In this report we present the experimental data and the step-by-step technique to evaluate the work functions of thermionic cathodes. The thermionic cathodes utilized here were small, almost button size, and were provided by the Spectra-Mat corporation [1]. The experiments were performed by one of us, Erik Jongewaard, at Stanford Linear Accelerator Laboratory (SLAC). We would like to emphasize that among other parameters, the cathode electron current emission work function is of paramount importance for selecting the cathode for many applications and specifically for the injector of linear induction accelerators similar to DARHT II and most recently for the new radiographic accelerators under consideration.

Current and future experiments at LCLS require x-ray pulse trains of variable time separation on the nanosecond scale. For instance, the cavity-based XFEL (CBXFEL) will use up to 4 pulses separated by 218.5 ns, the X-ray Laser Oscillator (XLO) will use 15 to 25 ns spaced pulses, and the Matter under Extreme Conditions (MEC) experiments use pulse trains separated by 5 nanoseconds or less. In this paper, we demonstrate an ultra-fast e-beam trajectory control method based on transverse electro-magnetic (TEM) striplines and state-of-the-art power sources, to enhance LCLS operations in these regimes.

DRIVEN ACCELERATORS D.C. Nguyen , C.E. Buechler, G.E. Dale, R.L. Fleming, M.A. Holloway, J.W. Lewellen, D. Patrick Los Alamos National Laboratory J. Neilson, V. Dolgashev, E.N. Jongewaard, E.A. Nanni, A. Sy and S. Tantawi SLAC National Accelerator Laboratory Abstract Small, lightweight, few-MeV electron accelerators that can operate with low-voltage power sources, e.g., solidstate transistors running on 50 VDC, instead of highvoltage klystrons, will provide a new tool to enhance existing applications of accelerators as well as to initiate new ones. Recent advances in gallium nitride (GaN) semiconductor technologies [1] have resulted in a new class of high-power RF solid-state devices called highelectron mobility transistors (HEMTs). These HEMTs are capable of generating a few hundred watts at S-, Cand X-bands at 10% duty factor. We have characterized a number of GaN HEMTs and verified they have suitable RF characteristics to power accelerator cavities. We have measured energy gain as a function of RF power in a single low- C-band cavity. The HEMT powered RF accelerators will be compact and efficient, and they can operate off the low-voltage DC power buses or batteries. These all-solid-state accelerators are also more robust, less likely to fail, and are easier to maintain and operate. In this poster, we present the design of a low-, 5.1-GHz cavity and beam dynamics simulations showing continuous energy gain in a ten-cavity C-band prototype.

Los Alamos National Laboratory, in collaboration with SLAC and Goddard Space Flight Center, have begun developing a high-duty-factor, MeV-range linear accelerator intended for use on satellites, specifically to probe the magnetosphere-ionosphere linkage. The design makes use of low-beta C-band cavities operating at moderate gradients, individually powered by 500-W RF amplifier chips. We present the current state of the design, and technology maturation efforts including RF amplifier performance studies, cavity tuner design and an initial acceleration test using a DC beam source and single RF cavity.

Design of a beam energy recovery system for application to the CPI VKS-8262S S-band klystron is presented. The multi stage pulsed depressed collector optics, mechanical and thermal design, energy recovery modulator, and experimental program will be highlighted.

Establishing good initial quantum efficiency (QE) and reliable in-situ cleaning for copper cathode in the RF gun is of critical importance for the RF gun operations. Recent studies on the SLAC RF gun test bed indicated that the pre-cleaning (plasma cleaning) in the test chamber followed by copper cathode exposure to air for cathode change leads to a very low initial QE in the RF gun, and also demonstrated that without the pre-cleaning good initial QE >4×10−5 can be routinely achieved in the RF gun with the cathodes of QE <1×10−7 measured in the test chamber. QE can decay over the time in the RF gun. The in-situ laser cleaning technique for copper cathodes in the RF gun is established and refined in comparison to previous cleaning at the linac coherent light source, resulting in an improved QE and emittance evolutions. The physics of the laser cleaning process is discussed. It is believed that the reflectivity change is one of the major factors for the QE boost with the laser cleaning.

Ultrafast electron probes are powerful tools, complementary to x-ray free-electron lasers, used to study structural dynamics in material, chemical, and biological sciences. High brightness, relativistic electron beams with femtosecond pulse duration can resolve details of the dynamic processes on atomic time and length scales. SLAC National Accelerator Laboratory recently launched the Ultrafast Electron Diffraction (UED) and microscopy Initiative aiming at developing the next generation ultrafast electron scattering instruments. As the first stage of the Initiative, a mega-electron-volt (MeV) UED system has been constructed and commissioned to serve ultrafast science experiments and instrumentation development. The system operates at 120-Hz repetition rate with outstanding performance. In this paper, we report on the SLAC MeV UED system and its performance, including the reciprocal space resolution, temporal resolution, and machine stability.

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Journal Article Development of MeV Ultrafast Electron Scattering Instruments at SLAC National Accelerator Laboratory Get access R K Li, R K Li SLAC National Accelerator Laboratory, 2575 Sand Hill Rd, Menlo Park, California 94025 USA Search for other works by this author on: Oxford Academic Google Scholar A H Reid, A H Reid SLAC National Accelerator Laboratory, 2575 Sand Hill Rd, Menlo Park, California 94025 USA Search for other works by this author on: Oxford Academic Google Scholar S P Weathersby, S P Weathersby SLAC National Accelerator Laboratory, 2575 Sand Hill Rd, Menlo Park, California 94025 USA Search for other works by this author on: Oxford Academic Google Scholar G Brown, G Brown SLAC National Accelerator Laboratory, 2575 Sand Hill Rd, Menlo Park, California 94025 USA Search for other works by this author on: Oxford Academic Google Scholar M Centurion, M Centurion University of Nebraska-Lincoln, 855 N 16th Street, Lincoln, Nebraska 68588, USA Search for other works by this author on: Oxford Academic Google Scholar T Chase, T Chase SLAC National Accelerator Laboratory, 2575 Sand Hill Rd, Menlo Park, California 94025 USA Search for other works by this author on: Oxford Academic Google Scholar R Coffee, R Coffee SLAC National Accelerator Laboratory, 2575 Sand Hill Rd, Menlo Park, California 94025 USA Search for other works by this author on: Oxford Academic Google Scholar J Corbett, J Corbett SLAC National Accelerator Laboratory, 2575 Sand Hill Rd, Menlo Park, California 94025 USA Search for other works by this author on: Oxford Academic Google Scholar J C Frisch, J C Frisch SLAC National Accelerator Laboratory, 2575 Sand Hill Rd, Menlo Park, California 94025 USA Search for other works by this author on: Oxford Academic Google Scholar M Guehr, M Guehr SLAC National Accelerator Laboratory, 2575 Sand Hill Rd, Menlo Park, California 94025 USA Search for other works by this author on: Oxford Academic Google Scholar ... Show more N Hartmann, N Hartmann SLAC National Accelerator Laboratory, 2575 Sand Hill Rd, Menlo Park, California 94025 USA Search for other works by this author on: Oxford Academic Google Scholar C Hast, C Hast SLAC National Accelerator Laboratory, 2575 Sand Hill Rd, Menlo Park, California 94025 USA Search for other works by this author on: Oxford Academic Google Scholar L V Ho, L V Ho SLAC National Accelerator Laboratory, 2575 Sand Hill Rd, Menlo Park, California 94025 USA Search for other works by this author on: Oxford Academic Google Scholar K R Jobe, K R Jobe SLAC National Accelerator Laboratory, 2575 Sand Hill Rd, Menlo Park, California 94025 USA Search for other works by this author on: Oxford Academic Google Scholar E N Jongewaard, E N Jongewaard SLAC National Accelerator Laboratory, 2575 Sand Hill Rd, Menlo Park, California 94025 USA Search for other works by this author on: Oxford Academic Google Scholar J R Lewandowski, J R Lewandowski SLAC National Accelerator Laboratory, 2575 Sand Hill Rd, Menlo Park, California 94025 USA Search for other works by this author on: Oxford Academic Google Scholar A M Lindenberg, A M Lindenberg SLAC National Accelerator Laboratory, 2575 Sand Hill Rd, Menlo Park, California 94025 USA Search for other works by this author on: Oxford Academic Google Scholar J E May, J E May SLAC National Accelerator Laboratory, 2575 Sand Hill Rd, Menlo Park, California 94025 USA Search for other works by this author on: Oxford Academic Google Scholar D McCormick, D McCormick SLAC National Accelerator Laboratory, 2575 Sand Hill Rd, Menlo Park, California 94025 USA Search for other works by this author on: Oxford Academic Google Scholar X Shen, X Shen SLAC National Accelerator Laboratory, 2575 Sand Hill Rd, Menlo Park, California 94025 USA Search for other works by this author on: Oxford Academic Google Scholar K Sokolowski-Tinten, K Sokolowski-Tinten University of Duisburg-Essen, LotharstraBe 1, 47048 Duisburg, Germany Search for other works by this author on: Oxford Academic Google Scholar T Vecchione, T Vecchione SLAC National Accelerator Laboratory, 2575 Sand Hill Rd, Menlo Park, California 94025 USA Search for other works by this author on: Oxford Academic Google Scholar J Wu, J Wu SLAC National Accelerator Laboratory, 2575 Sand Hill Rd, Menlo Park, California 94025 USA Search for other works by this author on: Oxford Academic Google Scholar J Yang, J Yang University of Nebraska-Lincoln, 855 N 16th Street, Lincoln, Nebraska 68588, USA Search for other works by this author on: Oxford Academic Google Scholar H A Diirr, H A Diirr SLAC National Accelerator Laboratory, 2575 Sand Hill Rd, Menlo Park, California 94025 USA Search for other works by this author on: Oxford Academic Google Scholar X J Wang X J Wang SLAC National Accelerator Laboratory, 2575 Sand Hill Rd, Menlo Park, California 94025 USA Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 21, Issue S3, 1 August 2015, Pages 1209–1210, https://doi.org/10.1017/S1431927615006832 Published: 23 September 2015

The Linac Coherent Light Source (LCLS) is the first x-ray laser user facility based upon a free electron laser (FEL). In addition to many other stringent requirements, the LCLS XFEL requires extraordinary beam quality to saturate at 1.5 angstroms within a 100 meter undulator.[1] This new light source is using the last kilometer of the three kilometer linac at SLAC to accelerate the beam to an energy as high as 13.6 GeV and required a new electron gun and injector to produce a very bright beam for acceleration. At the outset of the project it was recognized that existing RF guns had the potential to produce the desired beam but none had demonstrated it. This paper describes the analysis and design improvements of the BNL/SLAC/UCLA s-band gun leading to achievement of the LCLS performance goals.

The Linac Coherent Light Source (LCLS) has used three copper photocathodes since its commissioning in 2007. Two of three copper cathodes had low initial quantum efficiency (QE) (<1×10) in the LCLS radio frequency (RF) gun. The two cathodes were exposed to the plasma cleaning in the cathode test chamber before installation in the RF gun. Recent studies at the SLAC RF gun test bed at the Accelerator Structure Test Area (ASTA) reveals that the pre-cleaning in the test chamber followed by cathode exposure to air for installation in the gun is the major factor leading to the low initial QE. All four cathodes, without the plasma pre-cleaning prior to the installation in the gun, have demonstrated initial QE>4×10 at the ASTA. Systematic studies also demonstrate that high-power RF gun operation provides an initial QE boost. In-situ laser cleaning for three new cathodes in the RF gun is extensively investigated, and a robust laser cleaning procedure is established at the ASTA with improvements of previous cleaning recipe for the LCLS cathode. The QE was shown to reproducibly evolved to >1×10 from about 4×10 immediately following the laser cleaning over ~3 weeks, a time much shorter than a few months for the previous laser cleaning for the present LCLS cathode. The intrinsic emittance of copper cathodes is recovered to the normal value within 1-2 days following the laser cleaning, much shorter than 3 weeks for previous laser cleaning for the present LCLS cathode. The experimental results at the ASTA, including comparison with the previous cleaning for the present LCLS cathode, are presented in the paper. Physics of the laser cleaning process and the evolution of the QE is discussed. INITIAL QE WITHOUT IN-SITU CLEANING IN THE RF GUN An RF gun test bed located at the SLAC’s Accelerator Structure Test Area (ASTA) has been constructed [1] to study photocathodes for the Linac Coherent Light Source (LCLS) injector cathode operations. The beamline of the ASTA gun test bed duplicates the existing LCLS injector gun system [2], consisting of a chirp-pulse-amplifier laser tripled to 253 nm wavelength, LCLS-type RF gun, a solenoid for emittance compensation, one pair of magnet correctors, a Faraday cup to measure the bunch charge, and a YAG screen to measure beam size and intrinsic emittance. Similar to the LCLS injector, the drive laser is configured for normal incident injection to the photocathode surface using a 45 in-vacuum mirror. The final electron beam energy from the RF gun is about 5.5 MeV. The Cause for Low Initial QE in the RF Gun The LCLS located at the SLAC National Accelerator Laboratory has been successfully operated for users for about 5 years. Its copper-based photo-injector has produced an ultra-low emitance and ultra-fast electron beam for the x-ray free electron laser (XFEL). Since its commissioning in 2007, three identical copper cathodes have been used for the LCLS injector operations with different initial quantum efficiency (QE) values. As illustrated in Fig.1, the first and third (present) LCLS cathodes had unexpectedly very low initial QE [3], about 5×10, while the second one had 6.5×10 of initial QE as expected. Lately, it is realized that both the first and third LCLS cathodes were exposed to the plasma cleaning in the test chamber before cathode exposure to air for installation in the RF gun, while the second one did not have this cleaning process. Very low QE measured in the test chamber drives to proceed to plasma cleaning for the cathode prior to the installation in the gun. During the cathode installation in the LCLS RF gun, the cathodes have to be exposed to air for about 3 minutes for the cathode change due to the lack of loadlock system. The recent observations at the ASTA RF gun reveal that the laser-cleaned areas are much more susceptible to the air exposure than the non-cleaned areas do. The ASTA RF gun is vented to nitrogen and then exposed to air for about 3 minutes to mimic the LCLS cathode change before its vacuum starts to be pumped down. Figure 2 (left) and (right) shows the QE maps before and after the RF gun vacuum venting to air, respectively. Before the RF gun vacuum venting, areas A, B, C, D, E, F, G and H on the cathode have been cleaned by the intensive laser, while the circled center area is not exposed to any laser cleaning. In Fig. 2 (left), bunch charge from areas A, B and C have been evolved to about 7500 units for a given laser energy, equivalent to 1×10 of QE, while the cathode center area has about 3500 units of the bunch charge for the same laser energy, equivalent to about 4×10 of QE. After gun vacuum venting, bunch charge productions from the previously cleaned areas A, B, C, D, E, F, G, and H were dropped to 1500-2000 units shown in Fig. 2 (right), equivalent to about 1-2×10 of QE, but the QE of the cathode center area still remains unchanged, at 4×10. The observations indicate that the cleaned or activated surface is susceptible to the air-exposure, revealing the pre-cleaning is the cause for low initial QE measured in the RF gun. Total four cathodes are characterized, which are not exposed to the pre-cleaning prior to the installation in the ASTA RF gun. All four ASTA cathodes have good initial QE in the RF gun ranging from 4×10 to 8×10, as illustrated in Fig. 1. We conclude that the pre-cleaning ____________________________________________ *The work is supported by DOE under grant No. DE-AC02-76SF00515. Proceedings of FEL2014, Basel, Switzerland THP030 Electron Bunch Generation and Manipulation ISBN 978-3-95450-133-5 769 C op yr ig ht © 20 14 C C -B Y3. 0 an d by th e re sp ec tiv e au th or s process activates the cathode surface, extremely susceptible to the contaminations, such as air-exposure. Without plasma cleaning in the test chamber, high initial QE can be routinely achieved in the RF gun. Figure 1: Original QE of cathodes in the LCLS and ASTA RF guns. Figure 2: QE map before (left) and after (right) the gun vacuum venting. Before the gun venting, areas A to H are processed by laser cleaning while the center area is not cleaned by intensive laser. QE Impact from the High-Power RF Processing For LCLS operations all three photocathodes were characterized in a test chamber prior to the installation in the RF gun. The test chamber utilizes a broadband UV light source followed by a narrow-band monochromator to select the desired photon energy. The photoemission from the cathode under 2.6 kV/m of electric field is measured with nanometer. The QE measured in the test chamber is typically on the order of 1×10 at the desired photon energy of 4.91 eV (253 nm). The cathodes with the low QE observed in the test chamber are then directly installed in the ASTA RF gun. Surprisingly, following high-power RF processing all four cathodes in the gun have demonstrated an initial QE >4×10 under normal operation conditions, two orders of magnitude higher than in the test chamber. The Schottky effect enhances QE by less than an order of magnitude for a copper work function between 4.3-4.7 eV. The two orders of magnitude of QE enhancement in the RF gun therefore cannot be explained by the Schottky effect alone. It is noticed that the high-power RF operation boosts QE at the ASTA RF gun, as shown in Fig. 3. For a new cathode installed in the ASTA RF gun, the initial QE is 3×10 for the first day of electron beam operation. Then the QE increases by 50% after a few days of RF operations. It is logical to assume the RF processing conditions the cathode by removing surface contamination. In combination with the Schottky effect, a much higher QE is observed relative to measurements in a test chamber. Figure 3: QE map for a new cathode in the ASTA RF gun: 3×10 of peak QE for first day with turn-on electron beam (left), 4.5×10 for a week later (right). IN-SITU LASER-ASSISTED CLEANING DEVELOPMENTS IN THE ASTA RF GUN Laser-based cleaning techniques have been widely used to clean metal photocathodes, such as copper and Mg, for more than two decades. A high-intensity laser beam, interacting with the metal cathodes, may ablate the cathode surface, removing surface contamination and possibly changing the cathode reflectivity, thereby resulting in a QE increase. However, the laser cleaning may change cathode’s QE uniformity, thereby electron beam emittance, and also generate unwanted dark current or even deteriorate cathode’s crystal quality, if the laser cleaning process is too aggressive. The laser cleaning was performed for the present LCLS cathode in July 2011 [4]. The QE was evolved to 1×10 from 3×10 over a few months following the laser cleaning. Since then, 1×10 of the QE is essentially unchanged for three years to date for 24/7 users operation. The emittance was recovered to the normal value within three weeks following the laser cleaning. Although the previous LCLS laser cleaning was successful, two major concerns still remain. One concern is the reproducibility of the laser cleaning for different spots on the same cathodes and different cathodes. The other is the need to reduce emittance-recovery time and QE-evolution time following the laser cleaning with refinements of laser leaning process. These concerns are particularly important to deliver reliable and high quality electron beam for a high-impact machine for users, like the LCLS, which drives further studies at ASTA. THP030 Proceedings of FEL2014, Basel, Switzerland ISBN 978-3-95450-133-5 770 C op yr ig ht © 20 14 C C -B Y3. 0 an d by th e re sp ec tiv e au th or s Electron Bunch Generation and Manipulation Improvements of Laser Cleaning at the ASTA

An X-band multi-bunch test station is being built at LLNL to investigate the science and technology paths required to boost the current mono-energetic gamma-ray (MEGa-Ray) brightness by orders of magnitude. The test station will consist of a 5.5 cell X-band RF photoinjector, single accelerator section, and beam diagnostics. Beam quality must be exceedingly high in order to produce narrow-bandwidth gamma-rays, requiring a robust state of the art photoinjector. The photoinjector will be a high gradient (200 MV/m peak surface field on the cathode) standing wave structure, featuring a dual feed racetrack coupler, elliptical irises, and an optimized first cell length. A solidstate Scandinova modulator will power a single SLAC XL4 11.424 GHz 50 MW klystron. RF distribution will allow for full powering of the photoinjector with the balance of the RF powering a single accelerator section so that the electron parameters can be measured. The status of the facility will be presented including commissioning schedule and first experiment plans. Future experimental programs pertinent to Compton scattering R&D, high gradient structure testing, and light source development will be discussed.

An accelerator test stand has been constructed at SLAC to characterize laser-assisted photocathode processing, electron beam emission physics and front-end RF gun performance. The objective of the research program is to identify definitive ‘recipes’ for high-reliability photocathode preparation resulting in persistent high quantum efficiency and low beam emittance. In this paper we report on timing, optics and instrumentation for the Ti:Sapphire drive laser and diagnostics for the electron beam.

A new laser laboratory has been constructed at SLAC to test and characterize photocathode gun physics and develop diagnostics for ultrafast FEL applications. At the heart of the laboratory is a dual-purpose Ti:Sapphire oscillator/regen laser that can deliver either 1.5-3.5 ps or 25 fs pulse durations. The primary objectives of the photocathode research are to identify reliable Cu cathode cleaning recipes and to produce high quantum efficiency with low beam emittance. The ultrafast applications program is presently aimed at developing spectral- encoding systems for shot-to-shot pulse arrival time diagnostics with 10's of fs timing resolution. In this paper we review the laser system and update status of the physics programs.

In support of Compton scattering gamma-ray source efforts at LLNL, a multi-bunch test stand is being developed to investigate accelerator optimization for future upgrades. This test stand will enable work to explore the science and technology paths required to boost the current 10 Hz monoenergetic gamma-ray (MEGa-Ray) technology to an effective repetition rate exceeding 1 kHz, potentially increasing the average gamma-ray brightness by two orders of magnitude. Multiple bunches must be of exceedingly high quality to produce narrow-bandwidth gamma-rays. Modeling efforts will be presented, along with plans for a multi-bunch test stand at LLNL. The test stand will consist of a 5.5 cell X-band rf photoinjector, single accelerator section, and beam diagnostics. The photoinjector will be a high gradient standing wave structure, featuring a dual feed racetrack coupler. The accelerator will increase the electron energy so that the emittance can be measured using quadrupole scanning techniques. Multi-bunch diagnostics will be developed so that the beam quality can be measured and compared with theory. Design will be presented with modeling simulations, and layout plans.