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Multipactor in SPL Couplers

Multipactor in SPL Couplers. Amos Dexter, Graeme Burt and Richard Carter CERN 16 th March 2010. Questions to be addressed. Can d.c. biasing of the inner coaxial conductor be avoided How does the conical matching section affect conditions for multipactor and mitigation techniques.

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Multipactor in SPL Couplers

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  1. Multipactor in SPL Couplers Amos Dexter, Graeme Burt and Richard Carter CERN 16th March 2010

  2. Questions to be addressed • Can d.c. biasing of the inner coaxial conductor be avoided • How does the conical matching section affect conditions for multipactor and mitigation techniques

  3. What CERN knows J. Tückmantel et al. “Improvements to Power Couplers for the LEP2 Superconducting Cavities” PAC 1995 (in particular section IV “measures against multipacting”) For coaxial lines with impedances above 50 W multipacting only runs on the outer conductor. Injection of sideband RF at 2% power with offset (200kHz to 1 MHz) eliminated multipactor in a warm test stand but not a cold test stand! Perturbation with static magnetic fields influence the levels but did not give suppression. DC bias with voltages as little as 2.5 kV give complete suppression.

  4. Modelling with Particle Studio Particle studio is not fully integrated with MWS yet and it determines the field using an eigenmode solver hence the last release does not do travelling waves. We imported the travelling wave solution from MWS using an undocumented feature but found some (minor) interpolation errors. Particle studio multipactor diagnostics are very rudimentary and it not possible to extract neat trajectories. Our simulations were run with electron – electron interactions turned off. The method is to launch some particles, run for a long time and look at the average SEY.

  5. time 1 ms Particle Studio SEY Plot Determine average by dividing number of electrons leaving each junction by the number of junctions (from start time to finish time) If Average SEY > 1 for a time where at least 20 wall interaction have taken place then we probably have multipactor

  6. Tracking Code We wrote a Fortran Tracking code in 2003 so we could model multipactor in the half height CESR input waveguide and produce a paper for MULTICOM 03. A personalised code offers huge flexibility in searching for events and changing surface models. The original code used analytic expression for the fields in waveguides and for waveguide boundaries. Since January 2010 we have blown the dust of the code, (a) modified it to study multipactor in Coax with dc bias and periodic magnetic fields. (b) evolved a new program that reads field output files from the MWS time domain solver as Real and Imaginary components and hence can construct field solutions in the cone for any level of reflection. Dexter A.C., Seviour R., Goudket P., “Multipactor Trajectories, Unsymmetrical Orbits and Electron Drift in High Power Waveguides and Cavities” , 4th International Workshop on Multipactor, Corona and Passive Intermodulation in Space RF Hardware, ESTEC, Noordwijk, The Netherlands, 8-11 September 2003http://conferences.esa.int/03C26/index.html

  7. Multipactor identification with tracking code • For every phase and field start with current = 1 • On wall collision determine true secondary yield as function of energy • (or bounce probability at low energies) • After each wall interaction set current = current * yield (or reduce current by one minus bounce probability) • Terminate if current is less than 0.2 • Record multipactor if secondaries > 20 and current > 10 • (or current > peak_yield ^ 15) • Event count is the number of launch phases that support multipactor This approach does not follow the electron tree but just a representative path. This means that high energy electrons emitted from the surface should not be followed as they do not follow the representative path. This has the danger that multipactors with drift can be missed where fast electrons move back to the initiation point. This possibility is covered by Particle Studio type simulations.

  8. Issues with MWS output files • Standard output is on a regular mesh for the entire bounding box used in the simulation, no windowing is possible. • 2 mm output mesh generates two 1.6 GB files • 1 mm output mesh generates two 12.8 GB files (too big to easily transfer between Graeme’s and my laptops hence only used 2mm mesh so far) • No convenient geometry file was available hence we constructed a rough approximation to the boundary shape from discs, cones and cylinders. • The MWS output file gives zero field inside boundaries rather than a value that might enable smooth interpolation of the fields onto the boundaries.

  9. Multipactor in uniform coax (TW) Surface model has been set to make conditions for multipactor more favourable than for a fully conditioned surface coax outer radius (m) : 0.051 coax inner radius (m) : 0.02215 Secondary threshold (eV) : 10.0 Peak yield : 1.6 Energy at peak yield (eV) : 800.0 Average starting energy (eV) : 4.0 Secondary energy range (eV) : 3.0 Bounce probability : 0.0 Bounce threshold (eV) : 10.0 number of phase points : 720 forward wave field magnitude : 10.0d3 V/m forward wave increment : 2.0d3 V/m backward wave field magnitude: 0.0d3

  10. Multipactor as function of forward power With reflection low order multipactor walks in the large fields and extinguishes

  11. Multipactor for full reflection Even low power couplers are susceptible to multipactor at the start of the superconducting cavity fill.

  12. Standing wave vs travelling wave Levels are characterised by field hence move to quarter power

  13. Secondary Yield Are the general predictions so far surface model dependent? I. Bojko, N. Hilleret, and C. Scheuerlein “Influence of air exposures and thermal treatments on the secondary electron yield of copper.” Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films, 18(3):972-979, 200

  14. Material Models Our Yield Function follows Gopinath & Vaughan Secondary emission curves for copper inclusive of reflected electrons have been given by Cimino and Collins An approximation we have used for secondary emission (model 2) Primary energy EP

  15. Secondary Model Dependency if(model.eq.1) se = secondary_energy + (0.5d00-random()) * range if(model.eq.2) se = ( 0.4d00+1.3d00*kenergy**0.33)*sqrt( -log(random()) )

  16. D.C. Bias for SW A d.c. bias of 1000 Volts apparently eliminates multipactor for SW in the power range of interest.

  17. SW in cone Searching for multipactor in cone with RFpactor.for Tracking code found no multipactor in cone for forward power less than 6.0 MW. Most trajectories terminate in ridge before window.

  18. TW in cone Searching for multipactor in cone with RFpactor.for Fields from MWS have poor resolution hence this may not be multipactor Tracking code found no multipactor in cone for forward power less than 12.0 MW. Most trajectories terminate in ridge before window.

  19. Future Work • Investigate window region • Check for multipactor in corner trap at end of cone • Investigate whether magnetic field or offset RF can replace DC bias • Look for high energy electrons travelling back to straight coax acting as a source. • Use the parallel code VORPAL for multipactor simulations in particular following a cavity fill and tracking multipactor development as it absorbs RF power. Summary • Calculations so far indicate that the conical matching section does not create problems that did not already exist with the straight coax. • MWS simulations suggest a potential multipactor in the corner trap

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