• K.L. Ferguson, C.W. Bennett, W.B. Colson, J.R. Harris, J.W. Lewellen, S.P. Niles, B. Rusnak, R. Swent
  NPS, Monterey, California
• T.I. Smith
  Stanford University, Stanford, California

The new experimental facilities for the Naval Postgraduate School Beam Physics Lab are at the 95% completion level for exterior construction, and work has begun on the internal lab spaces. A general timeline for the commencement of first experiments is presented, along with an overview of the experimental path forward. The NPS-BPL is rated for considerably higher average powers (40 kW) than most university accelerator facilities, which presents unique challenges in both the physical and administrative realms. Design considerations, radiation approval processes and other “lessons learned” in a non-U.S. Department of Energy government facility are discussed.

• V. Petrillo
  Universita' degli Studi di Milano, Milano
• M. Ferrario
  INFN/LNF, Frascati (Roma)
• A.R. Rossi
  Istituto Nazionale di Fisica Nucleare, Milano

We study both analytically and numerically the effects of a laser pulse on the longitudinal phase space of an electron beam in the stage of extraction from the cathode. We show how the interaction can produce modulations in the longitudinal momentum distribution.

• J.T. Donohue
  CENBG, Gradignan
• J. Gardelle, P. Modin
  CESTA, Le Barp

At FEL 2009, we presented experimental results on coherent Smith-Purcell obtained at CESTA in the microwave frequency domain * . Those results strongly supported the two-dimensional theory proposed by Andrews and Brau some years ago ** , and were consistent with simulations performed with the PIC code "MAGIC". That experiment used a large current, 200 A, for a grating of width 10 cm. In a follow-up experiment, emittance slits were used to reduce the current to as low as 2 A, with a quite thin, flat, and wide beam. The gain as a function of current and also of vertical beam position was measured in detail. In particular, the start current for our set-up was found. In parallel, 2-D simulations of the experiment with "MAGIC" were extensively compared with the experimental results. Very good agreement between simulations and experiment is obtained. This lends confidence that simulations of a scaled-down version of our experiment will be a reliable guide for Terahertz frequency coherent Smith Purcell experiments. Such simulations suggest that radiation in the range 100-200GHz should be feasible.

* J. T. Donohue, J. Gardelle, L. Courtois and P Modin, Proceeedings of FEL 2009.
** H. L. Andrews and C. A. Brau, Phys. Rev. ST Accel. Beams 7, 070701 (2004).

• J.D. Jarvis, C.A. Brau, J.L. Davidson, B.L. Ivanov, J.L. Kohler
  Vanderbilt University, Nashville, TN
• H.L. Andrews
  LANL, Los Alamos, New Mexico

Recently, the theory of the Smith-Purcell free-electron laser has been confirmed by the experiments of Andrews, et al. [1], and of Gardelle, et al. [2] In addition, high-brightness cathodes have been developed using field-emission from arrays of diamond pyramids [3]. By combining these developments we have designed an ultracompact (“shirt-pocket”) free-electron laser and we have begun constructing the device. The electron beam comprises an array of 2-micron diamond-pyramid field emitters that overfills an einzel lens 200-microns wide and 1-mm long, fabricated using ps-laser machining. The beam is accelerated to 10 keV and focused in the short dimension over a lamellar metal grating with a period of 150 microns and a length of 10 mm. The predicted start current at a wavelength of {10}84 microns is 11 mA, which corresponds to 9 A/cm² at the cathode, before focusing. We have tested cathodes at 30 A/cm² and 600 mA total current; higher current density should be possible.

[1] Andrews, et al, JAP {10}5, 024904 (2009)
[2] Andrews, et al, PRST-AB 12, 080703 (2009)
[3] Gardelle, et al, PRST-AB 12, 110701 (2009)
[4] Jarvis, et al, JVSTB 27, 2264 (2009)
[5] Jarvis, Thesis, 2009

• 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.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.

• 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.

• 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.

• 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.

• 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.

• 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.

• H. Tomizawa, H. Dewa, H. Hanaki, A. Mizuno, T. Taniuchi
  JASRI/SPring-8, Hyogo-ken

A laser-induced Schottky-effect-gated photocathode gun has been developed since 2006. This new type of gun utilizes a laser’s coherency to realize a compact laser source using Z-polarization of the IR laser on the cathode. This Z-polarization scheme reduces the laser pulse energy by reducing the cathode work function due to Schottky effect. A hollow laser incidence scheme is applied with a hollow convex lens that is focused after passing the beam through a radial polarizer. According to our calculations (convex lens: NA=0.15; 60-% hollow ratio), a Z-field of 1 GV/m needs 1.26 MW at peak power for the fundamental wavelength (792 nm). Therefore, we expect that this laser-induced Schottky emission requires just a compact femtosecond laser oscillator. We observed the first emission with a hollow laser incidence scheme (copper cathode illuminated by THG: 264 nm as a pilot experiment). The net charge of 21 pC with 100-fs laser pulse (pulse energy: 2.5 μJ; spot diameter: 200 μm). The maximum cathode surface field was 97 MV/m. This new scheme of gun will be investigated on several metal photocathode materials by comparing radial and azimuthal polarizations at 264, 396,792 nm.