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International Workshop on HOM Damping in Superconducting RF Cavities

October 11 -13, 2010

701 Clark Hall, Cornell University

Agenda

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Monday, October 11, 2010

9:00 – 9:15: Welcome (M. Liepe)

9:15 – 11:00: HOM damping requirements for various projects
Session Chair: I. Ben-Zvi Task: Collect key HOM specs for different projects.

• HOM damping requirements for SRF deflecting cavities (A. Nassiri, 15 min)
  
• Higher order mode damping considerations for the SPL cavities at CERN (W. Weingarten, 15 min)
  
• HOMs in the Project X linac (V. Yakovlev, 15 min)
  
• HOM damping requirements for various projects (all, 30 min total)
  
  • Bunch length, bunch charge, beam current, number of cavities
    
  • Cavity frequency, number of cells, longitudinal loss factor at design bunch length
    
  • Single bunch HOM power spectrum
    
  • Average HOM power per cavity (k*Qb*I)
    
  • Worst case peak HOM power per cavity in case of resonant excitation of modes
    
  • Required damping (typical Q-values only!) of monopole, dipole, and quadrupole modes
    
• Discussion: HOM damping requirements (all, 30 min total)

11:00 – 11:30: Introduction to HOM damping
Session Chair: I. Ben-Zvi

• A comparison of the HOM damping efficiency for various SRF coupler schemes (F. Marhauser, 30 min)

11:30 – 1:00 PM: Working lunch

1:00 – 5:00 PM: Antenna/ loop HOM couplers Session Chair: J. Knobloch

• HOM Damper and Filter Design for 56MHz SRF Cavity for RHIC (Qiong Wu, 20 min)
  
• HOM Damping Properties of Fundamental Power Couplers in the Superconducting Electron Gun of the Energy Recovery LINAC at BNL (L. Hammons, 20 min)
  
• Capacitive-Antennae HOM Damper (H. Hahn, 20 min)
  
• New HOM coupler design for High Current Superconducting cavity (W. Xu, 20 min)
  
• Experience with 3.9 GHz loop couplers (T. Khabiboulline, 20 min)
  
• Heating in DESY style HOM couplers in cw operation (J. Sekutowicz, 20 min)
  
• Heating of HOM loop couplers in CW mode (W.Anders/A.Neumann, 20 min)
  
• HOM damping variations in SRF cavities (F. Marhauser, 20 min)
  
• Optimization of HOM Couplers using Different Time Domain Schemes (C. Potratz, 20 min)
  
• Computation of Coupler Damping Properties in Concatenated Arrangements (H.-W. Glock, 20 min)
  
• Discussion: antenna based HOM damping (all, 40 min)
  
  • Effective HOM damping frequency range
    
    • Coupling to high frequency modes?
      
  • Measured and/or simulated HOM Q-values for given cavity design vs. frequency (no BBU simulation results!)
    
    • Coupling to monopole, dipole, and quadrupole modes
      
    • How many antenna/loop couplers are required per cavity to guarantee effective damping for all polarization angles?
      
    • Design and results from DESY, TJNAF, BNL
      
  • Maximum HOM power handling and extraction
    
    • Estimate of the heat load to ~2K and all other intercept temperatures at full HOM power
      
  • Coupling to the fundamental mode and suppression
    
    • Thermal limitations, e.g. long pulse vertical test (DESY), cw-version for the CEBAF upgrade cavities
      
    • High thermal conduction feedthroughs
      
    • Niobium or Cu antenna? Impact on cost?
      
    • Filter design and tuning, especially with large number of couplers per cavity. Reliability/ success rate?
      
    • Filter always needed?
      
  • Cleanness challenges and solutions
    
    • Field emission
      
    • Trap sulfur during EP?
      
  • Extra beamline length required per cavity (compared to linac without HOM damping)
    
  • Mechanical / fabrication challenges and solutions
    
    • FNAL experience at 3.9 GHz and SNS experience
      
    • Multipacting
      
    • Mechanical failure
      
  • Cost vs. design and material choices
    
    • Niobium vs. normal conducting
      
    • Filter design and complexity
      
    • Cabling; load inside our outside of vacuum vessel?
      
  • Other challenges, limitations and solutions

Tuesday, October 12, 2010

9:00 – 10:00 AM: Waveguide HOM damping
Session Chair: S. Belomestnykh

• Waveguide HOM damping studies at JLAB (R. Rimmer, 30 min)
  
• Discussion: waveguide HOM damping (all, 30 min)
  
  • Effective HOM damping frequency range
    
    • Coupling to high frequency modes?
      
  • Measured and/or simulated HOM Q-values for given cavity design vs. frequency (no BBU simulation results!)
    
    • TJNAF designs and results
      
  • Maximum HOM power handling and extraction
    
    • Estimate of the heat load to ~2K and all other intercept temperatures at full HOM power
      
  • Coupling to the fundamental mode and suppression
    
  • Cleanness challenges and solutions
    
    • Cleaning of waveguide sections
      
  • Extra beamline length required per cavity (compared to linac without HOM damping)
    
  • Mechanical / fabrication challenges and solutions
    
  • Cost vs. design and material choices
    
    • Superconducting or normal conducting waveguide sections?
      
    • Number of waveguides per cavity required
      
    • Length of waveguide section
      
    • Absorber inside or outside of vacuum vessel?
      
    • Water cooling vs. cryogens; risks involved
      
    • Temperature of loads at end of waveguides
      
    • Shielding of IR radiation from warm load
      
    • Water cooling and mechanical cavity vibrations
      
  • Other challenges, limitations and solutions

10:00 – 12:00 PM: RF absorbing materials
Session Chair: M. Liepe

• RF absorber studies at Cornell, part 1 (V. Shemelin, 20 min)
  
• RF absorber studies at Cornell, including DC conductivity, part 2 (E. Chojnacki, 20 min)
  
• RF absorber studies at KEK (M. Sawamura, 20 min)
  
• Measurements of absorber materials from room temperature to 2K (F. Marhauser, 20 min)
  
• Discussion: HOM absorbing materials (all, 40 min)
  
  • Room temperature and cryogenic material complex mu & eps (temperature dependence of absorption) of various dissipative materials vs. frequency (ferrites, ceramic with carbon, CNT…)
    
  • DC conductivity of dissipative materials and its temperature dependence
    
  • Mechanical and thermal properties of dissipative materials
    
  • Vacuum properties of dissipative materials
    
  • Coatings and other methods to avid electrostatic charging of dissipative materials
    
  • Fabrication of dissipative materials and reliability of achieving specs
    
  • Fabrication cost of different dissipative materials

12:00 – 1:30 PM: Working lunch

1:30 – 5:30 PM: Beamline HOM loads
Session Chair: M. Liepe

• Ferrite HOM Load Surrounding a Ceramic Break (L. Hammons, 20 min)
  
• Absorbing materials for beamline absorbers: How good is good enough? (N. Valles, 20 min)
  
• Experience with the Cornell ERL beamline absorber prototype and future plans (E. Chojnacki, 30 min)
  
• Resonant HOM load made of a resistive material (V. Shemelin, 20 min)
  
• Test of the Beam Line Absorber at FLASH (J. Sekutowicz, 20 min)
  
• Cooling test of HOM absorber model for cERL in Japan (M. Sawamura, 30 min)
  
• Operation Experience of HOM absorbers at KEKB (T. Furuya, 20 min)
  
• Beamline absorber work at Muon, Inc (R. Johnson, 20 min)
  
• Design and Application of the High-Efficiency HOM Absorbers at PEP-II (A. Novokhatski, 20 min)
  
• Discussion: beamline absorbers (all, 40 min)
  
  • Effective HOM damping frequency range
    
  • Measured and/or simulated HOM Q-values for given cavity design vs. frequency (no BBU simulation results!)
    
    • Cornell, DESY, BNL, KEK designs
      
  • Maximum HOM power handling and extraction
    
    • What is the optimal operating temperature?
      
    • Heat transfer and thermal connections
      
    • Estimate of the heat load to ~2K and all other intercept temperatures at full HOM power
      
  • Coupling to the fundamental mode and suppression
    
  • Cleanness challenges and solutions
    
    • Cleaning of absorber materials
      
    • Risk of particle generation?
      
    • How to quantify the absence or presence of RF absorber material particulate generation that could spoil the Q of nearby SRF cavities?
      
    • Coatings?
      
  • Extra beamline length required per cavity (compared to linac without HOM damping)
    
  • Mechanical / fabrication challenges and solutions
    
    • Are bellow sections between cavities needed / desirable?
      
    • Heat intercept and static heat loads to cavities
      
    • Brazing, soldering, metallization of ceramics/ferrites to heat sinks.
      
    • Absorber tiles vs. rings
      
    • Accurate mechanical modeling that includes plastic deformation of material.
      
  • Cost vs. design and material choices
    
    • Thermal matching of heat sinks to ceramic/ferrites
      
    • Copper coating of beam pipe sections or stainless steel?
      
  • Other challenges, limitations and solutions

Wednesday, October 13, 2010

9:00 – 10:15 AM: RF simulation tools (2D, 3D)
Session Chair: E. Chojnacki

• ACE3P and HOM power flow in the Cornell ERL (Liling Xiao, 20 min)
  
• HOM simulations with ANSYS (S. Posen, 20 min)
  
• Higher Order Mode Heating Analysis for the ILC Superconducting Linacs (C.r Nantista, 20 min)
  
• Discussion: HOM simulations (all, 15 min)
  
  • Which problems need 3D models?
    
  • Which problems require only 2D?
    
  • Which 3D software allows complex mu & eps?
    
  • Which 3D software is up to the job?
    
  • Which 2D software allows complex mu & eps?
    
  • Which 2D software is up to the job?
    
  • FEM vs. FD codes
    
  • How high in frequency can/should one go?
    
  • How much can one trust the simulations? What safety factor should be included? Comparison of simulations and measurements.
    
  • Time domain vs. frequency domain
    
  • Choice of boundary conditions at cavity beam tube ends (open, electric, magnetic). What is realistic for a large linac installation?

10:15 – 11:30 AM: Measurement Methods (HOMs, material properties)
Session Chair: E. Chojnacki

• RF absorber studies using waveguides in transmission Part 1 | Part 2 (E. Chojnacki, 20 min; V. Shemelin, 20 min)
  
• HOM-BPMs at the 3.9 GHz Superconducting Cavities for FLASH and the European XFEL (R.M. Jones, 20 min)
  
• Experiments on HOM Spectrum Manipulation in a ILC 1.3 GHz Cavity (T. Khabiboulline, 20 min)
  
• Discussion: HOM measurements
  
  • Measurement methods of RF absorbing materials (complex mu & eps, mechanical properties…)
    
  • Cornell waveguide method, terminated waveguide (TJNAF), resonator methods, …which method gives reliable data at operating temperatures?
    
    • DC conductivity, mechanical and thermal methods
      
  • Measurements in cavities and cavity prototypes (copper)
    
    • How much can one trust HOM measurements on individual cavities?
      
    • Boundary conditions at beam tubes?
      
    • Are cold measurements needed?
      
    • Are beam measurements needed?
      
    • Are copper modes needed?

11:30 – 1:00 PM: Closeout

• Summary of HOM damping schemes currently available and fully developed, including parameter specs (frequency range, power handling capabilities...)
  
• Summary of future, potential performance of improved versions, including outstanding challenges/problems, potential solutions, and R&D path
  
• Volunteers to perform the R&D, the time frame, and report the results

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Supported by the National Science Foundation and the US Department of Energy
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