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RF Conditioning Study on CLIC T18 X-band Structure and L-band Cavity

This study explores RF conditioning history, breakdown rates, pulse heating effects, and evolution of breakdown locations in the CLIC T18 X-band structure and L-band 5-cell cavity. Analysis includes pulse shape and gradient dependencies, pulse heating simulations, field evolution times, and potential recovery of hot cells through conditioning. Various breakdown event characteristics and implications for future gradient limits are examined.

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RF Conditioning Study on CLIC T18 X-band Structure and L-band Cavity

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  1. Breakdown Study on the CLIC Designed T18 X-band Structure and Lband 5-Cell SW Cavity Faya Wang Jul.-09-2008

  2. Topics • T18 RF Conditioning History • BKD study on T18 • Pulse heating study on T18 • L-band cavity RF Conditioning History • BKD Study on Lband cavity • Summary

  3. Cumulated Phase Change T18 Structure Profile Field Amplitude 120°

  4. RF Conditioning Statistics Max average Unloaded Gradient at different pulse width: 120MV/m at 70ns for 6hrs (*152MV/m) 120MV/m at 100ns for 76hrs (*152MV/m) 120MV/m at 140ns for 47hrs (*152MV/m) 110MV/m at 190ns for 41hrs (*140MV/m) 120MV/m at 200ns for 21hrs (*152MV/m) 120MV/m at 210ns for 24hrs (*152MV/m) 110MV/m at 230ns for 78hrs (*140MV/m) ~1000 hours total conditioning from 14 Apr. 2008 to 26-Jun-2008 ~2148 breakdowns (119 per cell) *:Max accelerator gradient in the structure

  5. T18VG2.4_Disk structure RF process history begin at Apr.14 2008 The gradient is the average unloaded gradient for the full structure. The beginning 500hrs, maximum unloaded gradient is 110MV/m The BKD Rate is normalized to the structure length(29cm).

  6. The following 900hrs, maximum unloaded gradient is 120MV/m Short pulse higher gradient condition Pulse shape dependence BKD study. BKD pulse width dependence study at 110MV/m. BKD gradient dependence study at 230ns pulse width

  7. BKD Rate Characteristics at Different Conditioning Time RF BKD Rate Gradient Dependence for 230ns Pulse at Different Conditioning Time RF BKD Rate Pulse Width Dependence at Different Conditioning Time After 250hrs RF Condition G=108MV/m G=108MV/m After 500hrs RF Condition After 900hrs RF Condition G=110MV/m After 1200hrs RF Condition The Error of Breakdown rate for the last two point at 1200hrs is very large, because it only got few events for a week running. After 900hrs RF condition BKD rate has a gradient dependence ~ and pulse width dependence ~

  8. The following test after 1200hrs at 110MV/m@230ns shows BKD rate is very high up to 1.9e-5/pulse/m. Because the hot cells are dominating the breakdowns.

  9. Then, set at 105MV/m@230ns for 140hrs ( 7 BKD Events )

  10. BKD Distribution along Structure at Different Stage

  11. BKD Cell Distribution Evolution with RF Conditioning Time

  12. BKD Cell Distribution Evolution with Accumulated BKDs

  13. BKD Cell Distribution Evolution with Cell Dissipated Energy

  14. Breakdown Cell Distribution at Different Stage

  15. Breakdown Cell Distribution Dependence decreasing with Conditioning For the last point the fit is applied without these hot cells (cell # 6~10)

  16. Field Evolution time: the time for field collapsed to 5% of normal field level

  17. Field Evolution Time for All Recorded Breakdown Events Why there is multi-peak evolution time from data?

  18. Field Evolution Time at Different Stage with Gaussian Fit

  19. Field Evolution Time increasing with Conditioning Could be dominated by hot cell breakdown

  20. Pulse Heating BKD Study *:Max gradient in the structure Main pulse Pre pulse After pulse SLED output pulse

  21. P2 P1 0 0 t1 t2 Number of pulse to damage the surface at certain pulse heating Pulse Heating for a square pulse P(t) U is a constant, T0 surface temperature without pulse heating For step pulse case Very bad Fit *V.F.Kovalenko, "Termophysical Processes and Electrovacuum Devices", Moscow, SOVETSKOE RADIO (1975), pp. 160-193.

  22. For a constant breakdown rate, we have: From T18 experiment result: From pulse heating caused breakdown: *From pulse heating on thermal fatigue: *S. V. Kuzikov & M. E. Plotkin,” Theory of Thermal Fatigue Caused by RF Pulsed Heating”, Int J Infrared Milli Waves (2008) 29:298–311

  23. RF Conditioning history in pulse heating scale

  24. For 230ns square pulse, BKD rate gradient dependence in Pulse heating scale BKD rate pulse width dependence in Pulse heating scale

  25. L-band Warm conductor 5 cell πmode SW cavity RF conditioning results

  26. Structure Profile J. Wang

  27. RF Conditioning Statistics till Jan-4-2008 5Hz repeating frequency 1Hz repeating frequency ~Max unloaded Acc. Gradient: 15MV/m ~6167 Breakdown Events (1233 per cell)

  28. Cavity unloaded gradient Measurement results

  29. Hard Event Soft Event

  30. Interesting WG BKD KLY CIR WG Cavity

  31. Red dash line: normal store energy decay Red solid line: store energy decay for WG BKD Blue line: klystron output power Black solid line: decay curve for cavity at Q0 BKD spot in WG with klystron still on block RF power emitted from Cavity

  32. RF Conditioning History 5Hz 1Hz

  33. Breakdown rate vs Gradient For BKD rate pulse width dependence measurement has not been done yet.

  34. BKD Characteristics with RF conditioning Time 5Hz 1Hz

  35. Map BKD Location

  36. Beating frequency of stored energy for blocking different iris

  37. Summary - Questions • Why pulse width dependence changing with RF conditioning time (accumulated BKDs)? • Why BKD location distribution changing with time? • Why field evolution time becoming longer? • Why hot cells are in the middle of the T18? • Do we reach the gradient limit of T18 (can the hot cells recovery with further conditioning)? …… • Is L-band cavity processing over? • What does the soft event and hard event mean? ….

  38. T18 & Lband Cavity Data Summary

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