bunches have been found by means of the optimization procedure based on ASTRA code. The expected bunch parameters are reported in this paper and compared with the experimental results. Further the discussion on advantages and drawbacks of the injector operation in low charge regime is given.

• I. Zagorodnov, M. Dohlus
  DESY, Hamburg

The nonlinearities of the RF fields and the dispersion sections can be corrected with a higher harmonic RF module. In this paper we present an analytical solution for nonlinearity correction up to the third order in a multistage bunch compression and acceleration system without collective effects. A more general solution for a system with collective effects (space charge, wakefields, CSR effects) is found by iterative tracking procedure based on this analytical result. We apply the developed formalism to study two stage bunch compression in FLASH and three stage bunch compression in the European XFEL. Different charges are considered. Analytical estimations of RF tolerances are given.

• F.-J. Decker, R. Akre, A. Brachmann, Y.T. Ding, D. Dowell, P. Emma, A.S. Fisher, J.C. Frisch, A. Gilevich, P. Hering, Z. Huang, R.H. Iverson, H. Loos, M. Messerschmidt, H.-D. Nuhn, D.F. Ratner, W.F. Schlotter, T.J. Smith, J.L. Turner, J.J. Welch, W.E. White, J. Wu
  SLAC, Menlo Park, California

The Linac Coherent Light Source at SLAC is a hard X-ray FEL which was designed for single electron bunch operation. Although most user experiments are not interested in multiple bunches from an S-band linac due to their short (ns) separation, there are some advantages with multi-bunch operation. Starting with two bunches where the delayed light of one bunch is used to seed the light of a second bunch, to many more bunches to increase the likelihood of rare target collisions, multi-bunch operation would open more options for the LCLS. In the past the SLAC Linac has operated with a few dedicated bunches for the SLC (Stanford Linear Collider), and up to 1400 bunches for some fixed target experiments, so a few bunches for the LCLS seems possible even with the original single bunch design. This paper will describe how the current RF implementation supports multi-bunch operation. Initial experimental tests with two bunches are presented.

• S. Thorin, M. Eriksson, S. Werin
  MAX-lab, Lund
• D. Angal-Kalinin, J.W. McKenzie, B.L. Militsyn, P.H. Williams
  STFC/DL/ASTeC, Daresbury, Warrington, Cheshire

The MAX IV linac will be used both for injection and top up into two storage rings, and as a high brightness injector for a Short Pulse Facility (SPF) and an FEL (in phase 2). Compression is done in two double achromats with positive R56. The natural second order momentum compaction, T566, from the achromats is used together with weak sextupoles to linearise longitudinal phase space. In this proceeding we present the design of the achromat compressors and initial results from particle tracking through the MAX IV Injector in high brightness mode.

• F. Sannibale, B.J. Bailey, K.M. Baptiste, A.L. Catalano, D. Colomb, J.N. Corlett, S. De Santis, L.R. Doolittle, J. Feng, D. Filippetto, G. Huang, R. Kraft, D. Li, H.A. Padmore, C. F. Papadopoulos, G.J. Portmann, S. Prestemon, J. Qiang, J.W. Staples, M.E. Stuart, T. Vecchione, R.P. Wells, M.S. Zolotorev
  LBNL, Berkeley, California
• M. J. Messerly, M.A. Prantil
  LLNL, Livermore, California
• M. Yoon
  POSTECH, Pohang, Kyungbuk

The fabrication and installation at the Lawrence Berkeley National Laboratory of a high-brightness high-repetition rate photo-gun, based on a normal conducting 187 MHz (VHF) RF cavity operating in CW mode, is in its final phase. The cavity will generate an electric field at the cathode plane of ~20 MV/m to accelerate the electron bunches up to ~750 keV, with peak current, energy spread and transverse emittance suitable for FEL and ERL applications. The gun vacuum system has been designed for pressures compatible with high quantum efficiency but "delicate" semiconductor cathodes to generate up to a nC bunches at MHz repetition rate with present laser technology. Several photo-cathode/laser systems are under consideration, and in particular photo-cathodes based on K2CsSb are being developed and have already achieved a QE of 8% at 532 nm wavelength, or close to 20% including the Schottky barrier lowering. The cathode will be operated by a microjoule fiber laser in conjunction with refractive optics to create a flat top transverse profile, as well as a birefringent pulse stacker to create a flat top temporal profile. The present status and the plan for future activities are presented.

• C. F. Papadopoulos, J.N. Corlett, D. Filippetto, J. Qiang, F. Sannibale, J.W. Staples, M. Venturini, M.S. Zolotorev
  LBNL, Berkeley, California

The Advanced Photoinjector Experiment (APEX) is a part of the Next Generation Light Source (NGLS), a proposed soft x-ray FEL concept being studied at LBNL. The requirements for the beam delivered to the FELs pose restrictions on the beam parameters at the injector. In addition, different modes of operation of the machine may pose different requirements on the beam. In order to optimize the performance of the injector, a genetic multiobjective algorithm has been used. A genetic algorithm is used because of the inherent complexity of the beam dynamics at the energy range in question (0-30 MeV) and the large number of parameters available for optimization. On the other hand, the multiplicity of requirements on the beam, which include beam emittance, beam pulse length, energy chirp, as well as pulse shape and peak current, leads to a mutliobjective approach for the optimization technique. In this paper, we present the status of the optimization simulations, using the ASTRA particle-in-cell code. Different injector setups are presented and the resulting transport solutions are compared to each other and the requirements of the downstream sections of the accelerator.

• S. Di Mitri, S. Ferry
  ELETTRA, Basovizza

Horizontal scraping, geometric and energy collimation of the Fermi@elettra electron beam has been investigated analytically and with the elegant particle tracking code. Beam scraping in the first magnetic bunch length compressor has been characterized in terms of reduction of the transverse emittance and variation of the energy chirp induced by the succeeding linac longitudinal wake field. The locations of the geometric and energy collimators have been identified in the machine lattice. A novel definition of collimation efficiency is proposed that allowed us to identify a configuration of the collimation system that is a compromise between the collimation performance, optics design and available space.

• M. Veronese, R. Appio, T. Borden, G. Ciani, P. Craievich, R. De Monte, S. Di Mitri, M. Ferianis, G. Gaio, S. Grulja, G. Scalamera, M. Tudor
  ELETTRA, Basovizza

Both absolute and relative bunch length measurement are key information for FERMI@Elettra commissioning and operation. In this paper we present the relative Bunch Length Monitor (BLM) system that has been designed and implemented at Sincrotrone Trieste. The first BLM station has been installed downstream the first bunch magnetic compressor (BC1) of FERMI@Elettra. In this paper we report about the first operation of the BLM system; it is based on the power measurement of the coherent radiations. To allow for efficient performances in the extended range of the foreseen bunch lengths for FERMI@Elettra, the system has adopted a pyro detector for coherent edge radiation from the last dipole. Also, the coherent diffraction radiation generated in a ceramic gap located downstream of BC1 is detected by a set of mm-wave diodes. The design of the system, along with its layout, is presented as well as the first measurement results obtained from the FERMI@Elettra compressed bunches.

• S. Di Mitri, M. Cornacchia
  ELETTRA, Basovizza

The optics design of the first bunch compressor area in the FERMI@elettra linac is presented. Several constraints on the Twiss parameters are set by the preservation of beam quality in the first magnetic compressor, the optimization of diagnostics performance, the collimation process and the beam matching to the downstream lattice. A compact multi-purpose arrangement of magnetic and diagnostic elements is presented that, in principle, satisfies several different needs over a total length of 14m.

• P. Craievich, S. Biedron, M. Ferianis, D. La Civita
  ELETTRA, Basovizza
• D. Alesini, L. Palumbo
  INFN/LNF, Frascati (Roma)
• L. Ficcadenti
  Rome University La Sapienza, Roma
• M. Petronio, R. Vescovo
  DEEI, Trieste

A RF deflector is a useful tool to completely characterize the beam phase space by means of measurements of the bunch length and the transverse slice emittance. At FERMI@Elettra, a soft X-ray next-generation light source under development at the Sincrotrone Trieste laboratory in Trieste, Italy, we are installing low-energy and high-energy deflectors. In particular, two deflecting cavities will be positioned at two points in the linac. One will be placed at 1.2 GeV (high energy), just before the FEL process starts; the other at 250 MeV (low energy), after the first bunch compressor (BC1). This paper concerns only the low-energy deflector. The latter was built over the past year in collaboration with the SPARC project team at INFN-LNF-Frascati, Italy and the University of Rome. In this paper we will describe the RF measurements performed to characterize the standing wave cavity before the installation in the FERMI@Elettra linac, and we will compare them with the simulations done using the electromagnetic code HFSS.

• M. Takasaki, M. A. Bakr, Y.W. Choi, K. Ishida, T. Kii, N. Kimura, R. Kinjo, K. Masuda, K. Nagasaki, H. Ohgaki, T. Sonobe, S. Ueda, K. Yoshida
  Kyoto IAE, Kyoto

A triode rf gun has been developed aiming at drastic reduction of back-streaming electrons at the thermionic cathode. Thermionic rf gun shows some advantages over photocathode gun such as low cost, easy operation and high average current, which are suitable for oscillator FELs. However, use of thermionic rf gun leads to inherent back-bombardment effect, which not only limits the macro-pulse duration, but also degrades the electron beam quality. In order to reduce the back-streaming electrons, we developed a thermionic triode rf gun which employs coaxial rf cavity much shorter than rf wavelength as the first cell. The phase and amplitude of the electric field for the first cell are independently controlled from successive cells. The results from simulations showed that the back-bombardment power was expected to be reduced by more than 80% without loss of beam brightness. The coaxial rf cavity to be installed in the rf gun for KU-FEL has been developed and a cold test has been performed. In this paper, we will report on the cold test results and comparison of them with the designed performance as well.

• P. Stoltz, C. Nieter, C. Roark
  Tech-X, Boulder, Colorado
• J.D.A. Smith
  TXUK, Warrington

Multipacting is a potential limit on the power one can deliver to different components of an FEL source, including the power couplers and the electron source cathode. Simulation is a main tool in helping to understand and mitigate multipacting. We present recent work on benchmarking multipacting simulation, including comparison with other codes and with rectangular waveguide experiment.

• J.D. Jarvis, C.A. Brau, J.L. Davidson, N. Ghosh, B.L. Ivanov, J.L. Kohler
  Vanderbilt University, Nashville, TN

We report recent advances in the development of electron sources of extreme brightness approaching the quantum degenerate limit. These cathodes comprise either a diamond field emitter or carbon nanotube and an individual adsorbed atom or molecule. Both emitters are covalent carbon structures and thus have the benefits of high activation energy for atomic migration, chemical inertness, and high thermal conductivity. The single adsorbate produces surface states which result in dramatic resonant enhancement of the field emission current at the allowed energies of those states. The result is a beam with a narrow energy spread that is spatially localized to roughly the size of a single atom. Thus far, we have observed short lived (~1 sec) beams from residual gases of ~6 microamps corresponding to a normalized transverse brightness of ~3·10¹⁸ A/m²-str. Whereas conventional field emitters have a quantum degeneracy of <10^-4, we estimate the degeneracy of our observed beams to be ~0.1. The use of metal adsorbates should stabilize the effect, allow higher current operation, and provide a long lived source whose brightness approaches the quantum limit.

• A.P. Lee, S.B. Hung, W.K. Lau, A. Sadeghipanah
  NSRRC, Hsinchu
• N.Y. Huang
  NTHU, Hsinchu

The design of a high brightness driver linac for single pass high gain FEL experiments is being studied at NSRRC. Simulation of beam dynamics starting from the cathode of the 2998 MHz photo-injector to linac exit are performed by LiTrack and Elegant. To obtain a “flat-flat” distribution of electron bunches, the reverse tracking technique has been used to optimize the longitudinal dynamics. Results of beam dynamics and reverse tracking will be presented in this report.

• R.A. Bosch, K.J. Kleman
  UW-Madison/SRC, Madison, Wisconsin
• J. Wu
  SLAC, Menlo Park, California

Bunch compression for a free-electron laser (FEL) may cause growth of current and energy fluctuations at wavelengths shorter than the bunch length. This microbunching instability may disrupt FEL performance or it may be used to produce coherent radiation. We give analytic formulas that approximate microbunching growth and apply them to the Wisconsin FEL (WiFEL).

• A. Marinelli, J.B. Rosenzweig
  UCLA, Los Angeles, California

Longitudinal space-charge forces can be a major source of micro-bunching instability. We will discuss a three-dimensional theoretical model for the high frequency limit of space-charge interactions leading to density modulation at the optical scale. Particular emphasis will be given to the effect of transverse thermal motion on the angular distribution of micro-bunching and to its connection to the physics of Landau damping in longitudinal plasma oscillations. A comparison with the results of high resolution molecular dynamics simulations will also be discussed.

• H. Hama, F. Hinode, S. Kashiwagi, M. Kawai, F. Miyahara, T. Muto, K. Nanbu, Y. Tanaka
  Tohoku University, School of Scinece, Sendai

The t-ACTS (test Accelerator as Coherent THz Source) at Tohoku University will provide intense terahertz radiation employing novel sources such as an isochronous accumulator ring and a pre-bunched free electron laser. Stable production of the very short electron pulse is a key issue for the t-ACTS accelerator system, in which a thermionic RF gun is being used. Particularly observation of the longitudinal phase space of the beam extracted the gun is crucial for efficient bunch compression. Because of space charge effect, the beam has to be diagnosed within a short drift space. We have studied a novel energy spectrometer using Cherenkov radiation (Linear Focal Cherenkov ring camera, LFC-camera). Though the method is valid for the lower energy less than 3 MeV, the energy distribution can be measured immediately at the gun exit. In addition to the present status of the t-ACTS project, we describe the principle of LFC camera and discuss energy resolution, prospect of the direct measurement of the particle distribution in the longitudinal phase space as well.