SLAC, Menlo Park, California
Electron source and gun technology by its nature is a multi-disciplined endeavor requiring knowledge of beam dynamics with RF fields, static fields and space charge forces as well as the chemistry and surface science related to electron emission and ultra-high vacuum. The need for a broad range of disciplines results because the electrons undergo a sequence of processes involving emission, acceleration and optical matching. This talk describes the physical process of each step with the goal of estimating its lowest possible contribution to the total emittance. The physics of electron emission, space charge forces, and the electron optics of the RF and magnetic fields will be developed and the emittance growth assessed for the gun and low energy portion of the injector. The thermal emittance and other properties of metal and semi-conductor cathodes are briefly reviewed, and the affect these properties have upon the limiting emittance and the gun design will be summarized. And finally, the space charge emittance compensation technique and the Ferrario matching criteria for the booster linac are discussed and critiqued for their emittance limits.
PSI, Villigen
The Paul Scherrer Institut is planning to construct a free electron laser covering the wavelength range of 1-70 Å. This project, “SwissFEL” will use a C-band radio-frequency linac of variable energy, 2.1 GeV to 5.8 GeV. The laser will be equipped with two undulator lines. A short period (15 mm) in-vacuum undulator, ‘Aramis’ will provide hard X-ray radiation in the range 1 Å to 7 Å. A 40 mm period APPLE-type undulator ‘Athos’ will provide wavelengths from 7 Å to 7 nm. The accelerator will employ an S-band RF photo-gun and an S-band injector providing a low normalized slice emittance (~ 0.3 mm-mrad @ 200 pC) beam of 450 MeV. The initial photo-current of 22 Amperes is increased to 2.7 kA through the use of two magnetic chicane bunch compressors. Acceleration to full energy is provided by twenty-six C-band RF “modules” each consisting of four, 2 m long, C-band structures. We will describe the status of the project and in particular the design of the accelerator. The beam dynamics simulations which have led us to our base-line design will be discussed and a description of the basic RF module will be given. A schedule for the project realization will also be presented.
* Submitted by T. Garvey on behalf of the SwissFEL project group
ELETTRA, Basovizza
Diamond, Oxfordshire
University of Nova Gorica, Nova Gorica
JLAB, Newport News, Virginia
LBNL, Berkeley, California
ENEA C.R. Frascati, Frascati (Roma)
DEEI, Trieste
MAX-lab, Lund
Some experiences have recently been collected from the FERMI@elettra Free Electron Laser first bunch compressor area. This includes a magnetic compressor, diagnostics for the characterization of the longitudinal and transverse phase space and suitable optics for matching to the downstream part of the linac. We report on the beam dynamics investigations in comparison with the modeling as well as the high level software control that has allowed this experience.
NPS, Monterey, California
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.
LBNL, Berkeley, California
UCB, Berkeley, California
ANL, Argonne
The Next Generation Light Source (NGLS) is a design concept, under development at LBNL, for a 10‐beamline soft x‐ray FEL array powered by a 2.4 GeV superconducting linear accelerator, operating with a 1 MHz bunch repetition rate. The CW superconducting linear accelerator is supplied by a high-brightness, high-repetition-rate photocathode electron gun. Beam is distributed from the linac to the array of independently configurable FEL beamlines with nominal bunch rates up to 100 kHz, with even pulse spacing. Individual FELs may be configured for EEHG, HGHG, SASE, or oscillator mode of operation, and will produce high peak and average brightness x-rays with a flexible pulse format ranging from sub-femtoseconds to hundreds of femtoseconds.
CENBG, Gradignan
CESTA, Le Barp
Several years ago Andrews and Brau * presented a two-dimensional (2-D) theory for the production of coherent Smith-Purcell radiation by an initially continuous beam. An essential component of their analysis was the dispersion relation for a lamellar grating (i.e., rectangular profile) relating frequency and axial wave number k. Both simulations and an experiment performed at CESTA ** using a wide beam have confirmed the validity of their approach. However, all gratings are three-dimensional objects, and one may ask what modifications of the theory might be necessary. We present here our solution to the problem, which assumes a progressive wave in the direction of the grooves, with wave number q. A surprisingly simple modification of the Andrews and Brau 2-D dispersion relation is found. We have extensively tested our theory, both with simulations using the 3-D PIC code "MAGIC", and with measurements of the properties of the surface wave on the CESTA grating made using a network analyzer. Extremely good agreement is found, both with and without sidewalls on the grating.
* H. L. Andrews and C. A. Brau, Phys. Rev. ST Accel. Beams 7, 070701 (2004).
** J. Gardelle, L. Courtois, P. Modin and J.T. Donohue, Phys. Rev. ST Accel. Beams 12, 110701 (2009).
Fermilab, Batavia
A superconducting (SC) RF Test Accelerator at the New Muon Lab (NML) is currently under construction at Fermilab. Its design goals include the replication of the pulse train proscribed for the International Linear Collider (ILC) and operations with the prototypic beam of the base RF unit. At 3 nC per micropulse and with 3000 micropulses per macropulse at a 5-Hz rate and at 750 MeV, a 40-kW beam would be generated. An RF photoelectric gun based on the PITZ-Zeuthen design will generate the beam which has a lower emittance of about 1-2 pi mm mrad when run at 1 nC or less per micropulse based on tests at Zeuthen. This beam quality is sufficient, when properly bunch compressed, to provide the driving beam for an extreme ultraviolet (EUV) and soft x-ray (SXR) self-amplified spontaneous emission (SASE) free-electron laser (FEL) or seeded FEL. Estimates for the gain length and output power have been calculated for wavelengths from 80 to 12 nm (at 1.5 GeV) using the simple scaling formula of M. Xie. This wavelength regime with 200-fs bunch lengths would complement the hard x-ray SASE FEL project at SLAC in the USA.
SLAC, Menlo Park, California
USTC/NSRL, Hefei, Anhui
Lots of applications with mm wave need very high power (from tens of kW to MW), such as surface processing of metals and ceramics, heating magnetically confined plasma in thermonuclear fusion reactors, isotope separation and so on. Recently developed gyrotrons can provide up to 1 MW CW mm-wave source, however there are a number of limitations, needs of supper conducting magnet, cathode lifetime degradation because of very high current, almost approaching the upper limit of their power and frequency capabilities, and so on. It is thought that the electrostatic accelerator FEL (EA-FEL) will be a promising high power IR-mm source, because of its high average power generation, high-energy conversion efficiency and high spectral purity. The property of an EA as a high quality e-beam source for a FEL is crucial for attaining high brightness spontaneous emission radiation. The unique features of EA-FELs make them naturally fitting for a variety of applications in the present and in the near future. And few high power mm-IR EA FEL facilities have been successfully built around world. Here an EA of 3 MeV with beam current of 2 A is studied for a high average power (kWs) mm-THz source
MAX-lab, Lund
HZB, Berlin
Lund University, Division of Atomic Physics, Lund
The first generation of coherent harmonic radiation from the MAX-lab test FEL have recently been achieved. The 380 MeV electron beam has been seeded by a 263 nm Ti:Sapphire laser and coherent radiation in the harmonics 1 to 4 (263 66 nm) has been produced both in linear and circular polarization mode. The facility consists of a photo cathode RF gun, the MAX injector (two 95 MeV linacs placed in a recirculator), beam transport including compression optics and the two undulators (modulator and radiator) separated by a four magnet chicane for bunching control. The radiator undulator is of Apple type providing tunable polarization. The basic characterization of the source with dynamic studies of laser energy, undulator gap and chicane influence on the coherent harmonic signal will be reported.
ISIR, Osaka
Femtosecond electron beam, which is essential for pump-probe measurement, was generated with a 1.6-cell S-band photocathode rf gun. The rf gun was driven by femtosecond UV laser pulse (266 nm), which was generated with third-harmonic-generation (THG) of Ti:Sapphire femtosecond laser (800 nm). The longitudinal and transverse dynamics of the electron bunch generated by the UV laser was investigated. The bunch length was measured with the dependence of energy spread on acceleration phase in a linac, which was set at the downstream of the rf gun. Transverse emittance at the linac exit was also measured with Q-scan method.
DESY Zeuthen, Zeuthen
DESY, Hamburg
YerPhI, Yerevan
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.
DESY Zeuthen, Zeuthen
MBI, Berlin
DESY, Hamburg
UCLA, Los Angeles, California
HZB, Berlin
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.
DESY Zeuthen, Zeuthen
DESY, Hamburg
JINR, Dubna, Moscow Region
UCLA, Los Angeles, California
HZB, Berlin
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.
UMD, College Park, Maryland
NPS, Monterey, California
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).
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.
JASRI/SPring-8, Hyogo-ken
RIKEN/SPring-8, Hyogo
University of Tokyo, Tokyo
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.
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.
HZDR, Dresden
HZB, Berlin
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.
NPS, Monterey, California
Niowave, Inc., Lansing, Michigan
Boeing Company, Seattle, Washington State
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.
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.
MAX-lab, Lund
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.
LBNL, Berkeley, California
LLNL, Livermore, California
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.
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.
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.
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.
LLNL, Livermore, California
SLAC, Menlo Park, California
UCB, Berkeley, California
Recent progress in accelerator physics and laser technology have enabled the development of a new class of tunable x-ray and gamma-ray light sources based on Compton scattering between a high-brightness, relativistic electron beam and a high intensity laser pulse produced via CPA. A precision, tunable, monochromatic (< 0.4%) source driven by a compact, high-gradient X-band linac designed in collaboration with SLAC is under construction at LLNL. High-brightness (250 pC, 3.5 ps, 0.4 mm.mrad), relativistic electron bunches will interact with a Joule-class, 10 ps, diode-pumped CPA laser pulse to generate tunable γ-rays in the 0.5-2.5 MeV photon energy range. This gamma-ray source will be used to excite nuclear resonance fluorescence in various isotopes. A very compact version of the accelerator (2.5 m) will also be used to generate medical x-rays in the 15-25 keV range. Fields of endeavor include homeland security, stockpile science and surveillance, nuclear fuel assay, and waste imaging and assay. The source design, key parameters, and current status will be discussed, along with important applications, including nuclear resonance fluorescence and high precision medical imaging.
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.