WG D: HOM Couplers and Loads Summary

1. WG D: HOM Couplers and LoadsSummary G. Burt & J. Delayen

3. 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 • RF absorbing material: SiC produced by Coorstek (SC-35) • Broadband RF loss • ε = (50 – 25i) ε0 • μ = μ0 • Sufficient DC conductivity at 80 K • No measured particulate generation • Tested with high pressure nitrogen gas • Can be high pressure rinsed • Vacuum properties acceptable N. Valles – Cornell ERL Main Linac HOM load Research and Development

4. Initial SiC Sample Measurements • Initial RF Measurements give ε ~ (50 – 25i) ε0 • 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 N. Valles – Cornell ERL Main Linac HOM load Research and Development

5. Cryogenic Test of Power Absorption • 125 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 N. Valles – Cornell ERL Main Linac HOM load Research and Development

6. RF Absorber Measurement • Coax transmission measurement setup with prototype beam line load • Attenuation of the forward power is roughly consistent with the estimates for ε = (50 – 25i) ε0 N. Valles – Cornell ERL Main Linac HOM load Research and Development

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

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

10. RF Absorber (Ferrite: CMD10) • RF absorber is mounted on 77K thermal anchor. • Because brazing is included the manufacturing process, this ferrite was annealed at 900 ℃ before the measurement. To vacuum pump NetworkAnalyzer Liquid nitrogen d Input Transmission S21 S11 Ferrite Refection Coaxial waveguide RF Absorber (Ferrite) Imaginary part is larger than real part in HOM region We conclude this ferrite has sufficient absorbing ability at 77K. The permeability and permittivity were estimated by fitting. Higher Order Mode Diagnostics & Suppression in SC Cavities: Group D

11. Superconducting joint • 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 Hit the periphery ahead Base plate Flange Choke Flange Ti Indium t100mm Nb t2mm Knife edge t2mm 16th, 17th, 18th 8th 9th, 10th 12th, 13th Torque:25, 30, 35(N･m) • Transition temperature was observed at 3.4K in 8th~17thVT.This means indium had been exposed to the RF field. • Finally, we achieved 19MV/m, Qo=1.5×1010 No X-ray This Q-switch occurred by thermal quench of indium. To solve this, knife edge structure was introduced. Higher Order Mode Diagnostics & Suppression in SC Cavities: Group D 11

13. HOM coupler options

14. WG coupler

15. Coaxial with high pass filter

16. Coaxial performance

17. Comparison