SUPLE01
Arizona State University, Tempe, USA
LBNL, Berkeley, California, USA
SUPLE02
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
Cornell University, Ithaca, New York, USA
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.
SUPLE03
Northern Illinois University, DeKalb, Illinois, USA
Fermilab, Batavia, Illinois, USA
ANL, Lemont, Illinois, USA
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"
SUPLE04
Northern Illinois University, DeKalb, Illinois, USA
Fermilab, Batavia, Illinois, USA
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.
SUPLE06
Northern Illinois University, DeKalb, Illinois, USA
ANL, Lemont, Illinois, USA
Fermilab, Batavia, Illinois, USA
SUPLE07
Stony Brook University, Stony Brook, USA
BNL, Upton, New York, USA
The University of Texas at Austin, Austin, Texas, USA
UCLA, Los Angeles, California, USA
SBU, Stony Brook, USA
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.
SUPLE09
UCLA, Los Angeles, USA
SUPLE10
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
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.
SUPLE11
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
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.
SUPLE12
IIT, Chicago, Illinois, USA
UIC, Chicago, Illinois, USA
Euclid TechLabs, LLC, Solon, Ohio, USA
Michigan State University, East Lansing, Michigan, USA
Illinois Institute of Technology, Chicago, Illinois, USA
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).
SUPLE13
MIT/PSFC, Cambridge, Massachusetts, USA
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
SUPLE14
Northern Illinois University, DeKalb, Illinois, USA
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.
SUPLE16
Stony Brook University, Stony Brook, USA
BNL, Upton, New York, USA
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.
SUPLE17
NSCL, East Lansing, Michigan, USA
FRIB, East Lansing, Michigan, USA
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).
SUPLE18
UW-Madison/PD, Madison, Wisconsin, USA
ANL, Lemont, Illinois, USA