WG D: HOM Couplers and Loads Summary G. Burt & J. Delayen

Description of Beamline HOM Load HOM beamline absorber located between cavities in ERL linac at 40 to 80 K Based on the first generation ERL HOM load but greatly simplified and improved N. Valles – Cornell ERL Main Linac HOM load Research and Development 3 RF absorbing material: SiC produced by Coorstek (SC-35) Broadband RF loss – ε = (50 – 25i) ε 0 – μ = μ 0 Sufficient DC conductivity at 80 K No measured particulate generation – Tested with high pressure nitrogen gas – Can be high pressure rinsed Vacuum properties acceptable

Initial SiC Sample Measurements N. Valles – Cornell ERL Main Linac HOM load Research and Development 4 Initial RF Measurements give ε ~ (50 – 25i) ε 0 Discontinuities in measurement could be indications of Measurement error Inhomogeneity in material Effervescent (ghost) modes Good DC conductivity at cryogenic temperatures Key to avoiding charging HOM absorber by beam or field emission

Cryogenic Test of Power Absorption N. Valles – Cornell ERL Main Linac HOM load Research and Development W (limited by heater) applied to HOM absorber at cryogenic temperatures to simulate HOM heating Thermal cycled without any problems Thermal properties as expected ∆T over load very small at 125 W (<10K) ∆T shown below is for cooling gas

RF Absorber Measurement N. Valles – Cornell ERL Main Linac HOM load Research and Development 6 Coax transmission measurement setup with prototype beam line load Attenuation of the forward power is roughly consistent with the estimates for ε = (50 – 25i) ε 0

Introduction; Concept of Demountable Damped Cavity (DDC) 26 June 2012 Higher Order Mode Diagnostics & Suppression in SC Cavities: Group D 8 He Vessel Baseplate HOM Acc. Choke Coaxial waveguide RF absorber Accelerating Mode is reflected by choke filter. HOMs pass through the choke, and damped at the RF absorber

RF simulations 26 June 2012 Higher Order Mode Diagnostics & Suppression in SC Cavities: Group D 9 CST-Studio n=0 n=1 Resonate frequency of the cavity n=2 The coupling (Q coupling ) is about 790. L Top of inner conductor 15mm End of inner conductor Cavity iris

RF Absorber (Ferrite: CMD10) 26 June 2012 Higher Order Mode Diagnostics & Suppression in SC Cavities: Group D 10  RF absorber is mounted on 77K thermal anchor.  Because brazing is included the manufacturing process, this ferrite was annealed at 900 ℃ before the measurement. Ferrite To vacuum pump Liquid nitrogen Input Refection Transmission d S 11 S 21 Network Analyzer Coaxial waveguide RF Absorber (Ferrite) The permeability and permittivity were estimated by fitting. Imaginary part is larger than real part in HOM region We conclude this ferrite has sufficient absorbing ability at 77K.

Superconducting joint 26 June 2012 Higher Order Mode Diagnostics & Suppression in SC Cavities: Group D 11 Base plate Flange Choke Flange Indium Nb Ti 8th9th, 10th12th, 13th16th, 17th, 18th Hit the periphery ahead Torque:25, 30, 35(N･m) No X-ray t2mm t100mm  The magnetic field at the superconducting joint is 1/6 of the cavity’s maximum field.  Sealing material is indium. Its transition temperature is 3.4K.  Step by step, we changed the flange edge shape to achieve higher performance  Transition temperature was observed at 3.4K in 8 th ~17 th VT.This means indium had been exposed to the RF field.  Finally, we achieved 19MV/m, Qo=1.5×10 10 Knife edge This Q-switch occurred by thermal quench of indium. To solve this, knife edge structure was introduced. 11

HOM coupler options

WG coupler

Coaxial with high pass filter

Coaxial performance

Comparison