SUPLE —  Student Poster Session-Lake Erie   (01-Sep-19   14:00—16:00)
Paper Title Page
Photoluminescence Studies of Alkali-Antimonide Photocathodes  
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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.  
Enhanced Robustness of GaAs-Based Photocathodes Activation by Cs, Sb, and O2  
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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.
Start-to-End Simulation of the Drive-Beam Longitudinal Dynamics for Beam-Driven Wakefield Acceleration  
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  • W.H. Tan, P. Piot
    Northern Illinois University, DeKalb, Illinois, USA
  • P. Piot
    Fermilab, Batavia, Illinois, USA
  • A. Zholents, A. Zholents
    ANL, Lemont, Illinois, USA
  Funding: This work is supported by the U.S. Department of Energy, Office of Science under contracts No. DE-AC02-06CH11357 (via a laboratory- directed R&D program at ANL) and No. DE-SC0018656 at NIU.
Collinear beam-driven wakefield acceleration (WFA) relies on shaped driver beam to provide higher accelerating gradient at a smaller cost and physical footprint. This acceleration scheme is currently envisioned to accelerate electron beams capable of driving free-electron laser *. Start-to-end simulation of drive-bunch beam dynamics is crucial for the evaluation of the design of accelerators built upon WFA. We report the start-to-end longitudinal beam dynamics simulations of an accelerator beamline capable of producing high charge drive beam. The generated wakefield when it passes through a corrugated waveguide results in a transformer ratio of 5. This paper especially discusses the challenges and criteria associated with the generation of temporally-shaped driver beam, including the beam formation in the photoinjector, and the influence of energy chirp control on beam transport stability.
A. Zholents et al., "A Conceptual Design of a Compact Wakefield Accelerator for a High Repetition Rate Multi User X-ray Free-Electron Laser Facility"
Beam Dynamics Simulations for a Conduction-Cooled Superconducting RF Electron Source  
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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.
Ultrashort Laser Pulse Shaping and Characterization for Tailored Electron Bunch Generation  
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  • T. Xu, P. Piot
    Northern Illinois University, DeKalb, Illinois, USA
  • M.E. Conde, G. Ha, J.G. Power
    ANL, Lemont, Illinois, USA
  • P. Piot
    Fermilab, Batavia, Illinois, USA
  Temporally shaped laser pulses are desirable in various applications including emittance reduction and beam-driven acceleration. Pulse shaping techniques enable flexible controls over the longitudinal distribution of electron bunches emitted from the photocathode. While direct manipulation and measurement of an ultrashort pulse can be challenging in the time domain, both actions can be performed in the frequency domain. In this paper, we report the study and development of laser shaper and diagnostics at Argonne Wakefield Accelerator (AWA). Simulations of the shaping process for several sought-after shapes are presented along with the temporal diagnosis. Status of the experiment at the AWA facility is also discussed.  
Developing Criteria for Laser Transverse Instability in LWFA Simulations  
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  • Y. Yan, L.D. Amorim, P. Iapozzuto, V. Litvinenko, N. Vafaei-Najafabadi
    Stony Brook University, Stony Brook, USA
  • M. Babzien, M.G. Fedurin, Y.C. Jing, K. Kusche, M.A. Palmer, I. Pogorelsky, M.N. Polyanskiy
    BNL, Upton, New York, USA
  • M. Downer, J.R. Welch, R. Zgadzaj
    The University of Texas at Austin, Austin, Texas, USA
  • C. Joshi, W.B. Mori
    UCLA, Los Angeles, California, USA
  • P. Kumar, V. Samulyak
    SBU, Stony Brook, USA
  Funding: We acknowledge resources of NERSC facility, operated under Contract No. DE-AC02-5CH11231, and of SEAWULF at Stony Brook University as well as funding from SBU-BNL Seed Grants.
Laser-driven plasma wakefield acceleration (LWFA) is considered as a potential technology for future colliders and light sources. To make the best use of a laser’s power, the laser is expected to maintain a stable propagation. A transverse instability is observed in our previous simulations when a long, intense CO2 laser propagates inside a plasma*. This unstable motion is accompanied by strong transverse diffraction of the laser power and results in the disruption of the ion channel typically used for radiation generation**. We investigated the hosing-like instability using the Particle-in-Cell code OSIRIS*** by modeling the laser portion where this instability is seeded and then evolves. In this proceeding, a criteria will be described that allows for the characterization of the temporal and spatial evolution of this instability.
*J. Yan, et al. , AAC, IEEE, 2018.
** L. Nemos et al., PPCF, 58(3), 2016.
***R. A. Fonseca et al., Lecture Notes Computation Science (2331) 342, 2002.
Extreme Beams: Plasma-Optical Metrology and Plasma Photocathodes  
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  • 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.  
Design for HyRES Cathode Nanotip Electron Source  
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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.  
Ultrafast Nonlinear Photoemission from Alkali Antimonide Photocathodes  
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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.
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 icon Slides SUPLE11 [6.384 MB]  
Study of the Mean Transverse Energy and the Emission Mechanism of (N)UNCD Photocathodes  
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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).
STARRE Lab: The Sub-THz Accelerator Research Laboratory  
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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:
A High-Precision Emission Computational Model for Ultracold Electron Sources  
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  • A.J. Tencate, B. Erdelyi
    Northern Illinois University, DeKalb, Illinois, USA
  Funding: This work is supported by NSF award #1535401.
The high-intensity, high-brightness and precision frontiers for charged particle beams are an increasingly important focus for study. Ultimately for electron beam applications, including FELs and microscopy, the quality of the source is the limiting factor in the final quality of the beam. It is imperative to understand and develop a new generation of sub-Kelvin electron sources, and the current state of PIC codes are not precise enough to adequately treat this ultracold regime. Our novel computational framework is capable of modelling electron field emission from nanoscale structures on a substrate, with the precision to handle the ultracold regime. This is accomplished by integrating a newly developed Poisson integral solver capable of treating highly curved surfaces and an innovative collisional N-body integrator to propagate the emitted electron with prescribed accuracy. The electrons are generated from a distribution that accounts for quantum confinement and material properties and propagated to the cathode surface. We will discuss the novel techniques that we have developed and implemented, and show emission characteristics for several cathode designs.
slides icon Slides SUPLE14 [4.877 MB]  
Quantitative Analysis of Bulk GaAs Photocathode Using X-Ray Photoelectron Spectroscopy  
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  • 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.
Study of Photocathode Surface Damage due to Ion Back-Bombardment in High Current DC Gun  
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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.
Characterization and Performance of Plasma Window for Gas Flow Restriction in Different Geometries  
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  • A. Lajoie
    NSCL, East Lansing, Michigan, USA
  • J. Gao, F. Marti
    FRIB, East Lansing, Michigan, USA
  Funding: This work is supported by NSF Award PHY-1565546.
The plasma window is a DC cascaded arc whose function is to restrict gas flow from a high pressure region to a low pressure region without the use of any solid separation*. As a result, the plasma window allows a greater pressure to be maintained than otherwise possible. This is a beneficial characteristic for gas charge strippers for ion accelerators, since the higher pressures enable the stripper to be shorter and allow the same amount of stripping interactions**. The flow rate reduction is established by the increase in gas temperature from the power deposited into the plasma via the cathodes, resulting in a dramatically increased viscosity. The flow rate reduction, depends on the properties of the plasma, including the electron density and temperature, pressure, and electrical conductivity. Understanding these properties in multiple arc geometries - in this work having either 6 mm or 10 mm channel diameter - provides a means optimizing the plasma window for a given design. Determinations of the properties for different conditions are shown, and results are compared with a PLASIMO simulation, which has been shown to yield comparable properties to measurements in an argon arc***.
*A. Hershcovitch, Phys. Plasma 5, 2130 (1998).
**J. A. Nolen and F. Marti, Rev. Accel. Sci. Tech. 6, 221 (2013).
***G. M. W. Kroesen et al., Plas. Chem. and Plas. Proc. 10, 531 (1990).
slides icon Slides SUPLE17 [4.252 MB]  
Design of a Compact Wakefield Accelerator Based on a Corrugated Waveguide  
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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.