MOPLH —  Monday Poster Session-Lake Huron   (02-Sep-19   16:30—18:00)
Paper Title Page
THz-Driven MeV Electron Bunch Compression Experiments at SLAC UED  
  • M.A.K. Othman, M.C. Hoffmann, M.E. Kozina, R.K. Li, E.A. Nanni, X. Shen, E.J. Snively, X.J. Wang, S.P. Weathersby, C. Yoneda
    SLAC, Menlo Park, California, USA
  Funding: This work was supported by Department of Energy contract DE-AC02-76SF00515.
Intricate spatiotemporal energy-matter interactions are better understood through time-resolved measurement of ultrafast behavior at atomic scales. Ultrafast electron diffraction (UED) play a fundamental role in accessing these details and has received growing attention thanks to innovation in RF sources and detector, as well as the rich dynamics revealed by UED. Here we experimentally demonstrate THz-driven MeV electron bunch compression. The setup allows for compressing elongated electron beam bunches down to tens of femtoseconds by imposing an energy chirp on the bunch using high field single-cycle THz pulses. We show a novel design of a dispersion-free parallel-plate tapered compressor structure that provides focusing of THz pulses, and fully characterized with electro-optical sampling. The experimental setup provides clear advantages compared to conventional RF compressors in terms of timing synchronization. The compression setup not only reduces bunch length, but also diminishes time-of-arrival jitter between the optical pulse and the electron bunch at the sample which offers great opportunities towards sub-50 fs time-resolved pump-probe experiments.
MOPLH02 Study of Photocathode Surface Damage due to Ion Back-Bombardment in High Current DC Gun -1
SUPLE16   use link to see paper's listing under its alternate paper code  
  • 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.
Quantitative Analysis of Bulk GaAs Photocathode Using X-Ray Photoelectron Spectroscopy  
SUPLE15   use link to see paper's listing under its alternate paper code  
  • J.P. Biswas
    Stony Brook University, Stony Brook, USA
  • J. Cen, M. Gaowei, X. Tong, 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
GaAs photocathodes have been used as a polarized electron source for decades. The lifetime of GaAs-based photocathode is limited by the ion back bombardment in the high current DC gun due to damage of activation layer. To study the cathode QE degradation mechanism, as the first step, we have carried out X-ray photoelectron spectroscopy (XPS) measurement on n-type bulk GaAs to investigate the high temperature treatment and activation process in GaAs photocathode. We observed Arsenic oxides were completely reduced to noise level while small amount of oxides of Gallium was still present after the 2 hours 580°C heat treatment. Following XPS shows both metallic Cesium peak, and oxides of Cesium such as Cs2O after activation. In this proceeding, we describe the details of the experiment and show the data analysis.
MOPLH04 Design for HyRES Cathode Nanotip Electron Source -1
SUPLE09   use link to see paper's listing under its alternate paper code  
  • 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.  
Extreme Beams: Plasma-Optical Metrology and Plasma Photocathodes  
SUPLE08   use link to see paper's listing under its alternate paper code  
  • A. Sutherland
    SLAC, Menlo Park, California, USA
  Plasmas are capable of handling electric fields up to the TV/m level. Plasma based acceleration exploits these fields to shrink down the accelerator module length from the meter to the mm-scale at comparable energy gains. In addition to the accelerator section, the plasma photocathode (a.k.a. Trojan Horse) process can generate ultrashort, high-current electron beams with normalized emittance from the micrometer to the nanometer scale. Beams with such extreme parameters, and connected self-fields, are notoriously difficult to diagnose due to damage thresholds and sensitivity of state-of-the-art diagnostics. However, plasma-based diagnostics that exploit intriguing beam-gas-plasma dynamics allow metrology of such bunches in a regime far beyond the typical damage thresholds. These three approaches regarding bunch generation, acceleration, and metrology forms a consistent set of technology for extreme beams and have seen a first realization at SLAC’s FACET facility. This talk gives and overview on status and prospect of this technology, including a series of experiments approved for SLAC’s upcoming FACET-II facility.  
MOPLH06 Study of the Mean Transverse Energy and the Emission Mechanism of (N)UNCD Photocathodes -1
SUPLE12   use link to see paper's listing under its alternate paper code  
  • 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).
Use of Tellurium (Te) During GaAs Activation for Higher Charge Lifetime  
  • O.H. Rahman, M. Gaowei, W. Liu, E. Wang
    BNL, Upton, New York, USA
  • J.P. Biswaspresenter
    Stony Brook University, Stony Brook, USA
  Charge lifetime of GaAs photocathodes during operation is limited due to the extreme sensitivity of the surface NEA layer. This NEA layer, generally consisting of Cs and O2/NF3 , gets damaged by ion back bombardment and vacuum poisoning. In this work, we deposited Tellurium, in combination with the usual Cs and O2 layer, to create a more robust surface layer. Initial experiments show an increment of charge lifetime, from bulk GaAs using 532 nm laser, by a factor of 5-6. We also compare activation results using only Cs and Te with activations using Cs-O2-Te.  
poster icon Poster MOPLH07 [0.894 MB]  
MOPLH08 Tests of Cs-Free Operation of the SNS RF H Ion Sources -1
  • 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 icon Poster MOPLH08 [0.947 MB]  
MOPLH09 Photoluminescence Studies of Alkali-Antimonide Photocathodes -1
SUPLE01   use link to see paper's listing under its alternate paper code  
  • 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.  
MOPLH10 Field-Emission Electron Source Embedded in a Field-Enhanced Conduction-Cooled Superconducting RF Cavity -1
  • 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 icon Poster MOPLH10 [1.087 MB]  
MOPLH11 Nanostructured Photocathodes for Spin-Polarized Electron Beams -1
  • 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.
First Results on Flat Beam Electron Gun for Klylac  
  • A.V. Smirnov, A.Y. Murokh, D.Y. Shchegolkov
    RadiaBeam Systems, Santa Monica, California, USA
  • R.B. Agustsson, R.D. Berry, D.I. Gavryushkin, K.J. Hoyt
    RadiaBeam, Los Angeles, California, USA
  • A.A. Zavadtsev
    Nano, Moscow, Russia
  Funding: Work supported by the U.S. Department of Energy (award No. DE-SC0015721)
Linac-based system for borehole logging under development exploits KlyLac approach [1] combining sheet beam klystron (SBK) and linac sharing the same electron beam, vacuum volume, and direct RF feed with positive feedback. The KlyLac system to be energized by a high perveance, high aspect ratio, flat electron beam. Here we report design features of the diode type electron gun as well as specifics of the test setup and first results of the gun test. Among the results is the 24.2 A beam current measured at 24 kV voltage that corresponds to 6.5 micro-perveance agrees very well with our predictions.
[1] A. Smirnov R. Agustsson, S. Boucher, M. Harrison, A. Murokh, A. Yu. Smirnov, E.A. Savin, A.A. Zavadtev. "Klylac Prototyping For Borehole Logging", IPAC 2018, Vancouver, Canada
MOPLH13 STARRE Lab: The Sub-THz Accelerator Research Laboratory -1
SUPLE13   use link to see paper's listing under its alternate paper code  
  • 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:
MOPLH14 Ultrafast Nonlinear Photoemission from Alkali Antimonide Photocathodes -1
SUPLE10   use link to see paper's listing under its alternate paper code  
  • 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 icon Poster MOPLH14 [0.529 MB]  
Simulations of a Hybrid Plasma Photocathode Dielectric Wakefield Accelerator  
  • N.M. Cook, D.L. Bruhwiler
    RadiaSoft LLC, Boulder, Colorado, USA
  • G. Andonian, F.H. O’Shea
    RadiaBeam, Santa Monica, California, USA
  • G. Andonian
    UCLA, Los Angeles, California, USA
  • G. Ha
    ANL, Lemont, Illinois, USA
  • F.H. O’Shea
    Elettra-Sincrotrone Trieste S.C.p.A., Basovizza, Italy
  Funding: This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of High Energy Physics under Award Number DE-SC0017690.
Transmission electron microscopy (TEM) is a primary tool for materials characterization, with applications in chemistry, biology, and energy. Substantive improvements in beam brightness and temporal resolution are needed for the next generation of ultrafast electron microscopy (UEM) techniques. To achieve these goals, a novel, low cost, hybrid plasma photocathode scheme has been proposed for generating ultrashort, high brightness beams in the MeV energy range. This scheme, dubbed "Capillary Trojan Horse" (CTH), combines photoionization injection from the Trojan Horse strategy with a beam-driven dielectric wakefield accelerator scheme, supported by a gas-filled dielectric capillary. A proof-of-principle experiment is planned at the Argonne Wakefield Accelerator Facility. We present simulations of a high charge bunch train driving GV/m wakefields in a narrow dielectric tube. Realistic variations in the bunch charge, energy spread, and timing jitter are considered. We then model the injection of a high brightness witness bunch, with the goal of minimizing emittance growth due to varying spatiotemporal synchronization of the ionization laser as well as the presence of transverse wakes.
MOPLH16 Femtosecond Laser Microfabrication for Advanced Accelerator Applications -1
  • 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.
MOPLH17 Enhanced Robustness of GaAs-Based Photocathodes Activation by Cs, Sb, and O2 -1
SUPLE02   use link to see paper's listing under its alternate paper code  
  • 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 icon Poster MOPLH17 [0.928 MB]  
MOPLH19 Beam Dynamics Simulations for a Conduction-Cooled Superconducting RF Electron Source -1
SUPLE04   use link to see paper's listing under its alternate paper code  
  • 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.
Experiments with Metamaterial-Based Metallic Accelerating Structures  
MOZBB2   use link to access more material from this paper's primary paper code  
  • 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 icon Slides MOPLH20 [8.339 MB]  
Measuring the Mean Transverse Energy of Pump-Probe Photoemitted Electrons  
MOZBB6   use link to access more material from this paper's primary paper code  
SUPLE11   use link to access more material from this paper's primary paper code  
  • 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 icon Slides MOPLH21 [6.384 MB]  
MOPLH22 Focusing Studies of an Electron Beam in Diamond Field Emitter Array Cathodes -1
  • R.L. Fleming, H.L. Andrews, D. Gorelovpresenter, C. 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.
MOPLH23 An Analysis of Potential Compact Positron Beam Source -1
  • 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.  
MOPLH24 Towards the Optimization Of Photocathode Properties Via Surface Science Techniques: A Study On Cs3Sb Thin Film Growth -1
  • 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
MOPLH25 Characterization of Femtosecond-Laser-Induced Electron Emission from Diamond Nano-Tips -1
  • 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.
MOPLH26 Design of a Compact Wakefield Accelerator Based on a Corrugated Waveguide -1
SUPLE18   use link to see paper's listing under its alternate paper code  
  • 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.  
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 icon Slides MOPLH27 [6.859 MB]