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UK Jan 2010 LHC Crab cavity Update

UK Jan 2010 LHC Crab cavity Update. Ben Hall PhD student. Recent Activities. Participated in on-cell HOM damping studies at Jlab Understanding of geometrical dependencies for a simplified shape (cylindrical rods and limited rounding) Analytical Equivalent Circuit models

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UK Jan 2010 LHC Crab cavity Update

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  1. UK Jan 2010 LHC Crab cavity Update Ben Hall PhD student Lancaster Eng. 2010

  2. Recent Activities • Participated in on-cell HOM damping studies at Jlab • Understanding of geometrical dependencies for a simplified shape (cylindrical rods and limited rounding) • Analytical Equivalent Circuit models • Re-optimising full cavity shape

  3. Rod radius Gap Separation Beam-pipe radius Cavity radius

  4. Reaction to changing numbers from Rama • Minimum Beam-pipe radius is not known. • Depends on Beta at cavity location • Original design was 50 mm • Expected 40-50 mm • Maximum Cavity radius is not known • Depends on cavity location and surroundings • Expected to be 150-180 mm • Original design was 200 mm

  5. Implication of beam-pipe radius • Increased beam-pipe radius forces the rod separation further apart • An increased separation is not preferred as the peak surface fields increase for a given voltage (almost linearly). • For a simple cavity shape (not optimised) at Vt=3MV • Emax varies as 1.7 MV/mm separation change • Bmax varies as 3.4mT/mm Note: optimised rod shape will have significantly lower peak fields Lancaster Eng. 2010

  6. Implication of Maximum Cavity Radius • Decreased cavity outer radius leads to short distance between the outer wall and the rod. • A decrease in outer radius has little effect until the wall gets very close to the rods, where the peak fields take a sharp increase. • In the LHC cavity it may spike at a larger cavity radii as we have larger rod radii as we use a conical rod.

  7. Optimisation of LHC shape Improved shape both reduces peak surface currents and increases transverse voltage. Kidney shape offers the advantages of larger rods while maintaining the small Cavity radius. A peak magnetic field below 80mT at a deflecting voltage of 3MV has been achieved at a beam pipe radius of 50mm and outer radius of 150mm Further optimisation is being carried out to improve this further. Lancaster Eng. 2010

  8. Degenerate Mode Couplers This cavity doesn’t have a LOM and a SOM. Instead it has a degenerate LOM-like accelerating mode. As the crabbing mode has low fields on the outer conductor we can easily add waveguide couplers to the walls which damp this mode to an external Q ~100.

  9. Future Plans • Cavity/ Coupler designs Feb 2010 • Microphonics rod study March 2010 • Multipactor study April 2010 • Copper Prototype April 2010 • SRF Prototype Summer 2010 • Amos Dexter going to CERN this month to discuss LLRF design for LHC crabs.

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