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Effect of condensed gases on SRF cavities

Effect of condensed gases on SRF cavities. A. Romanenko. Overview. Most of gas molecules will condense on the walls of the cavity at the operating temperatures (2-4.2 K) Typical residual pressures at 2K - ~10 -11 Torr Effect on performance

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Effect of condensed gases on SRF cavities

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  1. Effect of condensed gases on SRF cavities A. Romanenko

  2. Overview • Most of gas molecules will condense on the walls of the cavity at the operating temperatures (2-4.2 K) • Typical residual pressures at 2K - ~10-11Torr • Effect on performance • Increase in the residual resistance (lower the Q0) • Lower the work function and increase the secondary electron yield = enhance field emission and multipacting A. Romanenko

  3. CESR Luminosity Upgrade Cavity Desorption of gases during warmup • R. Geng et al, Proc. of PAC’99 A. Romanenko

  4. Q. S. Shu et al, IEEE Trans. Magn., Vol 25, No. 2, 1989 Strong enhancement of field emission by ultrapure O2 injection A. Romanenko

  5. J. Knobloch and H. Padamsee, Proc. of SRF’97 Non-field emission dielectric losses Iris Equator Iris Ratio of the surface resistance after/before He gas introduction. Losses are following the E2 profile – mainly dielectric in nature Temperature map showing losses concentrated in the iris (high E field) area High ELECTRIC field surface area – important parameter A. Romanenko

  6. P. Kneisel, Proc. of SRF’95 Clean cavities (no field emission in the baseline test) A. Romanenko

  7. P. Kneisel, Proc. of SRF’95 Clean cavities – Q degradation detected at P >= 3 Torr 3 Torr 10 Torr A. Romanenko

  8. P. Kneisel, Proc. of SRF’95 Field emission gets worse (lower onset) if present before condensing gases Noticeable field emission enhancement was found to be present only if P>= 10-4Torr A. Romanenko

  9. Rough estimates based on Kneisel’s data Assuming all the gas molecules condense on the surface covering with the uniform layer: For 1.5 GHz single cells used by Kneisel: cavity volume V~ 2 x 10-3 m3 cavity surface S~8 x 10-2 m2 PV = NkT => N = PV/kT => surface density s = N/S = PV/(kTS) T = 300 K For field emission free cavities: P < 3 Torr => s < 2.2 x 1021 m-2 For “dirty” cavities: P < 10-4Torr => s < 7.3x1016m-2 – less than a monolayer A. Romanenko

  10. Proposed experiments Cavities now have higher Q0, higher gradients, may be more sensitive • Use high Q0 single cell 1.3 GHz cavity (limited at ~40 MV/m by quench, no field emission and high field Q-slope) • Pump down at room temperature to predetermined pressures and seal the cavity off • Cool down to T = 2K • Initially - 4 cavity tests: lowest pressure baseline, P=10-5, TBD, TBD • 4 cool down cycles => 2 weeks of vertical test stand time • 1 week if test 2 cavities at once A. Romanenko

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