Keyword: HOM
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MOYE5 In Situ Plasma Processing of Superconducting Cavities at JLab cavity, plasma, cryomodule, radiation 22
  • T. Powers, N.C. Brock, T.D. Ganey
    JLab, Newport News, Virginia, USA
  Jefferson Lab has an ongoing R&D program in plasma processing which is close to going into production processing in the CEBAF accelerator. Plasma processing is a common technique for removing hydrocarbons from surfaces, which increases the work function and reduces the secondary emission coefficient. Unlike helium processing which relies on ion bombardment of the field emitters, plasma processing uses free oxygen produced in the plasma to break down the hydrocarbons on the surface of the cavity. The initial focus of the effort was processing C100 cavities by injecting RF power into the HOM coupler ports. Results from processing cryomodule in the cryomodule test bunker as well as vertical test results will be presented. We plan to start processing cryomodules in the CEBAF tunnel within the next year. The goal will be to improve the operational gradients and the energy margin of the linacs. This work will describe the systems and methods used at JLAB for processing cavities using an argon oxygen gas mixture. Before and after plasma processing results will also be presented.  
slides icon Slides MOYE5 [2.679 MB]  
DOI • reference for this paper ※ doi:10.18429/JACoW-NAPAC2022-MOYE5  
About • Received ※ 01 August 2022 — Revised ※ 08 August 2022 — Accepted ※ 11 August 2022 — Issue date ※ 01 October 2022
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MOPA12 Commissioning of HOM Detectors in the First Cryomodule of the LCLS-II Linac cavity, MMI, alignment, cryomodule 69
  • J.A. Diaz Cruz
    UNM-ECE, Albuquerque, USA
  • B.T. Jacobson, N.R. Neveu, J.P. Sikora
    SLAC, Menlo Park, California, USA
  Long-range wakefields (LRWs) may cause emittance dilution effects. LWRs are especially unwanted at facilities with low emittance beams like the LCLS-II at SLAC. Dipolar higher-order modes (HOMs) are a set of LRWs that are excited by off-axis beams. Two 4-channel HOM detectors were built to measure the beam-induced HOM signals for TESLA-type superconducting RF (SRF) cavities; they were tested at the Fermilab Accelerator Science and Technology (FAST) facility and are now installed at SLAC. The HOM detectors were designed to investigate LRW effects on the beam and to help with beam alignment. This paper presents preliminary results of HOM measurements at the first cryomodule (CM01) of the LCLS-II linac and describes the relevant hardware and setup of the experiment.  
DOI • reference for this paper ※ doi:10.18429/JACoW-NAPAC2022-MOPA12  
About • Received ※ 09 August 2022 — Accepted ※ 20 August 2022 — Issue date ※ 31 August 2022  
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MOPA91 Plasma Processing of Superconducting Quarter-Wave Resonators Using a Higher-Order Mode plasma, cavity, SRF, cryomodule 267
  • W. Hartung, W. Chang, K. Elliott, S.H. Kim, T. Konomi, J.T. Popielarski, K. Saito, T. Xu
    FRIB, East Lansing, Michigan, USA
  The Facility for Rare Isotope Beams (FRIB) is a superconducting ion linac with acceleration provided by 104 quarter-wave resonators (QWRs) and 220 half-wave resonators (HWRs); FRIB user operations began in May 2022. Plasma cleaning is being developed as a method to mitigate possible future degradation of QWR or HWR performance: in-situ plasma cleaning represents an alternative to removal and disassembly of cryomodules for refurbishment of each cavity via repeat etching and rinsing. Initial measurements were done on a QWR and an HWR with room-temperature-matched input couplers to drive the plasma via the fundamental mode. Subsequent plasma cleaning tests were done on two additional FRIB QWRs using the fundamental power coupler (FPC) to drive the plasma. When using the FPC, a higher-order mode (HOM) at 5 times the accelerating mode frequency was used to drive the plasma. Use of the HOM allowed for less mismatch at the FPC and hence lower field in the coupler relative to the cavity. A neon-oxygen gas mixture was used for plasma generation. Before and after cold tests showed a significant reduction in field emission X-rays after plasma cleaning. Results will be presented.  
DOI • reference for this paper ※ doi:10.18429/JACoW-NAPAC2022-MOPA91  
About • Received ※ 12 August 2022 — Revised ※ 16 August 2022 — Accepted ※ 25 August 2022 — Issue date ※ 16 September 2022
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TUPA27 Longitudinal Feedback Dynamics in Storage Rings with Small Synchrotron Tunes feedback, simulation, synchrotron, cavity 405
  • R.R. Lindberg
    ANL, Lemont, Illinois, USA
  Funding: This work was supported by U.S. Dept. of Energy Office of Sciences under Contract No. DE-AC02-06CH11357.
We analyze the dynamics of multibunch longitudinal instabilities including bunch-by-bunch feedback under the assumption that the synchrotron tune is small. We find that increasing the feedback response does not always guarantee stability, even in the ideal case with no noise. As an example, we show that if the growth rate of a cavity-driven mode is of the order of the synchrotron frequency, then there are parameter regions for which the instability cannot be controlled by feedback irrespective of its gain. We verify these calculations with tracking simulations relevant to the APS-U, and find that the dynamics do not depend upon whether the longitudinal feedback relies on phase-sensing or energy-sensing technology. Hence, this choice should be dictated by measurement accuracy and noise considerations.
poster icon Poster TUPA27 [1.180 MB]  
DOI • reference for this paper ※ doi:10.18429/JACoW-NAPAC2022-TUPA27  
About • Received ※ 26 July 2022 — Revised ※ 05 August 2022 — Accepted ※ 07 August 2022 — Issue date ※ 26 August 2022
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WEPA09 A Parallel Automatic Simulation Tool for Cavity Shape Optimization cavity, simulation, SRF, gun 634
  • L. Ge, Z. Li, C.-K. Ng, L. Xiao
    SLAC, Menlo Park, California, USA
  • M. Beall, B.R. Downie, O. Klaas
    Simmetrix Inc., Clifton Park, USA
  Funding: U.S. Department of Energy under contract No. DE-SC0018715.
We present a parallel automatic shape optimization workflow for designing accelerator cavities. The newly developed 3D parallel optimization tool Opt3P based on discrete adjoint methods is used to determine the optimal accelerator cavity shape with the desired spectral response. Initial and updated models, meshes, and design velocities of design parameters for defining the cavity shape are generated with Simmetrix tools for mesh generation (MeshSim), geometry modification and query (GeomSim), and user interface tools (SimModeler). Two shape optimization examples using this automatic simulation workflow will be presented here. One is the TESLA cavity with higher-order-mode (HOM) couplers and the other is a superconducting rf (SRF) gun. The objective for the TESLA cavity is to minimize HOM damping factors and for the SRF gun to minimize the surface electric and magnetic fields while maintaining its operating mode frequency at a prescribed value. The results demonstrate that the automatic simulation tool allows an efficient shape optimization procedure with minimal manual operations. All simulations were performed on the NERSC supercomputer Cori system for solution speedup.
DOI • reference for this paper ※ doi:10.18429/JACoW-NAPAC2022-WEPA09  
About • Received ※ 03 August 2022 — Revised ※ 05 August 2022 — Accepted ※ 09 August 2022 — Issue date ※ 08 October 2022
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WEPA26 197 MHz Waveguide Loaded Crabbing Cavity Design for the Electron-Ion Collider cavity, GUI, impedance, electron 679
  • S.U. De Silva, J.R. Delayen
    ODU, Norfolk, Virginia, USA
  • J. Guo, R.A. Rimmer
    JLab, Newport News, Virginia, USA
  • Z. Li
    SLAC, Menlo Park, California, USA
  • B.P. Xiao
    BNL, Upton, New York, USA
  The Electron-Ion Collider will require crabbing systems at both hadron and electron storage rings in order to reach the desired luminosity goal. The 197 MHz crab cavity system is one of the critical rf systems of the col-lider. The crab cavity, based on the rf-dipole design, ex-plores the option of waveguide load damping to suppress the higher order modes and meet the tight impedance specifications. The cavity is designed with compact dog-bone waveguides with transitions to rectangular wave-guides and waveguide loads. This paper presents the compact 197 MHz crab cavity design with waveguide damping and other ancillaries.  
DOI • reference for this paper ※ doi:10.18429/JACoW-NAPAC2022-WEPA26  
About • Received ※ 08 August 2022 — Revised ※ 09 August 2022 — Accepted ※ 11 August 2022 — Issue date ※ 06 September 2022
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