NAPAC2019 - Table of Session: MOPLH (Monday Poster Session-Lake Huron)

Paper	Title	Page
MOPLH02	Study of Photocathode Surface Damage due to Ion Back-Bombardment in High Current DC Gun	174
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	J.P. Biswas Stony Brook University, Stony Brook, USA O.H. Rahman, E. Wang BNL, Upton, New York, USA
	Funding: This work was supported by Brookhaven Science Associates, LLC under Contract No. DE-SC0012704, with the U.S. DOE In high current DC gun, GaAs photocathode lifetime is limited by the ion back-bombardment. While gun operation ions are generated and accelerate back towards the cathode thus remove the activation layer’s material Cesium from the photocathode surface. We have developed an object-oriented code to simulate the ion generation due to dynamic gas pressure and ion trace in the electromagnetic field. The pressure profile varies from cathode position towards the transfer line behind the anode, which signifies the importance of dynamic simulation for ion back-bombardment study. In our surface damage study, we traced the energy and position of the ions on the photocathode surface and performed the Stopping and Range of Ions in Matter(SRIM) simulation to count the number of Cesium atoms removed from the surface due to single bunch impact. Cesium atom removal is directly related to the photocathode Quantum Efficiency(QE) decay. Our new dynamic simulation code can be used in any DC gun to study ion back-bombardment. We have used this new code to better understand the ion generation in prototype BNL 350 KV DC gun, and we have also estimated the normalized QE decay due to ion back-bombardment.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-NAPAC2019-MOPLH02
About •	paper received ※ 27 August 2019 paper accepted ※ 03 September 2019 issue date ※ 08 October 2019
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MOPLH04	Design for HyRES Cathode Nanotip Electron Source	177
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	R.M. Hessami, A.F. Amhaz, P. Musumeci UCLA, Los Angeles, USA
	A new ultrafast electron diffraction (UED) instrument is being developed by UCLA-Colorado University collaboration for the STROBE NSF Center with the goal of using electron and EUV photon beams to reveal the structural dynamics of materials in non-equilibrium states at fundamental atomic and temporal scales. This paper describes the design of the electron beamline of this instrument. In order to minimize the initial emittance, a nanotip photocathode, 25 nm in radius, will be used. This requires a redesign of the cathode and anode components of the electron gun to allow for the tip to be properly aligned. Solenoidal lenses are used to focus the beam transversely to a sub-micron spot at the sample and a radiofrequency (RF) cavity, driven by a continuous wave S-band RF source, longitudinally compresses the beam to below 100 fs, required for atomic resolution.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-NAPAC2019-MOPLH04
About •	paper received ※ 27 August 2019 paper accepted ※ 05 September 2019 issue date ※ 08 October 2019
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MOPLH06	Study of the Mean Transverse Energy and the Emission Mechanism of (N)UNCD Photocathodes	181
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	G. Chen IIT, Chicago, Illinois, USA G. Adhikari, W.A. Schroeder UIC, Chicago, Illinois, USA S.P. Antipov, E. Gomez Euclid TechLabs, LLC, Solon, Ohio, USA S.V. Baryshev, T. Nikhar Michigan State University, East Lansing, Michigan, USA L.K. Spentzouris Illinois Institute of Technology, Chicago, Illinois, USA
	Funding: This project is supported by NSF grant No. NSF-1739150, NSF-1535676, and NSF grant No. PHYS-1535279. Nitrogen incorporated ultrananocrystalline diamond ((N)UNCD) is promising for photocathode applications due to its high quantum efficiency (QE). The mean transverse energy (MTE) which, along with QE, defines the brightness of the emitted electron beam must be thoroughly characterized and understood for (N)UNCD. Our previous work* further corroborated the important role of graphitic grain boundaries (GB’s). UNCD consists of diamond (sp3-hybrized) grains and graphitic (sp2-hybrized) GB’s: GB’s are behind the high emissivity of (N)UNCD and therefore play a crucial role in defining and controlling the MTE. In this work, the MTE of two different (N)UNCD samples having different ratios of sp3/sp2 were measured versus the primary photon energies. As a reference, MTE of highly oriented pyrolytic graphite (HOPG, canonical sp2-hybrized graphite) was also measured. * G. Chen et al., Appl. Phys. Lett. 114, 093103 (2019).
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-NAPAC2019-MOPLH06
About •	paper received ※ 27 August 2019 paper accepted ※ 12 September 2019 issue date ※ 08 October 2019
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MOPLH08	Tests of Cs-Free Operation of the SNS RF H⁻ Ion Sources	184
	B. Han, S.M. Cousineau, S.N. Murray, T.R. Pennisi, M.P. Stockli, R.F. Welton ORNL, Oak Ridge, Tennessee, USA T.M. Sarmento, O.A. Tarvainen STFC/RAL/ISIS, Chilton, Didcot, Oxon, United Kingdom C. Stinson ORNL RAD, Oak Ridge, Tennessee, USA
	Funding: This work was performed at Oak Ridge National Laboratory, which is managed by UT-Battelle, LLC, under contract number DE-AC05-00OR22725 for the United States Department of Energy. Tests were performed at SNS in collaboration with visiting colleagues from ISIS, UK to evaluate the uncesiated beam performance of the SNS RF H⁻ ion sources. Two spare experimental sources, one with internal antenna and one with external antenna were used for the tests. The beam currents achieved with Cs-free operations accounted for about 1/3 to 1/2 of the beam currents produced with cesiated operations. ~17 mA uncesiated H⁻ current was demonstrated within the tested RF power range up to 65 kW with the internal antenna source and ~15 mA with up to 40 kW RF with the external antenna source. In Cs-free operations, the power supply for the electron dumping electrode was loaded down below its set voltage but was not too drastic to tamper the operation.
	Poster MOPLH08 [0.949 MB]
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-NAPAC2019-MOPLH08
About •	paper received ※ 27 August 2019 paper accepted ※ 02 September 2019 issue date ※ 08 October 2019
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MOPLH09	Photoluminescence Studies of Alkali-Antimonide Photocathodes	188
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	P. Saha, O. Chubenko, S.S. Karkare Arizona State University, Tempe, USA H.A. Padmore LBNL, Berkeley, California, USA
	Alkali-antimonide photocathodes have a very high quantum efficiency and a low intrinsic emittance, making them excellent electron sources for Energy Recovery Linacs, X-ray Free Electron Lasers, Electron Cooling, and Ultrafast Electron Diffraction applications. Despite numerous studies of their photoemission spectra, there has been nearly no conclusive experimental investigation of their basic electronic and optical properties (e.g. band gap, electron affinity, optical constants, etc.), which determine the nature of photoemission. Therefore, the systematic study and deep understanding of fundamental characteristics of alkali-antimonide photocathodes are required in order to develop next-generation electron sources with improved crystal and electronic structures to fit specific application. Here we report on the development of an experimental setup to measure photoluminescence (PL) spectra from alkali-antimonide photocathodes, enabling estimation of a material band gap and defect state energies, and provide preliminary results for Cs3Sb films.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-NAPAC2019-MOPLH09
About •	paper received ※ 27 August 2019 paper accepted ※ 04 September 2019 issue date ※ 08 October 2019
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MOPLH10	Field-Emission Electron Source Embedded in a Field-Enhanced Conduction-Cooled Superconducting RF Cavity	192
	D. Mihalcea, V. Korampally, A. McKeown, O. Mohsen, P. Piot, I. Salehinia Northern Illinois University, DeKalb, Illinois, USA R. Dhuley, M.G. Geelhoed, P. Piot, J.C.T. Thangaraj Fermilab, Batavia, Illinois, USA
	We present simulations and experimental progress toward the development of a high-current electron source with the potential to deliver high charge electron bunches at GHz-level repetition rates. To achieve these goals electrons are generated through field-emission and the cathode is immersed in a conduction-cooled superconducting 650-MHz RF cavity. The field-emitters consist of microscopic silicon pyramids and have a typical enhancement factor of about 500. To trigger field-emission, the peak field inside the RF cavity of about 6 MV/m is further enhanced by placing the field-emitters on the top of a superconducting Nb rod inserted in the RF cavity. So far, we cannot control the duration of the electron bunches which is of the order of RF period. Also, the present cryo-cooler power of about 2 W limits the beam current to microamp level.
	Poster MOPLH10 [1.063 MB]
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-NAPAC2019-MOPLH10
About •	paper received ※ 27 August 2019 paper accepted ※ 05 September 2019 issue date ※ 08 October 2019
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MOPLH11	Nanostructured Photocathodes for Spin-Polarized Electron Beams	196
	E.J. Montgomery, C. Jing, S. Poddar Euclid Beamlabs LLC, Bolingbrook, USA A. Afanasev GWU, Washington, USA R. Kumar, G.J. Salamo University of Arkansas, Fayetteville, Arkansas, USA S. Zhang JLab, Newport News, Virginia, USA
	Funding: Work supported by US DOE Office of Science, Office of Nuclear Physics, SBIR grant DESC0019559. CNM work supported by US DOE Office of Science, Basic Energy Sciences, contract DE-AC02-06CH11357. We present progress on incorporation of nanopillar arrays into spin-polarized gallium arsenide photocathodes in pursuit of record high tolerance to ion back-bombardment. Our goal is to exceed the 400 Coulomb record for a high polarization milliampere-class electron source set at Jefferson Laboratory in 2017, while maintaining high quantum efficiency (QE) and spin polarization with a superlattice. Because the Mie effect is resonant, uniformity and careful control over nanostructure geometry is key. We report excellent uniformity and straight sidewall geometry with improved optical absorption using a painstakingly optimized inductively coupled plasma reactive ion etch. We also report the application of Kerker theory to spin-polarized photocathode nanopillar arrays, setting new requirements on nanostructure dimensions to avoid spoiling spin polarization. Finally, we also report initial steps toward re-establishing U.S. production of strained superlattice photocathodes towards integration with nanopillar arrays.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-NAPAC2019-MOPLH11
About •	paper received ※ 03 September 2019 paper accepted ※ 12 September 2019 issue date ※ 08 October 2019
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MOPLH13	STARRE Lab: The Sub-THz Accelerator Research Laboratory	199
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	J.F. Picard, S.C. Schaub, R.J. Temkin MIT/PSFC, Cambridge, Massachusetts, USA
	Funding: Department of Energy, Office of HEP, DE- SC0015566; Office of Fusion Energy Sciences, DE-FC02-93ER54186; National Institutes of Health, NIBIB, EB004866 and EB001965; This work presents the development of the STARRE Lab, a facility at MIT for testing breakdown in high gradient accelerator structures at 110 GHz. The system utilizes a Laser-Driven Semiconductor Switch (LDSS) to modulate the output of a megawatt gyrotron, which generates 3 μs pulses at up to 6 Hz. The LDSS employs silicon (Si) and gallium arsenide (GaAs) wafers to produce nanosecond-scale pulses at the megawatt level from the gyrotron output. Photoconductivity is induced in the wafers using a 532 nm Nd:YAG laser, which produces 6 ns, 230 mJ pulses. A single Si wafer produces 110 GHz RF pulses with 9 ns width, while under the same conditions, a single GaAs wafer produces 24 ns 110 GHz RF pulses. In dual-wafer operation, which uses two active wafers, pulses of variable length down to 3 ns duration can be created at power levels greater than 300 kW. The switch has been successfully tested at incident 110 GHz RF power levels up to 720 kW.* The facility has been used to successfully test an advanced 110 GHz accelerator structure built by SLAC to gradients in excess of 220 MV/m. *J.F. Picard et al., Appl. Phys. Lett. 114, 164102 (2019); doi: https://doi.org/10.1063/1.5093639
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-NAPAC2019-MOPLH13
About •	paper received ※ 24 August 2019 paper accepted ※ 05 September 2019 issue date ※ 08 October 2019
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MOPLH14	Ultrafast Nonlinear Photoemission from Alkali Antimonide Photocathodes	203
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	W.H. Li, M.B. Andorf, I.V. Bazarov, L. Cultrera, C.J.R. Duncan, A. Galdi, J.M. Maxson, C.A. Pennington Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
	Funding: This work was supported by the U.S. National Science Foundation under Award No. PHY-1549132, the Center for Bright Beams. Alkali antimonides photocathodes are a popular choice of electron source for high average brightness beams, due to their high quantum efficiency (QE) and low mean transverse energy (MTE). This paper describes the first measurements of their nonlinear photoemission properties under sub-ps laser illumination. These measurements include wavelength-resolved power dependence, pulse length dependence, and temporal response. The transition between linear and nonlinear photoemission is observed through the wavelength-resolved scan, and implications of nonlinear photoemission are discussed.
	Poster MOPLH14 [0.543 MB]
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-NAPAC2019-MOPLH14
About •	paper received ※ 27 August 2019 paper accepted ※ 31 August 2019 issue date ※ 08 October 2019
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MOPLH16	Femtosecond Laser Microfabrication for Advanced Accelerator Applications	207
	S.P. Antipov, E. Dosov, E. Gomez, S.V. Kuzikov Euclid TechLabs, LLC, Solon, Ohio, USA A.A. Vikharev IAP/RAS, Nizhny Novgorod, Russia
	Funding: DOE SBIR Femtosecond laser microfabrication allows for precise dimension control and reduced thermal stress of the machined materials. It can be applied to a wide range of materials from copper to diamond. Combined with secondary operations like polishing laser microfabrication can be utilized in various state of the art components required for AAC community. In this paper we will review several applications of laser microfabrication for Advanced Accelerator research and development. These will include wakefield structures (corrugated metal and dielectric loaded), plasma capillaries, x-ray refractive optics, high power laser optical components: mirrors, phase plates.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-NAPAC2019-MOPLH16
About •	paper received ※ 28 August 2019 paper accepted ※ 31 August 2019 issue date ※ 08 October 2019
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MOPLH17	Enhanced Robustness of GaAs-Based Photocathodes Activation by Cs, Sb, and O2	210
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	J. Bae, L. Cultrera, A. Galdi, F. Ikponmwen Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA I.V. Bazarov, J.M. Maxson Cornell University, Ithaca, New York, USA
	Funding: This work is funded by Department of Energy: DE-SC0016203. Operational lifetime of GaAs photocathodes is the primary limit for applications as high current spin polarized electron sources in future nuclear physics facilities, such as Electron Ion Collider. Recently, ultrathin Cs₂Te on GaAs has shown a successful negative electron affinity (NEA) activation with an improved lifetime by a factor of 5 . In this work, we report activation of GaAs with Cs, Sb and oxygen. Four different methods of introducing oxygen during the growth was investigated. Cs-Sb-O activated GaAs has shown up to a factor of 40 and 13 improvement in charge extraction lifetime and dark lifetime, respectively. Bae, et al. (2018). Rugged spin-polarized electron sources based on negative electron affinity GaAs photocathode with robust Cs₂Te coating. Applied Physics Letters, 112(15), 154101.
	Poster MOPLH17 [0.926 MB]
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-NAPAC2019-MOPLH17
About •	paper received ※ 28 August 2019 paper accepted ※ 01 September 2019 issue date ※ 08 October 2019
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MOPLH19	Beam Dynamics Simulations for a Conduction-Cooled Superconducting RF Electron Source	213
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	O. Mohsen, V. Korampally, A. McKeown, D. Mihalcea, P. Piot, I. Salehinia Northern Illinois University, DeKalb, Illinois, USA R. Dhuley, M.G. Geelhoed, J.C.T. Thangaraj Fermilab, Batavia, Illinois, USA
	Funding: Work supported by DOE awards DE-SC0018367 with NIU and DE-AC02-07CH11359 The development of robust and portable high-average power electron sources is key to many societal applications. An approach toward such sources is the use of cryogen-free superconducting radiofrequency cavities. This paper presents beam-dynamics simulations for a proof-of-principle experiment on a cryogen-free SRF electron source being prototyped at Fermilab. The proposed design implement a geometry that enhances the electric field at the cathode surface to simultaneously extract and accelerate electrons. In this paper, we explore the beam dynamics considering both the case of field and photoemission mechanism.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-NAPAC2019-MOPLH19
About •	paper received ※ 02 September 2019 paper accepted ※ 05 September 2019 issue date ※ 08 October 2019
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MOPLH20	Experiments with Metamaterial-Based Metallic Accelerating Structures
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	X. Lu SLAC, Menlo Park, California, USA M.E. Conde, D.S. Doran, G. Ha, J.G. Power, J.H. Shao, E.E. Wisniewski ANL, Lemont, Illinois, USA C.-J. Jing Euclid TechLabs, LLC, Solon, Ohio, USA X. Lu, I. Mastovsky, J.F. Picard, M.A. Shapiro, R.J. Temkin MIT/PSFC, Cambridge, Massachusetts, USA M.M. Peng AAI/ANL, Lemont, Illinois, USA J. Seok UNIST, Ulsan, Republic of Korea
	Funding: U.S. Department of Energy, Office of Science, Office of High Energy Physics under Award No. DE-SC0015566 at MIT and No. DE-AC02-06CH11357 at ANL We present experimental studies of metamaterial (MTM) structures for wakefield acceleration. The MTM structure is an all-metal periodic structure with its period much smaller than the wavelength at X-band. The fundamental TM mode has a negative group velocity, so an electron beam traveling through the structure radiates by reversed Cherenkov radiation. Two experiments have been completed at the Argonne Wakefield Accelerator (AWA), namely the Stage-I and Stage-II experiments. Differences between the two experiments include: (1) Structure length (Stage-I 8 cm, Stage-II 20 cm); (2) Bunch number used to excite the structure (Stage-I up to 2 bunches, Stage-II up to 8 bunches). In the Stage-I experiment, two bunches with a total charge of 85 nC generated 80 MW of RF power in a 2 ns long pulse. In the Stage-II experiment, the highest peak power reached 380 MW in a 10 ns long pulse from a train of 8 bunches with a total charge of 224 nC. Acceleration of a witness bunch has not been demonstrated yet, but the extracted power can be transferred to a separate accelerator for two-beam acceleration or directly applied to a trailing witness bunch in the same structure for collinear acceleration.
	Slides MOZBB2 [8.172 MB]
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-NAPAC2019-MOZBB2
About •	paper received ※ 27 August 2019 paper accepted ※ 04 September 2019 issue date ※ 08 October 2019
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MOPLH21	Measuring the Mean Transverse Energy of Pump-Probe Photoemitted Electrons
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	C.M. Pierce, I.V. Bazarov, L. Cultrera, J.M. Maxson Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
	Funding: This work was supported by the U.S. National Science Foundation under Award PHY-1549132, the Center for Bright Beams. Low effective mass semiconductor photocathodes have historically failed to exhibit the sub-thermal mean transverse energies (MTEs) expected of them based on their band structure. However, conservation of transverse momentum across the vacuum interface, and therefore a low MTE in these materials, has been observed in time resolved ARPES. To help bridge this gap, we measured the MTE of the pump probe photoemitted electrons seen in the ARPES experiment using methods typical of accelerator physics. We compare the results of these measurements with those of both communities and discuss them in the context of photoemission physics. Kanasaki, J., Tanimura, H., & Tanimura, K. (2014). Imaging Energy-, Momentum-, and Time-Resolved Distributions of Photoinjected Hot Electrons in GaAs. Physical Review Letters, 113(23), 237401.
	Slides MOZBB6 [7.348 MB]
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-NAPAC2019-MOZBB6
About •	paper received ※ 28 August 2019 paper accepted ※ 31 August 2019 issue date ※ 08 October 2019
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MOPLH22	Focusing Studies of an Electron Beam in Diamond Field Emitter Array Cathodes	217
	R.L. Fleming, H.L. Andrews, D. Gorelovpresenter, C.-K. Huang, D. Kim, J.W. Lewellen, K.E. Nichols, V.N. Pavlenko, E.I. Simakov LANL, Los Alamos, New Mexico, USA
	Funding: Los Alamos National Laboratory LDRD Program We present the simulations and test results for focusing studies performed on diamond field emitter array cathodes. This design utilized a simple variable-focus solenoidal lens in conjunction with a scanning wire technique in order to measure the beam spot size. The spot size was measured by scanning a thin copper wire across the beam in 1 µm increments, with voltage being measured and averaged at each location in order to map the location and intensity of the beam. Scans were taken at different distances away from the magnetic center of the lens, and show good agreement with our simulations of the beam. Ultimately this has allowed us to focus the beam to a spot size of 5.72 µm with an average current of 15.78 µA.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-NAPAC2019-MOPLH22
About •	paper received ※ 27 August 2019 paper accepted ※ 05 September 2019 issue date ※ 08 October 2019
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MOPLH23	An Analysis of Potential Compact Positron Beam Source	220
	R.M. Hessami UCLA, Los Angeles, USA S.J. Gessner CERN, Geneva, Switzerland
	For positron studies in plasma wakefield accelerators such as AWAKE, the development of new, cheaper, and compact positron beam sources is necessary. Using an electrostatic trap with parameters similar to other experiments, this paper explores converting that trapped positron plasma into a usable beam. Bunching is initially accomplished by an electrostatic buncher and the beam is accelerated to 148 keV by pulsed electrostatic accelerators. This is necessary for injection into the beta-matched rf cavities operating at 600 MHz, which bring the positron beam to a transverse emittance of 1.3 pi mrad mm, a longitudinal emittance of 93.3 pi keV mm, stdz of 1.85 mm and an energy of 22 MeV. The beamline used here is far simpler and less expensive than those at many facilities, such as SLAC, allowing for a cheap source of positron beams, potentially opening up positron beam studies to many facilities that could not previously afford such a source.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-NAPAC2019-MOPLH23
About •	paper received ※ 28 August 2019 paper accepted ※ 03 September 2019 issue date ※ 08 October 2019
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MOPLH24	Towards the Optimization Of Photocathode Properties Via Surface Science Techniques: A Study On Cs3Sb Thin Film Growth	224
	A. Galdi, J. Balajka, W.J.I. DeBenedetti, M. Hines Cornell University, Ithaca, New York, USA I.V. Bazarov, L. Cultrera, F. Ikponmwen, J.M. Maxsonpresenter, S.A. McBride Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
	Funding: National Science Foundation Grant No. PHY-1549132 A better understanding of the properties of photocathode materials can be achieved by integrating advanced growth and surface science techniques in their synthesis and analysis. This is a main research theme of the Center for Bright Beams, whose goal is increasing the brightness of linear electron accelerators. Alkali antimonides are efficient photocathode materials and have very low intrinsic emittance at cryogenic temperatures.* A limiting factors is the surface roughness and chemical inhomogeneity of the films.** We studied the influence of growth parameters on the morphology and composition of Cs3Sb thin films. The films are codeposited using pure element sources and transferred via UHV suitcase to a STM/XPS analysis chamber, to study in particular the influence of substrate temperature and material. This platform can be expanded to more analysis and growth systems thanks to a specially designed sample holder and suitcase. An example is a new cryogenic instrument for intrinsic emittance measurements. * L. Cultrera et al., Phys. Rev. ST ’ Acc. Beams 18 (2015) 113401 ** G. Gevorkian et al., Phys. Rev. Accel. Beams, 21 (2018) 093401
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-NAPAC2019-MOPLH24
About •	paper received ※ 28 August 2019 paper accepted ※ 30 August 2019 issue date ※ 08 October 2019
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MOPLH25	Characterization of Femtosecond-Laser-Induced Electron Emission from Diamond Nano-Tips	228
	V.N. Pavlenko, H.L. Andrewspresenter, R.L. Fleming, D. Gorelov, D. Kim, E.I. Simakov LANL, Los Alamos, New Mexico, USA D.S. Black, K.J. Leedle Stanford University, Stanford, California, USA
	Funding: LANL Laboratory Directed Research and Development (LDRD). Nanocrystalline diamond is a promising material for electron emission applications, as it combines robustness of diamond and ability to easily conform to a pre-defined shape, even at nano-scale. However, its electron emission properties are yet to be fully understood. Recently, we started to investigate femtosecond-laser-induced strong-field photoemission from nanocrystalline diamond field emitters with very sharp (~10 nm apex) tips. Initial results show that the mechanism of electron emission at ~10¹⁰ W/cm² light intensities in the near UV to near IR range is more complex than in metals. We present our latest experimental results obtained at Stanford University, while LANL’s strong-field photoemission test stand is being commissioned. We show that strong-field photoemission occurs not only at the nano-tip’s apex, but also on flat diamond surfaces (e.g., pyramid sides), that is why extra care needs to be taken to differentiate between emission spots on the chip. Qualitatively, we discuss the models that explain the observed dependences of electron emission on the optical power, polarization of the light, etc.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-NAPAC2019-MOPLH25
About •	paper received ※ 27 August 2019 paper accepted ※ 06 September 2019 issue date ※ 08 October 2019
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MOPLH26	Design of a Compact Wakefield Accelerator Based on a Corrugated Waveguide	232
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	A.E. Siy UW-Madison/PD, Madison, Wisconsin, USA G.J. Waldschmidt, A. Zholents ANL, Lemont, Illinois, USA
	A compact wakefield accelerator is being developed at the Argonne National Laboratory for a future multiuser x-ray free electron laser facility. A cylindrical structure with a 2 mm internal diameter and fine corrugations on the wall will be used to create Čerenkov radiation. A "drive" bunch producing radiation at 180 GHz will create accelerating gradients on the order of 100 MV/m for the "witness" bunch. The corrugated structure will be approximately half meter long with the entire accelerator spanning a few tens of meters. An ultra-compact transition region between each corrugated structure has been designed to accommodate an output coupler, a notch filter, an integrated offset monitor, bellows, pumping and water cooling ports. The output coupler will extract on the order of a kilowatt of power from the Čerenkov radiation unused by the witness bunch. The integrated offset monitor is a novel diagnostic which will measure the cumulative offset of the electron beam in the corrugated structure upstream of the monitor. The specific details of the rf design will be presented here.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-NAPAC2019-MOPLH26
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MOPLH27	Conceptual Design of a Compact 500 MeV Short-Pulse Two-Beam Acceleration Demonstrator at Argonne Wakefield Accelerator
MOZBB3	use link to access more material from this paper's primary paper code
	J.H. Shao, M.E. Conde, D.S. Doran, G. Ha, J.G. Power ANL, Lemont, Illinois, USA C.-J. Jing Euclid TechLabs, LLC, Solon, Ohio, USA
	Short-pulse two-beam acceleration (SP-TBA) is an advanced acceleration concept that can potentially meet the luminosity and cost requirements in future linear colliders and XFELs. In this concept, a high charge drive beam travelling through a structure excites short wakefield field (<20 ns) which is used to accelerate a low charge main beam in a parallel structure. A SP-TBA program is under development at the Argonne Wakefield Accelerator (AWA) facility where 300 MW generated power, 150 MeV/m acceleration gradient, and simplified staging have been successfully achieved. Based on the ongoing effort of novel dielectric disk structure, fast kicker/septum, and improved beam quality, a fully-functional demonstrator that can fit into AWA’s current bunker is proposed to demonstrate key technologies required by SP-TBA based machines: GW power generation, >250 MV/m acceleration, drive beam distribution/transportation, successive main beam acceleration, etc. The 70 MeV drive beam will be decelerated by four power extractors in two stages so as to boost the main beam energy from 15 MeV to 500 MeV by the four corresponding accelerators. The conceptual design will be presented in detail.
	Slides MOZBB3 [6.853 MB]
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Paper

Title

Page

MOPLH02

Study of Photocathode Surface Damage due to Ion Back-Bombardment in High Current DC Gun

174

SUPLE16

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J.P. Biswas
Stony Brook University, Stony Brook, USA
O.H. Rahman, E. Wang
BNL, Upton, New York, USA

Funding: This work was supported by Brookhaven Science Associates, LLC under Contract No. DE-SC0012704, with the U.S. DOE
In high current DC gun, GaAs photocathode lifetime is limited by the ion back-bombardment. While gun operation ions are generated and accelerate back towards the cathode thus remove the activation layer’s material Cesium from the photocathode surface. We have developed an object-oriented code to simulate the ion generation due to dynamic gas pressure and ion trace in the electromagnetic field. The pressure profile varies from cathode position towards the transfer line behind the anode, which signifies the importance of dynamic simulation for ion back-bombardment study. In our surface damage study, we traced the energy and position of the ions on the photocathode surface and performed the Stopping and Range of Ions in Matter(SRIM) simulation to count the number of Cesium atoms removed from the surface due to single bunch impact. Cesium atom removal is directly related to the photocathode Quantum Efficiency(QE) decay. Our new dynamic simulation code can be used in any DC gun to study ion back-bombardment. We have used this new code to better understand the ion generation in prototype BNL 350 KV DC gun, and we have also estimated the normalized QE decay due to ion back-bombardment.

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MOPLH04

Design for HyRES Cathode Nanotip Electron Source

177

SUPLE09

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R.M. Hessami, A.F. Amhaz, P. Musumeci
UCLA, Los Angeles, USA

A new ultrafast electron diffraction (UED) instrument is being developed by UCLA-Colorado University collaboration for the STROBE NSF Center with the goal of using electron and EUV photon beams to reveal the structural dynamics of materials in non-equilibrium states at fundamental atomic and temporal scales. This paper describes the design of the electron beamline of this instrument. In order to minimize the initial emittance, a nanotip photocathode, 25 nm in radius, will be used. This requires a redesign of the cathode and anode components of the electron gun to allow for the tip to be properly aligned. Solenoidal lenses are used to focus the beam transversely to a sub-micron spot at the sample and a radiofrequency (RF) cavity, driven by a continuous wave S-band RF source, longitudinally compresses the beam to below 100 fs, required for atomic resolution.

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MOPLH06

Study of the Mean Transverse Energy and the Emission Mechanism of (N)UNCD Photocathodes

181

SUPLE12

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G. Chen
IIT, Chicago, Illinois, USA
G. Adhikari, W.A. Schroeder
UIC, Chicago, Illinois, USA
S.P. Antipov, E. Gomez
Euclid TechLabs, LLC, Solon, Ohio, USA
S.V. Baryshev, T. Nikhar
Michigan State University, East Lansing, Michigan, USA
L.K. Spentzouris
Illinois Institute of Technology, Chicago, Illinois, USA

Funding: This project is supported by NSF grant No. NSF-1739150, NSF-1535676, and NSF grant No. PHYS-1535279.
Nitrogen incorporated ultrananocrystalline diamond ((N)UNCD) is promising for photocathode applications due to its high quantum efficiency (QE). The mean transverse energy (MTE) which, along with QE, defines the brightness of the emitted electron beam must be thoroughly characterized and understood for (N)UNCD. Our previous work* further corroborated the important role of graphitic grain boundaries (GB’s). UNCD consists of diamond (sp3-hybrized) grains and graphitic (sp2-hybrized) GB’s: GB’s are behind the high emissivity of (N)UNCD and therefore play a crucial role in defining and controlling the MTE. In this work, the MTE of two different (N)UNCD samples having different ratios of sp3/sp2 were measured versus the primary photon energies. As a reference, MTE of highly oriented pyrolytic graphite (HOPG, canonical sp2-hybrized graphite) was also measured.
* G. Chen et al., Appl. Phys. Lett. 114, 093103 (2019).

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MOPLH08

Tests of Cs-Free Operation of the SNS RF H⁻ Ion Sources

184

B. Han, S.M. Cousineau, S.N. Murray, T.R. Pennisi, M.P. Stockli, R.F. Welton
ORNL, Oak Ridge, Tennessee, USA
T.M. Sarmento, O.A. Tarvainen
STFC/RAL/ISIS, Chilton, Didcot, Oxon, United Kingdom
C. Stinson
ORNL RAD, Oak Ridge, Tennessee, USA

Funding: This work was performed at Oak Ridge National Laboratory, which is managed by UT-Battelle, LLC, under contract number DE-AC05-00OR22725 for the United States Department of Energy.
Tests were performed at SNS in collaboration with visiting colleagues from ISIS, UK to evaluate the uncesiated beam performance of the SNS RF H⁻ ion sources. Two spare experimental sources, one with internal antenna and one with external antenna were used for the tests. The beam currents achieved with Cs-free operations accounted for about 1/3 to 1/2 of the beam currents produced with cesiated operations. ~17 mA uncesiated H⁻ current was demonstrated within the tested RF power range up to 65 kW with the internal antenna source and ~15 mA with up to 40 kW RF with the external antenna source. In Cs-free operations, the power supply for the electron dumping electrode was loaded down below its set voltage but was not too drastic to tamper the operation.

Poster MOPLH08 [0.949 MB]

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paper received ※ 27 August 2019 paper accepted ※ 02 September 2019 issue date ※ 08 October 2019

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MOPLH09

Photoluminescence Studies of Alkali-Antimonide Photocathodes

188

SUPLE01

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P. Saha, O. Chubenko, S.S. Karkare
Arizona State University, Tempe, USA
H.A. Padmore
LBNL, Berkeley, California, USA

Alkali-antimonide photocathodes have a very high quantum efficiency and a low intrinsic emittance, making them excellent electron sources for Energy Recovery Linacs, X-ray Free Electron Lasers, Electron Cooling, and Ultrafast Electron Diffraction applications. Despite numerous studies of their photoemission spectra, there has been nearly no conclusive experimental investigation of their basic electronic and optical properties (e.g. band gap, electron affinity, optical constants, etc.), which determine the nature of photoemission. Therefore, the systematic study and deep understanding of fundamental characteristics of alkali-antimonide photocathodes are required in order to develop next-generation electron sources with improved crystal and electronic structures to fit specific application. Here we report on the development of an experimental setup to measure photoluminescence (PL) spectra from alkali-antimonide photocathodes, enabling estimation of a material band gap and defect state energies, and provide preliminary results for Cs3Sb films.

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paper received ※ 27 August 2019 paper accepted ※ 04 September 2019 issue date ※ 08 October 2019

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MOPLH10

Field-Emission Electron Source Embedded in a Field-Enhanced Conduction-Cooled Superconducting RF Cavity

192

D. Mihalcea, V. Korampally, A. McKeown, O. Mohsen, P. Piot, I. Salehinia
Northern Illinois University, DeKalb, Illinois, USA
R. Dhuley, M.G. Geelhoed, P. Piot, J.C.T. Thangaraj
Fermilab, Batavia, Illinois, USA

We present simulations and experimental progress toward the development of a high-current electron source with the potential to deliver high charge electron bunches at GHz-level repetition rates. To achieve these goals electrons are generated through field-emission and the cathode is immersed in a conduction-cooled superconducting 650-MHz RF cavity. The field-emitters consist of microscopic silicon pyramids and have a typical enhancement factor of about 500. To trigger field-emission, the peak field inside the RF cavity of about 6 MV/m is further enhanced by placing the field-emitters on the top of a superconducting Nb rod inserted in the RF cavity. So far, we cannot control the duration of the electron bunches which is of the order of RF period. Also, the present cryo-cooler power of about 2 W limits the beam current to microamp level.

Poster MOPLH10 [1.063 MB]

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paper received ※ 27 August 2019 paper accepted ※ 05 September 2019 issue date ※ 08 October 2019

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MOPLH11

Nanostructured Photocathodes for Spin-Polarized Electron Beams

196

E.J. Montgomery, C. Jing, S. Poddar
Euclid Beamlabs LLC, Bolingbrook, USA
A. Afanasev
GWU, Washington, USA
R. Kumar, G.J. Salamo
University of Arkansas, Fayetteville, Arkansas, USA
S. Zhang
JLab, Newport News, Virginia, USA

Funding: Work supported by US DOE Office of Science, Office of Nuclear Physics, SBIR grant DESC0019559. CNM work supported by US DOE Office of Science, Basic Energy Sciences, contract DE-AC02-06CH11357.
We present progress on incorporation of nanopillar arrays into spin-polarized gallium arsenide photocathodes in pursuit of record high tolerance to ion back-bombardment. Our goal is to exceed the 400 Coulomb record for a high polarization milliampere-class electron source set at Jefferson Laboratory in 2017, while maintaining high quantum efficiency (QE) and spin polarization with a superlattice. Because the Mie effect is resonant, uniformity and careful control over nanostructure geometry is key. We report excellent uniformity and straight sidewall geometry with improved optical absorption using a painstakingly optimized inductively coupled plasma reactive ion etch. We also report the application of Kerker theory to spin-polarized photocathode nanopillar arrays, setting new requirements on nanostructure dimensions to avoid spoiling spin polarization. Finally, we also report initial steps toward re-establishing U.S. production of strained superlattice photocathodes towards integration with nanopillar arrays.

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paper received ※ 03 September 2019 paper accepted ※ 12 September 2019 issue date ※ 08 October 2019

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MOPLH13

STARRE Lab: The Sub-THz Accelerator Research Laboratory

199

SUPLE13

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J.F. Picard, S.C. Schaub, R.J. Temkin
MIT/PSFC, Cambridge, Massachusetts, USA

Funding: Department of Energy, Office of HEP, DE- SC0015566; Office of Fusion Energy Sciences, DE-FC02-93ER54186; National Institutes of Health, NIBIB, EB004866 and EB001965;
This work presents the development of the STARRE Lab, a facility at MIT for testing breakdown in high gradient accelerator structures at 110 GHz. The system utilizes a Laser-Driven Semiconductor Switch (LDSS) to modulate the output of a megawatt gyrotron, which generates 3 μs pulses at up to 6 Hz. The LDSS employs silicon (Si) and gallium arsenide (GaAs) wafers to produce nanosecond-scale pulses at the megawatt level from the gyrotron output. Photoconductivity is induced in the wafers using a 532 nm Nd:YAG laser, which produces 6 ns, 230 mJ pulses. A single Si wafer produces 110 GHz RF pulses with 9 ns width, while under the same conditions, a single GaAs wafer produces 24 ns 110 GHz RF pulses. In dual-wafer operation, which uses two active wafers, pulses of variable length down to 3 ns duration can be created at power levels greater than 300 kW. The switch has been successfully tested at incident 110 GHz RF power levels up to 720 kW.* The facility has been used to successfully test an advanced 110 GHz accelerator structure built by SLAC to gradients in excess of 220 MV/m.
*J.F. Picard et al., Appl. Phys. Lett. 114, 164102 (2019); doi: https://doi.org/10.1063/1.5093639

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paper received ※ 24 August 2019 paper accepted ※ 05 September 2019 issue date ※ 08 October 2019

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MOPLH14

Ultrafast Nonlinear Photoemission from Alkali Antimonide Photocathodes

203

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W.H. Li, M.B. Andorf, I.V. Bazarov, L. Cultrera, C.J.R. Duncan, A. Galdi, J.M. Maxson, C.A. Pennington
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA

Funding: This work was supported by the U.S. National Science Foundation under Award No. PHY-1549132, the Center for Bright Beams.
Alkali antimonides photocathodes are a popular choice of electron source for high average brightness beams, due to their high quantum efficiency (QE) and low mean transverse energy (MTE). This paper describes the first measurements of their nonlinear photoemission properties under sub-ps laser illumination. These measurements include wavelength-resolved power dependence, pulse length dependence, and temporal response. The transition between linear and nonlinear photoemission is observed through the wavelength-resolved scan, and implications of nonlinear photoemission are discussed.

Poster MOPLH14 [0.543 MB]

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reference for this paper ※ https://doi.org/10.18429/JACoW-NAPAC2019-MOPLH14

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paper received ※ 27 August 2019 paper accepted ※ 31 August 2019 issue date ※ 08 October 2019

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MOPLH16

Femtosecond Laser Microfabrication for Advanced Accelerator Applications

207

S.P. Antipov, E. Dosov, E. Gomez, S.V. Kuzikov
Euclid TechLabs, LLC, Solon, Ohio, USA
A.A. Vikharev
IAP/RAS, Nizhny Novgorod, Russia

Funding: DOE SBIR
Femtosecond laser microfabrication allows for precise dimension control and reduced thermal stress of the machined materials. It can be applied to a wide range of materials from copper to diamond. Combined with secondary operations like polishing laser microfabrication can be utilized in various state of the art components required for AAC community. In this paper we will review several applications of laser microfabrication for Advanced Accelerator research and development. These will include wakefield structures (corrugated metal and dielectric loaded), plasma capillaries, x-ray refractive optics, high power laser optical components: mirrors, phase plates.

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paper received ※ 28 August 2019 paper accepted ※ 31 August 2019 issue date ※ 08 October 2019

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MOPLH17

Enhanced Robustness of GaAs-Based Photocathodes Activation by Cs, Sb, and O2

210

SUPLE02

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J. Bae, L. Cultrera, A. Galdi, F. Ikponmwen
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
I.V. Bazarov, J.M. Maxson
Cornell University, Ithaca, New York, USA

Funding: This work is funded by Department of Energy: DE-SC0016203.
Operational lifetime of GaAs photocathodes is the primary limit for applications as high current spin polarized electron sources in future nuclear physics facilities, such as Electron Ion Collider. Recently, ultrathin Cs₂Te on GaAs has shown a successful negative electron affinity (NEA) activation with an improved lifetime by a factor of 5 *. In this work, we report activation of GaAs with Cs, Sb and oxygen. Four different methods of introducing oxygen during the growth was investigated. Cs-Sb-O activated GaAs has shown up to a factor of 40 and 13 improvement in charge extraction lifetime and dark lifetime, respectively.
* Bae, et al. (2018). Rugged spin-polarized electron sources based on negative electron affinity GaAs photocathode with robust Cs₂Te coating. Applied Physics Letters, 112(15), 154101.

Poster MOPLH17 [0.926 MB]

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paper received ※ 28 August 2019 paper accepted ※ 01 September 2019 issue date ※ 08 October 2019

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MOPLH19

Beam Dynamics Simulations for a Conduction-Cooled Superconducting RF Electron Source

213

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O. Mohsen, V. Korampally, A. McKeown, D. Mihalcea, P. Piot, I. Salehinia
Northern Illinois University, DeKalb, Illinois, USA
R. Dhuley, M.G. Geelhoed, J.C.T. Thangaraj
Fermilab, Batavia, Illinois, USA

Funding: Work supported by DOE awards DE-SC0018367 with NIU and DE-AC02-07CH11359
The development of robust and portable high-average power electron sources is key to many societal applications. An approach toward such sources is the use of cryogen-free superconducting radiofrequency cavities. This paper presents beam-dynamics simulations for a proof-of-principle experiment on a cryogen-free SRF electron source being prototyped at Fermilab. The proposed design implement a geometry that enhances the electric field at the cathode surface to simultaneously extract and accelerate electrons. In this paper, we explore the beam dynamics considering both the case of field and photoemission mechanism.

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reference for this paper ※ https://doi.org/10.18429/JACoW-NAPAC2019-MOPLH19

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MOPLH20

Experiments with Metamaterial-Based Metallic Accelerating Structures

MOZBB2

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X. Lu
SLAC, Menlo Park, California, USA
M.E. Conde, D.S. Doran, G. Ha, J.G. Power, J.H. Shao, E.E. Wisniewski
ANL, Lemont, Illinois, USA
C.-J. Jing
Euclid TechLabs, LLC, Solon, Ohio, USA
X. Lu, I. Mastovsky, J.F. Picard, M.A. Shapiro, R.J. Temkin
MIT/PSFC, Cambridge, Massachusetts, USA
M.M. Peng
AAI/ANL, Lemont, Illinois, USA
J. Seok
UNIST, Ulsan, Republic of Korea

Funding: U.S. Department of Energy, Office of Science, Office of High Energy Physics under Award No. DE-SC0015566 at MIT and No. DE-AC02-06CH11357 at ANL
We present experimental studies of metamaterial (MTM) structures for wakefield acceleration. The MTM structure is an all-metal periodic structure with its period much smaller than the wavelength at X-band. The fundamental TM mode has a negative group velocity, so an electron beam traveling through the structure radiates by reversed Cherenkov radiation. Two experiments have been completed at the Argonne Wakefield Accelerator (AWA), namely the Stage-I and Stage-II experiments. Differences between the two experiments include: (1) Structure length (Stage-I 8 cm, Stage-II 20 cm); (2) Bunch number used to excite the structure (Stage-I up to 2 bunches, Stage-II up to 8 bunches). In the Stage-I experiment, two bunches with a total charge of 85 nC generated 80 MW of RF power in a 2 ns long pulse. In the Stage-II experiment, the highest peak power reached 380 MW in a 10 ns long pulse from a train of 8 bunches with a total charge of 224 nC. Acceleration of a witness bunch has not been demonstrated yet, but the extracted power can be transferred to a separate accelerator for two-beam acceleration or directly applied to a trailing witness bunch in the same structure for collinear acceleration.

Slides MOZBB2 [8.172 MB]

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paper received ※ 27 August 2019 paper accepted ※ 04 September 2019 issue date ※ 08 October 2019

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MOPLH21

Measuring the Mean Transverse Energy of Pump-Probe Photoemitted Electrons

MOZBB6

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SUPLE11

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C.M. Pierce, I.V. Bazarov, L. Cultrera, J.M. Maxson
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA

Funding: This work was supported by the U.S. National Science Foundation under Award PHY-1549132, the Center for Bright Beams.
Low effective mass semiconductor photocathodes have historically failed to exhibit the sub-thermal mean transverse energies (MTEs) expected of them based on their band structure. However, conservation of transverse momentum across the vacuum interface, and therefore a low MTE in these materials, has been observed in time resolved ARPES*. To help bridge this gap, we measured the MTE of the pump probe photoemitted electrons seen in the ARPES experiment using methods typical of accelerator physics. We compare the results of these measurements with those of both communities and discuss them in the context of photoemission physics.
* Kanasaki, J., Tanimura, H., & Tanimura, K. (2014). Imaging Energy-, Momentum-, and Time-Resolved Distributions of Photoinjected Hot Electrons in GaAs. Physical Review Letters, 113(23), 237401.

Slides MOZBB6 [7.348 MB]

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MOPLH22

Focusing Studies of an Electron Beam in Diamond Field Emitter Array Cathodes

217

R.L. Fleming, H.L. Andrews, D. Gorelovpresenter, C.-K. Huang, D. Kim, J.W. Lewellen, K.E. Nichols, V.N. Pavlenko, E.I. Simakov
LANL, Los Alamos, New Mexico, USA

Funding: Los Alamos National Laboratory LDRD Program
We present the simulations and test results for focusing studies performed on diamond field emitter array cathodes. This design utilized a simple variable-focus solenoidal lens in conjunction with a scanning wire technique in order to measure the beam spot size. The spot size was measured by scanning a thin copper wire across the beam in 1 µm increments, with voltage being measured and averaged at each location in order to map the location and intensity of the beam. Scans were taken at different distances away from the magnetic center of the lens, and show good agreement with our simulations of the beam. Ultimately this has allowed us to focus the beam to a spot size of 5.72 µm with an average current of 15.78 µA.

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paper received ※ 27 August 2019 paper accepted ※ 05 September 2019 issue date ※ 08 October 2019

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MOPLH23

An Analysis of Potential Compact Positron Beam Source

220

R.M. Hessami
UCLA, Los Angeles, USA
S.J. Gessner
CERN, Geneva, Switzerland

For positron studies in plasma wakefield accelerators such as AWAKE, the development of new, cheaper, and compact positron beam sources is necessary. Using an electrostatic trap with parameters similar to other experiments, this paper explores converting that trapped positron plasma into a usable beam. Bunching is initially accomplished by an electrostatic buncher and the beam is accelerated to 148 keV by pulsed electrostatic accelerators. This is necessary for injection into the beta-matched rf cavities operating at 600 MHz, which bring the positron beam to a transverse emittance of 1.3 pi mrad mm, a longitudinal emittance of 93.3 pi keV mm, stdz of 1.85 mm and an energy of 22 MeV. The beamline used here is far simpler and less expensive than those at many facilities, such as SLAC, allowing for a cheap source of positron beams, potentially opening up positron beam studies to many facilities that could not previously afford such a source.

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paper received ※ 28 August 2019 paper accepted ※ 03 September 2019 issue date ※ 08 October 2019

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MOPLH24

Towards the Optimization Of Photocathode Properties Via Surface Science Techniques: A Study On Cs3Sb Thin Film Growth

224

A. Galdi, J. Balajka, W.J.I. DeBenedetti, M. Hines
Cornell University, Ithaca, New York, USA
I.V. Bazarov, L. Cultrera, F. Ikponmwen, J.M. Maxsonpresenter, S.A. McBride
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA

Funding: National Science Foundation Grant No. PHY-1549132
A better understanding of the properties of photocathode materials can be achieved by integrating advanced growth and surface science techniques in their synthesis and analysis. This is a main research theme of the Center for Bright Beams, whose goal is increasing the brightness of linear electron accelerators. Alkali antimonides are efficient photocathode materials and have very low intrinsic emittance at cryogenic temperatures.* A limiting factors is the surface roughness and chemical inhomogeneity of the films.** We studied the influence of growth parameters on the morphology and composition of Cs3Sb thin films. The films are codeposited using pure element sources and transferred via UHV suitcase to a STM/XPS analysis chamber, to study in particular the influence of substrate temperature and material. This platform can be expanded to more analysis and growth systems thanks to a specially designed sample holder and suitcase. An example is a new cryogenic instrument for intrinsic emittance measurements.
* L. Cultrera et al., Phys. Rev. ST ’ Acc. Beams 18 (2015) 113401
** G. Gevorkian et al., Phys. Rev. Accel. Beams, 21 (2018) 093401

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-NAPAC2019-MOPLH24

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paper received ※ 28 August 2019 paper accepted ※ 30 August 2019 issue date ※ 08 October 2019

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MOPLH25

Characterization of Femtosecond-Laser-Induced Electron Emission from Diamond Nano-Tips

228

V.N. Pavlenko, H.L. Andrewspresenter, R.L. Fleming, D. Gorelov, D. Kim, E.I. Simakov
LANL, Los Alamos, New Mexico, USA
D.S. Black, K.J. Leedle
Stanford University, Stanford, California, USA

Funding: LANL Laboratory Directed Research and Development (LDRD).
Nanocrystalline diamond is a promising material for electron emission applications, as it combines robustness of diamond and ability to easily conform to a pre-defined shape, even at nano-scale. However, its electron emission properties are yet to be fully understood. Recently, we started to investigate femtosecond-laser-induced strong-field photoemission from nanocrystalline diamond field emitters with very sharp (~10 nm apex) tips. Initial results show that the mechanism of electron emission at ~10¹⁰ W/cm² light intensities in the near UV to near IR range is more complex than in metals. We present our latest experimental results obtained at Stanford University, while LANL’s strong-field photoemission test stand is being commissioned. We show that strong-field photoemission occurs not only at the nano-tip’s apex, but also on flat diamond surfaces (e.g., pyramid sides), that is why extra care needs to be taken to differentiate between emission spots on the chip. Qualitatively, we discuss the models that explain the observed dependences of electron emission on the optical power, polarization of the light, etc.

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reference for this paper ※ https://doi.org/10.18429/JACoW-NAPAC2019-MOPLH25

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paper received ※ 27 August 2019 paper accepted ※ 06 September 2019 issue date ※ 08 October 2019

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MOPLH26

Design of a Compact Wakefield Accelerator Based on a Corrugated Waveguide

232

SUPLE18

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A.E. Siy
UW-Madison/PD, Madison, Wisconsin, USA
G.J. Waldschmidt, A. Zholents
ANL, Lemont, Illinois, USA

A compact wakefield accelerator is being developed at the Argonne National Laboratory for a future multiuser x-ray free electron laser facility. A cylindrical structure with a 2 mm internal diameter and fine corrugations on the wall will be used to create Čerenkov radiation. A "drive" bunch producing radiation at 180 GHz will create accelerating gradients on the order of 100 MV/m for the "witness" bunch. The corrugated structure will be approximately half meter long with the entire accelerator spanning a few tens of meters. An ultra-compact transition region between each corrugated structure has been designed to accommodate an output coupler, a notch filter, an integrated offset monitor, bellows, pumping and water cooling ports. The output coupler will extract on the order of a kilowatt of power from the Čerenkov radiation unused by the witness bunch. The integrated offset monitor is a novel diagnostic which will measure the cumulative offset of the electron beam in the corrugated structure upstream of the monitor. The specific details of the rf design will be presented here.

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reference for this paper ※ https://doi.org/10.18429/JACoW-NAPAC2019-MOPLH26

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paper received ※ 27 August 2019 paper accepted ※ 12 September 2019 issue date ※ 08 October 2019

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MOPLH27

Conceptual Design of a Compact 500 MeV Short-Pulse Two-Beam Acceleration Demonstrator at Argonne Wakefield Accelerator

MOZBB3

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J.H. Shao, M.E. Conde, D.S. Doran, G. Ha, J.G. Power
ANL, Lemont, Illinois, USA
C.-J. Jing
Euclid TechLabs, LLC, Solon, Ohio, USA

Short-pulse two-beam acceleration (SP-TBA) is an advanced acceleration concept that can potentially meet the luminosity and cost requirements in future linear colliders and XFELs. In this concept, a high charge drive beam travelling through a structure excites short wakefield field (<20 ns) which is used to accelerate a low charge main beam in a parallel structure. A SP-TBA program is under development at the Argonne Wakefield Accelerator (AWA) facility where 300 MW generated power, 150 MeV/m acceleration gradient, and simplified staging have been successfully achieved. Based on the ongoing effort of novel dielectric disk structure, fast kicker/septum, and improved beam quality, a fully-functional demonstrator that can fit into AWA’s current bunker is proposed to demonstrate key technologies required by SP-TBA based machines: GW power generation, >250 MV/m acceleration, drive beam distribution/transportation, successive main beam acceleration, etc. The 70 MeV drive beam will be decelerated by four power extractors in two stages so as to boost the main beam energy from 15 MeV to 500 MeV by the four corresponding accelerators. The conceptual design will be presented in detail.

Slides MOZBB3 [6.853 MB]

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