Keyword: multipactoring
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MOPOB53 Simulation of Ping-Pong Multipactor with Continuous Electron Seeding ion, simulation, electron, resonance 181
 
  • M. Siddiqi, R.A. Kishek
    UMD, College Park, Maryland, USA
 
  Funding: National Science Foundation grant No. PHY1535519
Multipactor is a discharge induced by the impact of electrons on a surface due to radio-frequency (RF) electromagnetic fields and secondary electron emission (SEE). Depending on the impact energy and RF phase of the incident electron, a growth in the electron density is possible. Multipactor can lead to device breakdown in many applications, such as particle accelerator structures and rf systems, satellite communication equipment, and microwave components. Multipactor can also be a precursor for electron cloud effects. Due to the critical need to mitigate multipactor, a more comprehensive theory has been introduced that views multipactor as a global effect that can be analyzed through the concepts of iterative maps and nonlinear dynamics *. In order to test this novel approach, multipactor is simulated in a parallel-plate waveguide using the WARP particle-in-cell code. Different parameters are varied in the simulation to determine the conditions that add to multipactor growth, such as geometry dimensions, electron seeding scenarios, and an applied DC electric field. These computational results and their implications on the further development of this theory will be presented.
*R.A. Kishek, Physics of Plasmas 20, 056702 (2013).
 
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-NAPAC2016-MOPOB53  
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WEPOB31 Dark Current Study of a Standing Wave Disk-Loaded Waveguide Structure at 17 GHz ion, electron, simulation, experiment 971
 
  • H. Xu, M.A. Shapiro, R.J. Temkin
    MIT/PSFC, Cambridge, Massachusetts, USA
 
  Funding: US DoE, Office of High Energy Physics
We present calculations of the dark current in a high gradient accelerator with the intent of understanding its role in breakdown. The initial source of the dark current is the field emission of electrons. For a 17 GHz single-cell standing wave disk-loaded waveguide structure, the 3D particle-in-cell simulation shows that only a small portion of the charge emitted reaches the current monitors at the ends of the structure, while most of the current collides on the structure surfaces, causing secondary electron emission. In the simulation, a two-point multipactor process is observed on the side wall of the cell due to the low electric field on the surface. The multipactor approaches a steady state within nanoseconds when the electric field is suppressed by the electron cloud formed so that the average secondary electron yield is reduced. This multipactor current can cause the ionization of the metal material and surface outgassing, leading to breakdown. We report first results from an experiment designed to extract dark current directly from an accelerator cell from the side through two slits. First results show that the dark current behavior deviates from the field emission theory.
 
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-NAPAC2016-WEPOB31  
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WEPOB67 K2CsSb Photocathode Performance in QWR SRF Gun cathode, ion, gun, vacuum 1042
 
  • E. Wang, Y. Hao, Y.C. Jing, V. Litvinenko, I. Pinayev, T. Rao, J. Skaritka, G. Wang, T. Xin
    BNL, Upton, Long Island, New York, USA
 
  In 2016 run of Coherent Electron Cooling, we have successfully tested the performance of a number of K2CsSb cathodes. These cathodes with QE of 6%-10% were fabricated in Instrumentation Division, a few miles away, transported to RHIC tunnel under UHV conditions, attached to the CeC gun, kept in storage, and inserted in the gun as needed. A maximum bunch charge of 4.6 nC was generated in the gun when the QE was 1.8 %. With careful conditioning at increasing accelerating fields, it was possible to maintain the QE of several cathodes for more than a week. For the cathodes that experienced degradation, the primary cause was multipacting when the power into the gun was increased. In the initial runs, the entire 20 mm substrate face was coated with the cathode material causing cathode induced multipacting. For subsequent measurements, the substrate was masked to coat only the central 9 mm of the substrate. By optimizing the procedure for boosting the power to the gun and covering all viewports to minimize dark current, we were able to minimize QE degradation. In this paper we discuss the cathode preparation, transfer to the gun and operational experience with the cathodes in 112 MHz gun.  
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-NAPAC2016-WEPOB67  
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THPOA20 Simulation of Multipacting with Space Charge Effect in PIP-II 650 MHz Cavities ion, simulation, space-charge, cavity 1142
 
  • G.V. Romanov
    Fermilab, Batavia, Illinois, USA
 
  The central element of the Proton Improvement Plan -II at Fermilab is a new 800 MeV superconducting linac, injecting into the existing Booster. Multipacting affects superconducting RF cavities in the entire range from high energy elliptical cavities to coaxial resonators for low-beta part of the linac. The extensive simulations of multipacting in the cavities with updated material properties and comparison of the results with experimental data are routinely performed during electromagnetic design at Fermilab. This work is focused on multipacting study in the low-beta and high-beta 650 MHz elliptical cavities. The new advanced computing capabilities made it possible to take the space charge effect into account in this study. The results of the simulations and new features of multipacting due to the space charge effect are discussed.  
poster icon Poster THPOA20 [3.572 MB]  
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-NAPAC2016-THPOA20  
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THPOA21 Multipacting in HOM Coupler of LCLS-II 1.3 GHz SC Cavity ion, HOM, cavity, electron 1146
 
  • G.V. Romanov, T.N. Khabiboulline, A. Lunin
    Fermilab, Batavia, Illinois, USA
 
  During high power tests of the 1.3 GHz LCLS-2 cavity on the test stand at Fermilab an anomalous rise of temperature of the pickup antenna in the higher order mode (HOM) coupler was detected in accelerating gradient range of 5-10 MV/m. It was suggested that the multipacting in the HOM coupler may be a cause of this temperature rise. In this work the suggestion was studied, and the conditions and the location, where multipacting can develop, were found.  
poster icon Poster THPOA21 [4.786 MB]  
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-NAPAC2016-THPOA21  
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