IPAC2021 - Classification: MC2: Photon Sources and Electron Accelerators / T02 Electron Sources

Paper	Title	Page
TUPAB033	Photocathode Stress Test Bench at INFN LASA	1413
	D. Sertore, D. Giove, G. Guerini Rocco, L. Monaco INFN/LASA, Segrate (MI), Italy A. Bacci, F. Canella, S. Cialdi, I. Drebot, D. Giannotti, L. Serafini INFN-Milano, Milano, Italy D. Cipriani, E. Suerra Università degli Studi di Milano, Milano, Italy G. Galzerano POLIMI, Milano, Italy
	In the framework of the preparatory activities to the BriXSino project, a test bench for testing Cs₂Te photocathode at 100 MHz laser repetition rate has been installed at INFN LASA. This high repetition operation mode is foreseen to be the base operation mode of BriXSino and a qualification of the Cs₂Te photocathodes is a key component. While we are not at full specification due to the limited HV of the present DC gun, we discuss the status of the test bench and the initial results.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2021-TUPAB033
About •	paper received ※ 19 May 2021 paper accepted ※ 27 May 2021 issue date ※ 19 August 2021
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TUPAB034	Development of Multi-Alkali Antimonides Photocathodes for High-Brightness RF Photoinjectors	1416
	S.K. Mohanty, M. Krasilnikov, A. Oppelt, H.J. Qian, F. Stephan DESY Zeuthen, Zeuthen, Germany G. Guerini Rocco, C. Pagani Università degli Studi di Milano & INFN, Segrate, Italy W. Hillert University of Hamburg, Institut für Experimentalphysik, Hamburg, Germany P. Michelato, L. Monaco, D. Sertore INFN/LASA, Segrate (MI), Italy
	Multi-alkali antimonide-based photocathodes are suitable candidate for the electron sources of next-generation high brightness RF photoinjectors due to their excellent photoemissive properties especially, like low thermal emittances and high sensitivity to visible light. The former stands out, paving the way towards CW operations. Based on the previous successful development of Cesium Telluride photocathodes, we are now channelling our efforts toward an R&D activity focused on KCsSb and NaKSb(Cs) photocathodes. Parallel to that R&D activity, we have installed a new dedicated photocathode production system at the INFN-LASA to start the preparation of these photocathodes for their test in the PITZ photoinjector at DESY in Zeuthen. In this paper, detailed experimental results obtained from the KCsSb, along with a preliminary result from the NaKSb(Cs) photocathode material as well as the status of the overall project are presented.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2021-TUPAB034
About •	paper received ※ 19 May 2021 paper accepted ※ 21 June 2021 issue date ※ 31 August 2021
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WEXC03	Review of Superconducting Radio Frequency Gun	2556
	R. Xiang HZDR, Dresden, Germany
	The success of proposed high power free-electron lasers (FELs) and energy recovery linac (ERL) largely depends on the development of the electron source, which requires the best beam quality and CW operation. An elegant way to realize this average brilliance is to combine the high beam quality of mature normal conducting radio frequency photoinjector with the quick developing superconducting radio frequency technology, to build superconducting rf photoinjectors (SRF guns). In last decade, several SRF gun programs based on different approaches have achieved promising progress, even succeeded in routine operation at BNL and HZDR [,]. In the near future SRF guns are expected to play an important role for hard X-ray FEL facilities. In this contribution, we will review the design concepts, parameters, and the status of the major SRF gun projects. I. Petrushina et al., Phys. Rev. Lett. 124, 244801 **J. Teichert at al., Phys. Rev. Accel. Beams 24, 033401
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2021-WEXC03
About •	paper received ※ 19 May 2021 paper accepted ※ 28 June 2021 issue date ※ 23 August 2021
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WEXC07	Nucleation of Single Crystal Photocathode on Atomically Thin Graphene Substrate Using Co-Deposition of Cesium Telluride
	M. Gaowei, J. Cen, J. Sinsheimer, J. Walsh BNL, Upton, New York, USA A.M. Alexander, F. Liu, V.N. Pavlenko, J. Smedley, H. Yamaguchi LANL, Los Alamos, New Mexico, USA J.P. Biswas Stony Brook University, Stony Brook, USA D.P. Juarez-Lopez The University of Liverpool, Liverpool, United Kingdom S. Mistry HZB, Berlin, Germany
	Funding: Work supported by Brookhaven Science Associates, LLC under Contract No. DE-SC0012704 with the U.S. Department of Energy. For the past decades, cesium telluride (CsTe) has been chosen as the electron source material for high bunch charge, high repetition rate superconducting radio frequency electron injectors. The application of cesium telluride photocathode has been reported by accelerators all over the world. Alkali based semiconductor photocathode material has always been vapor deposited thin films, with amorphous or very limited crystalline phases. The fragility of alkali-based photocathode partially comes from its disordered and unstable structure. The limited crystallinity also limits the quantum efficiency to improve. Therefore, growing large crystal or even single crystal of the alkali-based photocathode material is the goal of many scientific projects these days. Nucleation of cesium telluride crystalline phase was observed via co-deposition method on atomically thin graphene substrate, which is is a recognized sign of the first step of the formation of single crystal. In situ and operando X-ray characterization has been performed on this process and the results in the evolution of crystal structure, chemical stoichiometry as well as the surface morphology and quantum efficiency are reported.
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WEPAB092	Redesign of the Jefferson Lab -300 kV DC Photo-Gun for High Bunch Charge Operations	2802
	S.A.K. Wijethunga, J.R. Delayen, G.A. Krafft, G.G. Palacios Serrano ODU, Norfolk, Virginia, USA J.F. Benesch, J.R. Delayen, C. Hernandez-Garcia, G.A. Krafft, M.A. Mamun, M. Poelker, R. Suleiman JLab, Newport News, Virginia, USA
	Funding: The U.S. Department of Energy, Office of Science, Office of Nuclear Physics under contract DE-AC05-06OR23177, JSA initiatives fund program and Laboratory Directed Research and Development program. Production of high bunch charge beams for the Electron-Ion Collider (EIC) is a challenging task. High bunch charge (a few nC) electron beam studies at Jefferson Lab using an inverted insulator DC high voltage photo-gun showed evidence of space charge limitations starting at 0.3 nC, limiting the maximum delivered bunch charge to 0.7 nC for beam at -225 kV, 75 ps (FWHM) pulse width, and 1.64 mm (rms) laser spot size. The low extracted charge is due to the modest longitudinal electric field (Ez) at the photocathode leading to beam loss at the anode and downstream beam pipe. To reach the few nC high bunch charge goal, and to correct the beam deflection exerted by the non-symmetric nature of the inverted insulator photo-gun the existing photo-gun was modified. This contribution discusses the electrostatic design of the modified photo-gun obtained using CST Studio Suite’s electromagnetic field solver. Beam dynamics simulations performed using General Particle Tracer (GPT) with the resulting electrostatic field map obtained from the modified electrodes confirmed the validity of the new design.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2021-WEPAB092
About •	paper received ※ 20 May 2021 paper accepted ※ 02 June 2021 issue date ※ 17 August 2021
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WEPAB096	RF Testbed for Cryogenic Photoemission Studies	2810
	G.E. Lawler, A. Fukasawa, N. Majernik, J.B. Rosenzweig, A. Suraj, M. Yadav UCLA, Los Angeles, California, USA Z. Li SLAC, Menlo Park, California, USA M. Yadav The University of Liverpool, Liverpool, United Kingdom M. Yadav Cockcroft Institute, Warrington, Cheshire, United Kingdom
	Funding: This work was supported by the Center for Bright Beams, National Science Foundation Grant No. PHY-1549132 and DOE Contract DE-SC0020409 Producing higher brightness beams at the cathode is one of the main focuses for future electron beam applications. For photocathodes operating close to their emission threshold, the cathode lattice temperature begins to dominate the minimum achievable intrinsic emittance. At UCLA, we are designing a radiofrequency (RF) test bed for measuring the temperature dependence of the mean transverse energy (MTE) and quantum efficiency for a number of candidate cathode materials. We intend to quantify the attainable brightness improvements at the cathode from cryogenic operation and establish a proof-of-principle cryogenic RF gun for future studies of a 1.6 cell cryogenic photoinjector for the UCLA ultra compact XFEL concept (UC-XFEL). The test bed will use a C-band 0.5-cell RF gun designed to operate down to 40K, producing an on-axis accelerating field of 120 MV/m. The cryogenic system uses conduction cooling and a load-lock system is being designed for transport and storage of air-sensitive high brightness cathodes.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2021-WEPAB096
About •	paper received ※ 19 May 2021 paper accepted ※ 24 June 2021 issue date ※ 15 August 2021
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WEPAB099	Near-Threshold Nonlinear Photoemission From Cu(100)	2822
	C.J. Knill, S.S. Karkare Arizona State University, Tempe, USA H.A. Padmore LBNL, Berkeley, California, USA
	Funding: National Science Foundation Grant No. PHY-1549132 Photocathodes that have a low mean transverse energy (MTE) are crucial to the development of compact X-ray Free Electron Lasers (XFEL) and ultrafast electron diffraction (UED) experiments. For FELs, low MTE cathodes result in a lower requirement for electron energy when lasing at a defined energy, and for a defined electron energy result in lasing at higher energy. For UED experiments, low MTE cathodes give a longer coherence length, allowing measurements on larger unit cell materials. A record low MTE of 5 meV has been recently demonstrated from a Cu (100) surface when measured near the photoemission threshold and cooled down to 30 K with liquid Helium []. For UED and XFEL applications that require a high charge density, the low quantum efficiency of Cu (100) near threshold necessitates the use of a high laser fluence to achieve the desired charge density []. At high laser fluences the MTE is limited by nonlinear effects, and therefore it is necessary to investigate near photoemission threshold at these high laser fluences. In this paper we report on nonlinear, near-threshold photoemission from a Cu (100) cathode, and its effect on the MTE. S. Karkare et al, Phys. Rev. Lett. 125, 054801 (2020) ** J. Bae et al, J. Appl. Phys., 124, 244903 (2018)
	Poster WEPAB099 [0.829 MB]
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2021-WEPAB099
About •	paper received ※ 19 May 2021 paper accepted ※ 21 July 2021 issue date ※ 29 August 2021
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WEPAB100	Heat Dissipation of Photocathodes at High Laser Intensities for a New DC Electron Source	2826
	M.A. Dehn, K. Aulenbacher IKP, Mainz, Germany K. Aulenbacher HIM, Mainz, Germany K. Aulenbacher GSI, Darmstadt, Germany
	Funding: This project was supported by the German science ministry BMBF through the Verbundforschung Laser intensities of 1W or more are required to extract average beam currents of more than 10mA from photocathodes. Most of this laser power is converted into thermal load within the cathode and has to be dissipated to avoid excessive heating of the cathode and thus a significant reduction in lifetime. At Johannes Gutenberg-University Mainz, we are developing a new high current DC electron source operating at an energy of 100keV, where an efficient heat dissipation of the photocathode is achieved by a mechanical design of the supporting structure.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2021-WEPAB100
About •	paper received ※ 19 May 2021 paper accepted ※ 24 June 2021 issue date ※ 30 August 2021
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WEPAB101	An Improved Model for Photoemission of Space Charge Dominated Picosecond Electron Bunches: Theory and Experiment	2829
	S.M. Polozov, V.I. Rashchikov MEPhI, Moscow, Russia M. Krasilnikov DESY Zeuthen, Zeuthen, Germany
	The emission of a short highly charged electron bunch in a radiofrequency photogun is discussed. The traditional space charge limited emission numerical model is extended by an introduction of positively charged ions arising in the cathode region and dynamically changing during the emission. Estimates on the time characteristics of the charge migrating process in the semiconductor region are given. The numerical results are compared with the results of other numerical models and with experimental observations at the Photo Injector Test facility at DESY in Zeuthen (PITZ).
	Poster WEPAB101 [1.601 MB]
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2021-WEPAB101
About •	paper received ※ 08 May 2021 paper accepted ※ 07 June 2021 issue date ※ 19 August 2021
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WEPAB102	Half-Metal Spin Filter for Highly Polarized Emission from GaAs Photocathodes	2833
	S. Poddar, C.-J. Jing, E.J. Montgomery Euclid Beamlabs, Bolingbrook, USA P. Lukashev University of Northern Iowa, Cedar Falls, Iowa, USA C. Palmstrøm UCSB, Santa Barbara, California, USA M.L. Stutzman, S. Zhang JLab, Newport News, Virginia, USA
	Funding: Work supported by Department of Energy grant number DE-SC0020564. GaAs-based photocathodes are one of the major sources of spin-polarized electrons and are crucial for the upcoming Electron-Ion collider experiments which includes study of proton spin and spin parity violation in the standard model. The theoretical polarization limit in unstrained GaAs photocathodes is 50 % but only 35 % is routinely achieved in experiments. Spin selective filtering allows to boost the spin polarization beyond the 50 % theoretical limit. In this work, first-principle electronic calculations using standard Density Functional Theory are performed to predict possible Heusler alloy half-metal candidates to be used as spin-filter. Simulations are also performed to investigate the half-metallicity as function of the magnetic spin direction. Several devices are experimentally fabricated using dedicated Molecular Beam Epitaxy growth system. We implemented Quantum Efficiency and Polarization testing of these half-metal/GaAs heterostructures using a dedicated Mott polarimeter system. Photoemission can also be seen on magnetically switching the spin-filter direction accompanied by a change in sign of the asymmetry which is a qualitative proof of the spin-filtering effect.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2021-WEPAB102
About •	paper received ※ 20 May 2021 paper accepted ※ 28 July 2021 issue date ※ 27 August 2021
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WEPAB103	Systematic Beam Parameter Studies at the Injector Section of FLUTE	2837
	T. Schmelzer, E. Bründermann, D. Hoffmann, I. Križnar, S. Marsching, A.-S. Müller, M.J. Nasse, R. Ruprecht, J. Schäfer, M. Schuh, N.J. Smale, P. Wesolowski, T. Windbichler KIT, Karlsruhe, Germany
	Funding: This work is supported by the DFG-funded Doctoral School "Karlsruhe School of Elementary and Astroparticle Physics: Science and Technology (KSETA)" FLUTE (Ferninfrarot Linac- und Test-Experiment) is a compact linac-based test facility for accelerator R&D and source of intense THz radiation for photon science. In preparation for the next experiments, the electron beam of the injector section of FLUTE has been characterized. In systematic studies the electron beam parameters, e.g., beam energy and emittance, are measured with several diagnostic systems. This knowledge allows the establishment of different operation settings and the optimization of electron beam parameters for future experiments.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2021-WEPAB103
About •	paper received ※ 19 May 2021 paper accepted ※ 01 September 2021 issue date ※ 13 August 2021
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WEPAB104	Improving the Operational Lifetime of the CEBAF Photo-Gun by Anode Biasing	2840
	J.T. Yoskowitz, G.A. Krafft, G.G. Palacios Serrano, S.A.K. Wijethunga ODU, Norfolk, Virginia, USA J.M. Grames, J. Hansknecht, C. Hernandez-Garcia, M. Poelker, M.L. Stutzman, R. Suleiman JLab, Newport News, Virginia, USA S.B. van der Geer Pulsar Physics, Eindhoven, The Netherlands
	Funding: U.S. Department of Energy, Office of Science, Office of Nuclear Physics under contract DE-AC05-06OR23177. The operating lifetime of GaAs-based photocathodes in DC high voltage electron photo-guns is dominated by the ionization rate of residual beamline gas molecules. In this work, experiments were performed to quantify the improvement in photocathode charge lifetime by biasing the photo-gun anode with a positive voltage, which repels ions generated downstream of the anode. The photo-cathode charge lifetime improved by almost a factor of two when the anode was biased compared to the usual grounded configuration. Simulations were performed using the particle tracking code General Particle Tracer (GPT) with a new custom element. The simulation results showed that both the number and energy of ions play a role in the pattern of QE degradation. The experiment results and conclusions supported by GPT simulations will be presented.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2021-WEPAB104
About •	paper received ※ 20 May 2021 paper accepted ※ 02 June 2021 issue date ※ 18 August 2021
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WEPAB106	Study on Durability Improvement of Cs-Te Photocathode by Means of Alkali Halide Protective Films	2847
	K. Ezawa, R. Fukuoka, Y. Koshiba, T. Tamba, M. Washio Waseda University, Tokyo, Japan K. Sakaue The University of Tokyo, Graduate School of Engineering, Bunkyo, Japan
	We have been conducting basic and applied research for generating high quality electron beams, using 1.6 cell laser photocathode RF-gun. In our laboratory, Cesium Telluride (Cs-Te), one of the semiconductor photocathodes, is used as an electron source for accelerator experiments. This semiconductor photocathode is known for high quantum efficiency (Q.E.) about 5~10% and 3-month 1/e lifetime. High Q.E. photocathodes can reduce the power requirement of the laser system, and long lifetime photocathodes can decrease the maintenance frequency, contributing to an efficient experimental environment. For these reasons, high Q.E. and long lifetime photocathodes are necessary in accelerator experiments. In order to produce robust photocathodes and extend the lifetime, we have conducted covering Cs-Te photocathodes with CsBr and CsI protective films. In this conference, we report the thickness dependency on the lifetime of Cs-Te photocathodes when we intentionally exposed oxygen gas to coated and non-coated Cs-Te photocathodes.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2021-WEPAB106
About •	paper received ※ 19 May 2021 paper accepted ※ 02 June 2021 issue date ※ 15 August 2021
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WEPAB109	Initial Study of GaN Thin Films for Photocathodes Prepared by Magnetron Sputtering on Copper Substrates	2850
	M. Vogel, X. Jiang, C. Wang University Siegen, Siegen, Germany P. Murcek, J. Schaber, R. Xiang HZDR, Dresden, Germany
	Funding: This research is funded by the Federal Ministry of Education and Research of Germany in the framework of BETH (project number 05K19PSB). On the path for high brightness electron beams, Gallium Nitride (GaN) is one promising candidate for a photo-cathode material. In this contribution, we report on the continuation of the study to optimize the crystallization quality and crystallography of Mg-doped GaN samples on copper substrates that are synthesized by RF magnetron sputtering. SEM and XRD results show that the pretreatment methods and the sputtering conditions (temperature, sputtering power, and partial pressure of the reactive gas) can both affect the morphology and crystal quality of GaN films. The initial QE measurements of these samples are done in our newly build in-situ QE measurement system and the first results of QE analyses done at Helmholtz-Zentrum Dresden-Rossendorf (HZDR) are presented in a dedicated contribution. Part of this work was performed at the Micro- and Nanoanalytics Facility (MNaF) of the University of Siegen.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2021-WEPAB109
About •	paper received ※ 19 May 2021 paper accepted ※ 09 June 2021 issue date ※ 27 August 2021
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WEPAB110	Solid-State Driven X-Band Linac for Electron Microscopy	2853
	A. Dhar, E.A. Nanni, M.A.K. Othman, S.G. Tantawi SLAC, Menlo Park, California, USA
	Funding: This work was supported by the Department of Energy Contract No. DE-AC02-76SF00515. Microcrystal electron diffraction (MicroED) is a technique used by scientists to image molecular crystals with cryo-electron microscopy (cryo-EM). However, cryo-EMs remain expensive, limiting MicroED’s accessibility. Current cryo-EMs accelerate electrons to 200-300 keV using DC electron guns with a nA of current and low emittance. However at higher voltages these DC guns rapidly grow in size. Replacing these electron guns with a compact linac powered by solid-state sources could lower cost while maintaining beam quality, thereby increasing accessibility. Utilizing compact high shunt impedance X-band structures ensures that each RF cycle contains at most a few electrons, preserving beam coherence. CW operation of the RF linac is possible with distributed solid-state architectures* that use 100W solid-state amplifiers at X-band frequencies. We present an initial design for a prototype low-cost CW RF linac for high-throughput MicroED producing 200 keV electrons with a standing-wave architecture where each cell is individually powered by a solid-state amplifier. This design also provides an upgrade path for future compact MeV-scale sources on the order of 1 meter in size. * Jones, C. G. et al. ACS central science 4, 1587-1592 (2018). ** D. C. Nguyen et al, Proc. 9th International Particle Accelerator Conference (IPAC’18), no. 9, pp. 520-523
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2021-WEPAB110
About •	paper received ※ 19 May 2021 paper accepted ※ 24 June 2021 issue date ※ 10 August 2021
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WEPAB111	Controlled Degradation by Oxygen Exposure in the Performance of a Ag (100) Single-Crystal Photocathode	2856
	L.A.J. Soomary, C.P. Welsch The University of Liverpool, Liverpool, United Kingdom L.B. Jones, T.C.Q. Noakes STFC/DL/ASTeC, Daresbury, Warrington, Cheshire, United Kingdom
	The search for high-performance photocathode electron sources is a priority in the accelerator science community. The surface characteristics of a photocathode define many important factors of the photoemission including the work function, intrinsic emittance, and quantum efficiency of the photocathode. These factors in turn define the electron beam performance which is measurable as normalized emittance, brightness, and energy spread. Strategies for improving these parameters vary, but understanding and influencing the relevant cathode surface physics which underpin these attributes is a primary focus for the electron source community. As such, pure metal photocathodes and their performance at UV wavelengths are of interest as seen at the LCLS at SLAC and CLARA at Daresbury. We present performance data for an Ag (100) single-crystal photocathode under illumination at 266 nm wavelength, with known levels of surface roughness, using our Transverse Energy Spread Spectrometer (TESS)** both at room and cryogenic temperatures. Crucially our data shows the effect of progressive degradation in the photo-cathode performance as a consequence of exposure to controlled levels of oxygen. * D.H. Dowell, et al., Nucl. Instr. and Meth. A (2010), doi:10.1016/j.nima.2010.03.104 Appl. Phys. Lett. 89, 224103 (2006); doi:10.1063/1.2387968 * Proc. FEL’13, TUPPS033, 290-293
	Poster WEPAB111 [0.866 MB]
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2021-WEPAB111
About •	paper received ※ 20 May 2021 paper accepted ※ 22 June 2021 issue date ※ 31 August 2021
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WEPAB112	Performance Characterisation of a Cu (100) Single-Crystal Photocathode	2860
	L.A.J. Soomary, D.P. Juarez-Lopez, C.P. Welsch The University of Liverpool, Liverpool, United Kingdom L.B. Jones, T.C.Q. Noakes STFC/DL/ASTeC, Daresbury, Warrington, Cheshire, United Kingdom
	The search for high performance photocathode electron sources is a priority in the accelerator science community. The surface characteristics of a photocathode define important factors of the photoemission including the intrinsic emittance, the quantum efficiency and the work function of the photocathode. These factors in turn define the electron beam performance which are measurable as emittance, brightness and energy spread. We have used ASTeC’s Multiprobe (SAPI)* to characterise and analyse photocathode performance using multiple techniques including XPS, STM, and LEED imaging, and their Transverse Energy Spread Spectrometer (TESS)** to measure mean transverse energy (MTE). We present characterisation measurements for a Cu (100) single-crystal photocathode sample with data from SAPI confirming the crystallographic face and showing surface composition and roughness, supported by data from TESS showing the photocathode electron beam energy spread. * B.L. Militsyn, 4-th EuCARD2 WP12.5 meeting, Warsaw, 14-15 March 2017 **Proc. FEL’13, TUPPS033, 290-293
	Poster WEPAB112 [0.814 MB]
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2021-WEPAB112
About •	paper received ※ 19 May 2021 paper accepted ※ 12 July 2021 issue date ※ 22 August 2021
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THPAB082	Recent Operational Experience with Thermionic RF Guns at the APS	3959
	Y. Sun, M. Borland, G.I. Fystro, X. Huang, H. Shang ANL, Lemont, Illinois, USA
	Funding: Work supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357 The electron beam at the Argonne Advanced Photon Source (APS) is generated from an S-band thermionic RF gun. There are two locations at the frontend of the linac where thermionic RF guns are installed – RG1 and RG2. Three so-called generation-III guns are available, two are installed at RG1 and RG2, one is a spare. In recent years, these guns are showing signs of aging after over a couple of decades of operations. RF trips started to occur, and we had to reduce the nominal operating rf power to alleviate the problem. In addition, beam generated by RG1 suffers from low transportation efficiency from the gun to the linac, and beam trajectory is unstable which results in charge instabilities. Recently, APS obtained a new type of prototype gun and it was beam commissioned in the linac. In this paper, we report our operational experience with these thermionic rf guns including thermionic-cathode beam extraction, gun front-end optimization for maximum charge transmission through the linac, linac lattice setup to match beam for injection into the Particle Accumulator Ring (PAR) and optimization for maximum PAR injection efficiency.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2021-THPAB082
About •	paper received ※ 19 May 2021 paper accepted ※ 28 July 2021 issue date ※ 26 August 2021
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THPAB331	High-Power Test of a Highly Over-Coupled X-Band RF Gun Driven by Short RF Pulses	4432
	J.H. Shao, D.S. Doran, W. Liu, J.G. Power, C. Whiteford, E.E. Wisniewski ANL, Lemont, Illinois, USA C.-J. Jing, S.V. Kuzikov Euclid TechLabs, Solon, Ohio, USA X. Lu, P. Piot, W.H. Tan Northern Illinois University, DeKalb, Illinois, USA
	Beam brightness, a key figure of merit of RF photocathode guns, can be improved by increasing the cathode surface field which suppresses emittance growth from space charge. The surface field in normal-conducting structures is mainly limited by RF breakdown and it has been experimentally discovered that RF breakdown rate exponentially depends on RF pulse length. A highly over-coupled 1.5-cell X-band photocathode gun has been developed to be powered by 9 ns RF pulses with 3 ns rising time, 3 ns flat-top, and 3 ns falling time generated by an X-band metallic power extractor. In the recent experiment at Argonne Wakefield Accelerator facility, cathode surface field up to ~350 MV/m with a low breakdown rate has been obtained under ~250 MW input power. Strong beam loading from dark current was observed during RF conditioning and quickly recovered to a negligible level after the gun reached the maximum gradient. Detailed high-power test results and data analysis will be reported in this manuscript.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2021-THPAB331
About •	paper received ※ 25 May 2021 paper accepted ※ 14 July 2021 issue date ※ 23 August 2021
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THPAB344	Magneto-Optical Trap Cathode for High Brightness Applications	4466
	V.S. Yu, C.E. Hansel, G.E. Lawler, M. Mills, J.B. Rosenzweig UCLA, Los Angeles, California, USA J.I. Mann PBPL, Los Angeles, USA
	Funding: This work was performed with support of the US Department of Energy under Contract No. DE-SC0020409 and the National Science Foundation under Grant No. PHY-1549132 Electron bunches extracted from magneto-optical traps (MOTs) via femtosecond photo-ionization and electrostatic acceleration can have significantly lower transverse emittance than emissions from traditional metal cathodes. Such MOT cathodes, however, have two drawbacks: the need for multiple trapping lasers and the limit to ~MV/m fields. Designs exist for MOTs which only require one trapping laser. Our RF simulations in High-Frequency Structure Simulator (HFSS) indicate that the cone MOT is the only one compatible with high gradient RF cavities. We present the combination of the two, an RF cavity with a cone-MOT as part of its geometry. It only requires one trapping laser and can use much higher fields. The geometry of the chamber is compatible with a wide range of MOT species, which allows the search for one which is compatible with copper cavities.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2021-THPAB344
About •	paper received ※ 19 May 2021 paper accepted ※ 29 July 2021 issue date ※ 12 August 2021
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FRXA04	Commissioning results of LCLS-II MHz repetition rate electron source
	F. Zhou, C. Adolphsen, A.L. Benwell, G.W. Brown, D. Dowell, M.P. Dunning, S. Gilevich, K. Grouev, B.T. Jacobson, X. Liu, A. Miahnahri, J.F. Schmerge, T. Vecchione SLAC, Menlo Park, California, USA G. Huang, F. Sannibale LBNL, Berkeley, California, USA
	Funding: work supported by DOE under grant No. DE-AC02-76SF00515 A 4 GeV 1.3 GHz superconducting linac is being constructed at SLAC as part of the X-ray free electron laser project (LCLS-II). The first 3-meter of the electron source that includes a normal conducting 185.7MHz CW RF gun, 2-cell 1.3 GHz CW RF buncher, and a loadlock system for cathode changes was designed and built by LBNL based on their experience with similar one for advanced photo-injector experiment program. The electron beam is designed to operate at a high repetition rate, up to 1 MHz. Since summer of 2018 we started LCLS-II injector source commissioning immediately after the major installation completion. This paper presents major commissioning results including achievements of ultra-high vacuum, RF processing to CW nominal power, dark current characterization and mitigation, and high-brightness electron beam measurements.
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Paper

Title

Page

Photocathode Stress Test Bench at INFN LASA

1413

D. Sertore, D. Giove, G. Guerini Rocco, L. Monaco
INFN/LASA, Segrate (MI), Italy
A. Bacci, F. Canella, S. Cialdi, I. Drebot, D. Giannotti, L. Serafini
INFN-Milano, Milano, Italy
D. Cipriani, E. Suerra
Università degli Studi di Milano, Milano, Italy
G. Galzerano
POLIMI, Milano, Italy

In the framework of the preparatory activities to the BriXSino project, a test bench for testing Cs₂Te photocathode at 100 MHz laser repetition rate has been installed at INFN LASA. This high repetition operation mode is foreseen to be the base operation mode of BriXSino and a qualification of the Cs₂Te photocathodes is a key component. While we are not at full specification due to the limited HV of the present DC gun, we discuss the status of the test bench and the initial results.

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reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2021-TUPAB033

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paper received ※ 19 May 2021 paper accepted ※ 27 May 2021 issue date ※ 19 August 2021

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TUPAB034

Development of Multi-Alkali Antimonides Photocathodes for High-Brightness RF Photoinjectors

1416

S.K. Mohanty, M. Krasilnikov, A. Oppelt, H.J. Qian, F. Stephan
DESY Zeuthen, Zeuthen, Germany
G. Guerini Rocco, C. Pagani
Università degli Studi di Milano & INFN, Segrate, Italy
W. Hillert
University of Hamburg, Institut für Experimentalphysik, Hamburg, Germany
P. Michelato, L. Monaco, D. Sertore
INFN/LASA, Segrate (MI), Italy

Multi-alkali antimonide-based photocathodes are suitable candidate for the electron sources of next-generation high brightness RF photoinjectors due to their excellent photoemissive properties especially, like low thermal emittances and high sensitivity to visible light. The former stands out, paving the way towards CW operations. Based on the previous successful development of Cesium Telluride photocathodes, we are now channelling our efforts toward an R&D activity focused on KCsSb and NaKSb(Cs) photocathodes. Parallel to that R&D activity, we have installed a new dedicated photocathode production system at the INFN-LASA to start the preparation of these photocathodes for their test in the PITZ photoinjector at DESY in Zeuthen. In this paper, detailed experimental results obtained from the KCsSb, along with a preliminary result from the NaKSb(Cs) photocathode material as well as the status of the overall project are presented.

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reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2021-TUPAB034

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paper received ※ 19 May 2021 paper accepted ※ 21 June 2021 issue date ※ 31 August 2021

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WEXC03

Review of Superconducting Radio Frequency Gun

2556

R. Xiang
HZDR, Dresden, Germany

The success of proposed high power free-electron lasers (FELs) and energy recovery linac (ERL) largely depends on the development of the electron source, which requires the best beam quality and CW operation. An elegant way to realize this average brilliance is to combine the high beam quality of mature normal conducting radio frequency photoinjector with the quick developing superconducting radio frequency technology, to build superconducting rf photoinjectors (SRF guns). In last decade, several SRF gun programs based on different approaches have achieved promising progress, even succeeded in routine operation at BNL and HZDR [*,**]. In the near future SRF guns are expected to play an important role for hard X-ray FEL facilities. In this contribution, we will review the design concepts, parameters, and the status of the major SRF gun projects.
*I. Petrushina et al., Phys. Rev. Lett. 124, 244801
**J. Teichert at al., Phys. Rev. Accel. Beams 24, 033401

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paper received ※ 19 May 2021 paper accepted ※ 28 June 2021 issue date ※ 23 August 2021

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WEXC07

Nucleation of Single Crystal Photocathode on Atomically Thin Graphene Substrate Using Co-Deposition of Cesium Telluride

M. Gaowei, J. Cen, J. Sinsheimer, J. Walsh
BNL, Upton, New York, USA
A.M. Alexander, F. Liu, V.N. Pavlenko, J. Smedley, H. Yamaguchi
LANL, Los Alamos, New Mexico, USA
J.P. Biswas
Stony Brook University, Stony Brook, USA
D.P. Juarez-Lopez
The University of Liverpool, Liverpool, United Kingdom
S. Mistry
HZB, Berlin, Germany

Funding: Work supported by Brookhaven Science Associates, LLC under Contract No. DE-SC0012704 with the U.S. Department of Energy.
For the past decades, cesium telluride (CsTe) has been chosen as the electron source material for high bunch charge, high repetition rate superconducting radio frequency electron injectors. The application of cesium telluride photocathode has been reported by accelerators all over the world. Alkali based semiconductor photocathode material has always been vapor deposited thin films, with amorphous or very limited crystalline phases. The fragility of alkali-based photocathode partially comes from its disordered and unstable structure. The limited crystallinity also limits the quantum efficiency to improve. Therefore, growing large crystal or even single crystal of the alkali-based photocathode material is the goal of many scientific projects these days. Nucleation of cesium telluride crystalline phase was observed via co-deposition method on atomically thin graphene substrate, which is is a recognized sign of the first step of the formation of single crystal. In situ and operando X-ray characterization has been performed on this process and the results in the evolution of crystal structure, chemical stoichiometry as well as the surface morphology and quantum efficiency are reported.

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WEPAB092

Redesign of the Jefferson Lab -300 kV DC Photo-Gun for High Bunch Charge Operations

2802

S.A.K. Wijethunga, J.R. Delayen, G.A. Krafft, G.G. Palacios Serrano
ODU, Norfolk, Virginia, USA
J.F. Benesch, J.R. Delayen, C. Hernandez-Garcia, G.A. Krafft, M.A. Mamun, M. Poelker, R. Suleiman
JLab, Newport News, Virginia, USA

Funding: The U.S. Department of Energy, Office of Science, Office of Nuclear Physics under contract DE-AC05-06OR23177, JSA initiatives fund program and Laboratory Directed Research and Development program.
Production of high bunch charge beams for the Electron-Ion Collider (EIC) is a challenging task. High bunch charge (a few nC) electron beam studies at Jefferson Lab using an inverted insulator DC high voltage photo-gun showed evidence of space charge limitations starting at 0.3 nC, limiting the maximum delivered bunch charge to 0.7 nC for beam at -225 kV, 75 ps (FWHM) pulse width, and 1.64 mm (rms) laser spot size. The low extracted charge is due to the modest longitudinal electric field (Ez) at the photocathode leading to beam loss at the anode and downstream beam pipe. To reach the few nC high bunch charge goal, and to correct the beam deflection exerted by the non-symmetric nature of the inverted insulator photo-gun the existing photo-gun was modified. This contribution discusses the electrostatic design of the modified photo-gun obtained using CST Studio Suite’s electromagnetic field solver. Beam dynamics simulations performed using General Particle Tracer (GPT) with the resulting electrostatic field map obtained from the modified electrodes confirmed the validity of the new design.

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reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2021-WEPAB092

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paper received ※ 20 May 2021 paper accepted ※ 02 June 2021 issue date ※ 17 August 2021

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WEPAB096

RF Testbed for Cryogenic Photoemission Studies

2810

G.E. Lawler, A. Fukasawa, N. Majernik, J.B. Rosenzweig, A. Suraj, M. Yadav
UCLA, Los Angeles, California, USA
Z. Li
SLAC, Menlo Park, California, USA
M. Yadav
The University of Liverpool, Liverpool, United Kingdom
M. Yadav
Cockcroft Institute, Warrington, Cheshire, United Kingdom

Funding: This work was supported by the Center for Bright Beams, National Science Foundation Grant No. PHY-1549132 and DOE Contract DE-SC0020409
Producing higher brightness beams at the cathode is one of the main focuses for future electron beam applications. For photocathodes operating close to their emission threshold, the cathode lattice temperature begins to dominate the minimum achievable intrinsic emittance. At UCLA, we are designing a radiofrequency (RF) test bed for measuring the temperature dependence of the mean transverse energy (MTE) and quantum efficiency for a number of candidate cathode materials. We intend to quantify the attainable brightness improvements at the cathode from cryogenic operation and establish a proof-of-principle cryogenic RF gun for future studies of a 1.6 cell cryogenic photoinjector for the UCLA ultra compact XFEL concept (UC-XFEL). The test bed will use a C-band 0.5-cell RF gun designed to operate down to 40K, producing an on-axis accelerating field of 120 MV/m. The cryogenic system uses conduction cooling and a load-lock system is being designed for transport and storage of air-sensitive high brightness cathodes.

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reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2021-WEPAB096

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paper received ※ 19 May 2021 paper accepted ※ 24 June 2021 issue date ※ 15 August 2021

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WEPAB099

Near-Threshold Nonlinear Photoemission From Cu(100)

2822

C.J. Knill, S.S. Karkare
Arizona State University, Tempe, USA
H.A. Padmore
LBNL, Berkeley, California, USA

Funding: National Science Foundation Grant No. PHY-1549132
Photocathodes that have a low mean transverse energy (MTE) are crucial to the development of compact X-ray Free Electron Lasers (XFEL) and ultrafast electron diffraction (UED) experiments. For FELs, low MTE cathodes result in a lower requirement for electron energy when lasing at a defined energy, and for a defined electron energy result in lasing at higher energy. For UED experiments, low MTE cathodes give a longer coherence length, allowing measurements on larger unit cell materials. A record low MTE of 5 meV has been recently demonstrated from a Cu (100) surface when measured near the photoemission threshold and cooled down to 30 K with liquid Helium [*]. For UED and XFEL applications that require a high charge density, the low quantum efficiency of Cu (100) near threshold necessitates the use of a high laser fluence to achieve the desired charge density [**]. At high laser fluences the MTE is limited by nonlinear effects, and therefore it is necessary to investigate near photoemission threshold at these high laser fluences. In this paper we report on nonlinear, near-threshold photoemission from a Cu (100) cathode, and its effect on the MTE.
* S. Karkare et al, Phys. Rev. Lett. 125, 054801 (2020)
** J. Bae et al, J. Appl. Phys., 124, 244903 (2018)

Poster WEPAB099 [0.829 MB]

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reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2021-WEPAB099

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paper received ※ 19 May 2021 paper accepted ※ 21 July 2021 issue date ※ 29 August 2021

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WEPAB100

Heat Dissipation of Photocathodes at High Laser Intensities for a New DC Electron Source

2826

M.A. Dehn, K. Aulenbacher
IKP, Mainz, Germany
K. Aulenbacher
HIM, Mainz, Germany
K. Aulenbacher
GSI, Darmstadt, Germany

Funding: This project was supported by the German science ministry BMBF through the Verbundforschung
Laser intensities of 1W or more are required to extract average beam currents of more than 10mA from photocathodes. Most of this laser power is converted into thermal load within the cathode and has to be dissipated to avoid excessive heating of the cathode and thus a significant reduction in lifetime. At Johannes Gutenberg-University Mainz, we are developing a new high current DC electron source operating at an energy of 100keV, where an efficient heat dissipation of the photocathode is achieved by a mechanical design of the supporting structure.

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reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2021-WEPAB100

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paper received ※ 19 May 2021 paper accepted ※ 24 June 2021 issue date ※ 30 August 2021

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WEPAB101

An Improved Model for Photoemission of Space Charge Dominated Picosecond Electron Bunches: Theory and Experiment

2829

S.M. Polozov, V.I. Rashchikov
MEPhI, Moscow, Russia
M. Krasilnikov
DESY Zeuthen, Zeuthen, Germany

The emission of a short highly charged electron bunch in a radiofrequency photogun is discussed. The traditional space charge limited emission numerical model is extended by an introduction of positively charged ions arising in the cathode region and dynamically changing during the emission. Estimates on the time characteristics of the charge migrating process in the semiconductor region are given. The numerical results are compared with the results of other numerical models and with experimental observations at the Photo Injector Test facility at DESY in Zeuthen (PITZ).

Poster WEPAB101 [1.601 MB]

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reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2021-WEPAB101

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paper received ※ 08 May 2021 paper accepted ※ 07 June 2021 issue date ※ 19 August 2021

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WEPAB102

Half-Metal Spin Filter for Highly Polarized Emission from GaAs Photocathodes

2833

S. Poddar, C.-J. Jing, E.J. Montgomery
Euclid Beamlabs, Bolingbrook, USA
P. Lukashev
University of Northern Iowa, Cedar Falls, Iowa, USA
C. Palmstrøm
UCSB, Santa Barbara, California, USA
M.L. Stutzman, S. Zhang
JLab, Newport News, Virginia, USA

Funding: Work supported by Department of Energy grant number DE-SC0020564.
GaAs-based photocathodes are one of the major sources of spin-polarized electrons and are crucial for the upcoming Electron-Ion collider experiments which includes study of proton spin and spin parity violation in the standard model. The theoretical polarization limit in unstrained GaAs photocathodes is 50 % but only 35 % is routinely achieved in experiments. Spin selective filtering allows to boost the spin polarization beyond the 50 % theoretical limit. In this work, first-principle electronic calculations using standard Density Functional Theory are performed to predict possible Heusler alloy half-metal candidates to be used as spin-filter. Simulations are also performed to investigate the half-metallicity as function of the magnetic spin direction. Several devices are experimentally fabricated using dedicated Molecular Beam Epitaxy growth system. We implemented Quantum Efficiency and Polarization testing of these half-metal/GaAs heterostructures using a dedicated Mott polarimeter system. Photoemission can also be seen on magnetically switching the spin-filter direction accompanied by a change in sign of the asymmetry which is a qualitative proof of the spin-filtering effect.

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paper received ※ 20 May 2021 paper accepted ※ 28 July 2021 issue date ※ 27 August 2021

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WEPAB103

Systematic Beam Parameter Studies at the Injector Section of FLUTE

2837

T. Schmelzer, E. Bründermann, D. Hoffmann, I. Križnar, S. Marsching, A.-S. Müller, M.J. Nasse, R. Ruprecht, J. Schäfer, M. Schuh, N.J. Smale, P. Wesolowski, T. Windbichler
KIT, Karlsruhe, Germany

Funding: This work is supported by the DFG-funded Doctoral School "Karlsruhe School of Elementary and Astroparticle Physics: Science and Technology (KSETA)"
FLUTE (Ferninfrarot Linac- und Test-Experiment) is a compact linac-based test facility for accelerator R&D and source of intense THz radiation for photon science. In preparation for the next experiments, the electron beam of the injector section of FLUTE has been characterized. In systematic studies the electron beam parameters, e.g., beam energy and emittance, are measured with several diagnostic systems. This knowledge allows the establishment of different operation settings and the optimization of electron beam parameters for future experiments.

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reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2021-WEPAB103

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paper received ※ 19 May 2021 paper accepted ※ 01 September 2021 issue date ※ 13 August 2021

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WEPAB104

Improving the Operational Lifetime of the CEBAF Photo-Gun by Anode Biasing

2840

J.T. Yoskowitz, G.A. Krafft, G.G. Palacios Serrano, S.A.K. Wijethunga
ODU, Norfolk, Virginia, USA
J.M. Grames, J. Hansknecht, C. Hernandez-Garcia, M. Poelker, M.L. Stutzman, R. Suleiman
JLab, Newport News, Virginia, USA
S.B. van der Geer
Pulsar Physics, Eindhoven, The Netherlands

Funding: U.S. Department of Energy, Office of Science, Office of Nuclear Physics under contract DE-AC05-06OR23177.
The operating lifetime of GaAs-based photocathodes in DC high voltage electron photo-guns is dominated by the ionization rate of residual beamline gas molecules. In this work, experiments were performed to quantify the improvement in photocathode charge lifetime by biasing the photo-gun anode with a positive voltage, which repels ions generated downstream of the anode. The photo-cathode charge lifetime improved by almost a factor of two when the anode was biased compared to the usual grounded configuration. Simulations were performed using the particle tracking code General Particle Tracer (GPT) with a new custom element. The simulation results showed that both the number and energy of ions play a role in the pattern of QE degradation. The experiment results and conclusions supported by GPT simulations will be presented.

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paper received ※ 20 May 2021 paper accepted ※ 02 June 2021 issue date ※ 18 August 2021

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WEPAB106

Study on Durability Improvement of Cs-Te Photocathode by Means of Alkali Halide Protective Films

2847

K. Ezawa, R. Fukuoka, Y. Koshiba, T. Tamba, M. Washio
Waseda University, Tokyo, Japan
K. Sakaue
The University of Tokyo, Graduate School of Engineering, Bunkyo, Japan

We have been conducting basic and applied research for generating high quality electron beams, using 1.6 cell laser photocathode RF-gun. In our laboratory, Cesium Telluride (Cs-Te), one of the semiconductor photocathodes, is used as an electron source for accelerator experiments. This semiconductor photocathode is known for high quantum efficiency (Q.E.) about 5~10% and 3-month 1/e lifetime. High Q.E. photocathodes can reduce the power requirement of the laser system, and long lifetime photocathodes can decrease the maintenance frequency, contributing to an efficient experimental environment. For these reasons, high Q.E. and long lifetime photocathodes are necessary in accelerator experiments. In order to produce robust photocathodes and extend the lifetime, we have conducted covering Cs-Te photocathodes with CsBr and CsI protective films. In this conference, we report the thickness dependency on the lifetime of Cs-Te photocathodes when we intentionally exposed oxygen gas to coated and non-coated Cs-Te photocathodes.

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reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2021-WEPAB106

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paper received ※ 19 May 2021 paper accepted ※ 02 June 2021 issue date ※ 15 August 2021

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WEPAB109

Initial Study of GaN Thin Films for Photocathodes Prepared by Magnetron Sputtering on Copper Substrates

2850

M. Vogel, X. Jiang, C. Wang
University Siegen, Siegen, Germany
P. Murcek, J. Schaber, R. Xiang
HZDR, Dresden, Germany

Funding: This research is funded by the Federal Ministry of Education and Research of Germany in the framework of BETH (project number 05K19PSB).
On the path for high brightness electron beams, Gallium Nitride (GaN) is one promising candidate for a photo-cathode material. In this contribution, we report on the continuation of the study to optimize the crystallization quality and crystallography of Mg-doped GaN samples on copper substrates that are synthesized by RF magnetron sputtering. SEM and XRD results show that the pretreatment methods and the sputtering conditions (temperature, sputtering power, and partial pressure of the reactive gas) can both affect the morphology and crystal quality of GaN films. The initial QE measurements of these samples are done in our newly build in-situ QE measurement system and the first results of QE analyses done at Helmholtz-Zentrum Dresden-Rossendorf (HZDR) are presented in a dedicated contribution.
Part of this work was performed at the Micro- and Nanoanalytics Facility (MNaF) of the University of Siegen.

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paper received ※ 19 May 2021 paper accepted ※ 09 June 2021 issue date ※ 27 August 2021

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WEPAB110

Solid-State Driven X-Band Linac for Electron Microscopy

2853

A. Dhar, E.A. Nanni, M.A.K. Othman, S.G. Tantawi
SLAC, Menlo Park, California, USA

Funding: This work was supported by the Department of Energy Contract No. DE-AC02-76SF00515.
Microcrystal electron diffraction (MicroED) is a technique used by scientists to image molecular crystals with cryo-electron microscopy (cryo-EM)*. However, cryo-EMs remain expensive, limiting MicroED’s accessibility. Current cryo-EMs accelerate electrons to 200-300 keV using DC electron guns with a nA of current and low emittance. However at higher voltages these DC guns rapidly grow in size. Replacing these electron guns with a compact linac powered by solid-state sources could lower cost while maintaining beam quality, thereby increasing accessibility. Utilizing compact high shunt impedance X-band structures ensures that each RF cycle contains at most a few electrons, preserving beam coherence. CW operation of the RF linac is possible with distributed solid-state architectures** that use 100W solid-state amplifiers at X-band frequencies. We present an initial design for a prototype low-cost CW RF linac for high-throughput MicroED producing 200 keV electrons with a standing-wave architecture where each cell is individually powered by a solid-state amplifier. This design also provides an upgrade path for future compact MeV-scale sources on the order of 1 meter in size.
* Jones, C. G. et al. ACS central science 4, 1587-1592 (2018).
** D. C. Nguyen et al, Proc. 9th International Particle Accelerator Conference (IPAC’18), no. 9, pp. 520-523

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paper received ※ 19 May 2021 paper accepted ※ 24 June 2021 issue date ※ 10 August 2021

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WEPAB111

Controlled Degradation by Oxygen Exposure in the Performance of a Ag (100) Single-Crystal Photocathode

2856

L.A.J. Soomary, C.P. Welsch
The University of Liverpool, Liverpool, United Kingdom
L.B. Jones, T.C.Q. Noakes
STFC/DL/ASTeC, Daresbury, Warrington, Cheshire, United Kingdom

The search for high-performance photocathode electron sources is a priority in the accelerator science community. The surface characteristics of a photocathode define many important factors of the photoemission including the work function, intrinsic emittance, and quantum efficiency of the photocathode. These factors in turn define the electron beam performance which is measurable as normalized emittance, brightness, and energy spread*. Strategies for improving these parameters vary, but understanding and influencing the relevant cathode surface physics which underpin these attributes is a primary focus for the electron source community**. As such, pure metal photocathodes and their performance at UV wavelengths are of interest as seen at the LCLS at SLAC and CLARA at Daresbury. We present performance data for an Ag (100) single-crystal photocathode under illumination at 266 nm wavelength, with known levels of surface roughness, using our Transverse Energy Spread Spectrometer (TESS)*** both at room and cryogenic temperatures. Crucially our data shows the effect of progressive degradation in the photo-cathode performance as a consequence of exposure to controlled levels of oxygen.
* D.H. Dowell, et al., Nucl. Instr. and Meth. A (2010), doi:10.1016/j.nima.2010.03.104
** Appl. Phys. Lett. 89, 224103 (2006); doi:10.1063/1.2387968
*** Proc. FEL’13, TUPPS033, 290-293

Poster WEPAB111 [0.866 MB]

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reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2021-WEPAB111

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paper received ※ 20 May 2021 paper accepted ※ 22 June 2021 issue date ※ 31 August 2021

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WEPAB112

Performance Characterisation of a Cu (100) Single-Crystal Photocathode

2860

L.A.J. Soomary, D.P. Juarez-Lopez, C.P. Welsch
The University of Liverpool, Liverpool, United Kingdom
L.B. Jones, T.C.Q. Noakes
STFC/DL/ASTeC, Daresbury, Warrington, Cheshire, United Kingdom

The search for high performance photocathode electron sources is a priority in the accelerator science community. The surface characteristics of a photocathode define important factors of the photoemission including the intrinsic emittance, the quantum efficiency and the work function of the photocathode. These factors in turn define the electron beam performance which are measurable as emittance, brightness and energy spread. We have used ASTeC’s Multiprobe (SAPI)* to characterise and analyse photocathode performance using multiple techniques including XPS, STM, and LEED imaging, and their Transverse Energy Spread Spectrometer (TESS)** to measure mean transverse energy (MTE). We present characterisation measurements for a Cu (100) single-crystal photocathode sample with data from SAPI confirming the crystallographic face and showing surface composition and roughness, supported by data from TESS showing the photocathode electron beam energy spread.
* B.L. Militsyn, 4-th EuCARD2 WP12.5 meeting, Warsaw, 14-15 March 2017
**Proc. FEL’13, TUPPS033, 290-293

Poster WEPAB112 [0.814 MB]

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reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2021-WEPAB112

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paper received ※ 19 May 2021 paper accepted ※ 12 July 2021 issue date ※ 22 August 2021

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THPAB082

Recent Operational Experience with Thermionic RF Guns at the APS

3959

Y. Sun, M. Borland, G.I. Fystro, X. Huang, H. Shang
ANL, Lemont, Illinois, USA

Funding: Work supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357
The electron beam at the Argonne Advanced Photon Source (APS) is generated from an S-band thermionic RF gun. There are two locations at the frontend of the linac where thermionic RF guns are installed – RG1 and RG2. Three so-called generation-III guns are available, two are installed at RG1 and RG2, one is a spare. In recent years, these guns are showing signs of aging after over a couple of decades of operations. RF trips started to occur, and we had to reduce the nominal operating rf power to alleviate the problem. In addition, beam generated by RG1 suffers from low transportation efficiency from the gun to the linac, and beam trajectory is unstable which results in charge instabilities. Recently, APS obtained a new type of prototype gun and it was beam commissioned in the linac. In this paper, we report our operational experience with these thermionic rf guns including thermionic-cathode beam extraction, gun front-end optimization for maximum charge transmission through the linac, linac lattice setup to match beam for injection into the Particle Accumulator Ring (PAR) and optimization for maximum PAR injection efficiency.

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reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2021-THPAB082

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paper received ※ 19 May 2021 paper accepted ※ 28 July 2021 issue date ※ 26 August 2021

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THPAB331

High-Power Test of a Highly Over-Coupled X-Band RF Gun Driven by Short RF Pulses

4432

J.H. Shao, D.S. Doran, W. Liu, J.G. Power, C. Whiteford, E.E. Wisniewski
ANL, Lemont, Illinois, USA
C.-J. Jing, S.V. Kuzikov
Euclid TechLabs, Solon, Ohio, USA
X. Lu, P. Piot, W.H. Tan
Northern Illinois University, DeKalb, Illinois, USA

Beam brightness, a key figure of merit of RF photocathode guns, can be improved by increasing the cathode surface field which suppresses emittance growth from space charge. The surface field in normal-conducting structures is mainly limited by RF breakdown and it has been experimentally discovered that RF breakdown rate exponentially depends on RF pulse length. A highly over-coupled 1.5-cell X-band photocathode gun has been developed to be powered by 9 ns RF pulses with 3 ns rising time, 3 ns flat-top, and 3 ns falling time generated by an X-band metallic power extractor. In the recent experiment at Argonne Wakefield Accelerator facility, cathode surface field up to ~350 MV/m with a low breakdown rate has been obtained under ~250 MW input power. Strong beam loading from dark current was observed during RF conditioning and quickly recovered to a negligible level after the gun reached the maximum gradient. Detailed high-power test results and data analysis will be reported in this manuscript.

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reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2021-THPAB331

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paper received ※ 25 May 2021 paper accepted ※ 14 July 2021 issue date ※ 23 August 2021

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THPAB344

Magneto-Optical Trap Cathode for High Brightness Applications

4466

V.S. Yu, C.E. Hansel, G.E. Lawler, M. Mills, J.B. Rosenzweig
UCLA, Los Angeles, California, USA
J.I. Mann
PBPL, Los Angeles, USA

Funding: This work was performed with support of the US Department of Energy under Contract No. DE-SC0020409 and the National Science Foundation under Grant No. PHY-1549132
Electron bunches extracted from magneto-optical traps (MOTs) via femtosecond photo-ionization and electrostatic acceleration can have significantly lower transverse emittance than emissions from traditional metal cathodes. Such MOT cathodes, however, have two drawbacks: the need for multiple trapping lasers and the limit to ~MV/m fields. Designs exist for MOTs which only require one trapping laser. Our RF simulations in High-Frequency Structure Simulator (HFSS) indicate that the cone MOT is the only one compatible with high gradient RF cavities. We present the combination of the two, an RF cavity with a cone-MOT as part of its geometry. It only requires one trapping laser and can use much higher fields. The geometry of the chamber is compatible with a wide range of MOT species, which allows the search for one which is compatible with copper cavities.

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reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2021-THPAB344

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paper received ※ 19 May 2021 paper accepted ※ 29 July 2021 issue date ※ 12 August 2021

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FRXA04

Commissioning results of LCLS-II MHz repetition rate electron source

F. Zhou, C. Adolphsen, A.L. Benwell, G.W. Brown, D. Dowell, M.P. Dunning, S. Gilevich, K. Grouev, B.T. Jacobson, X. Liu, A. Miahnahri, J.F. Schmerge, T. Vecchione
SLAC, Menlo Park, California, USA
G. Huang, F. Sannibale
LBNL, Berkeley, California, USA

Funding: work supported by DOE under grant No. DE-AC02-76SF00515
A 4 GeV 1.3 GHz superconducting linac is being constructed at SLAC as part of the X-ray free electron laser project (LCLS-II). The first 3-meter of the electron source that includes a normal conducting 185.7MHz CW RF gun, 2-cell 1.3 GHz CW RF buncher, and a loadlock system for cathode changes was designed and built by LBNL based on their experience with similar one for advanced photo-injector experiment program. The electron beam is designed to operate at a high repetition rate, up to 1 MHz. Since summer of 2018 we started LCLS-II injector source commissioning immediately after the major installation completion. This paper presents major commissioning results including achievements of ultra-high vacuum, RF processing to CW nominal power, dark current characterization and mitigation, and high-brightness electron beam measurements.

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