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High-gradient test at Nextef

High-gradient test at Nextef. Shuji Matsumoto For KEK Nextef Group 2010.5.3. Contents.

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High-gradient test at Nextef

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  1. High-gradient test at Nextef Shuji Matsumoto For KEK Nextef Group 2010.5.3 44th X-band structure collaboration Meeting, CERN

  2. Contents Nextef, KEK’s X-band 100MW-class structure test stand, started its operation in 2007. Since then, we have been testing a series of CLIC prototype structures here, T18_Disk#2(2008), TD18Quad#5 (2009) and currently we run the facility for TD18_Disk#2. I am going to report on the testing of 1) TD18_Quad#5 2) TD18_Disk#2 44th X-band structure collaboration Meeting, CERN

  3. T18 parameters for 100MV/m in average gradient Riccardo info on 090202 TD18_Quad 55.5MW  100MV/m no-loaded with Tfill = 32.8ns Note we applied calibration 62.7MW input →100MV/m for TD18_Disk#2 (Faya Wang). Ep/Eacc of TD18_Quad should be similar to that of T18_Disk 44th X-band structure collaboration Meeting, CERN

  4. TD18_Quad#5 44th X-band structure collaboration Meeting, CERN

  5. High power processing and testsof TD18_Quad#5 • TD18_Quad#5 was set in a vacuum chamber and the chamber was installed in Shield-A of Nextef in Aug. 2009. • Processing and various tests were performed from Sep. to Nov. 2009. • The input power has been restricted rather soon at about 40MW due to frequent BDs in the accelerating structure. • Total RFON time is about 1000hrs. 44th X-band structure collaboration Meeting, CERN

  6. 173ns 113ns 293ns 44th X-band structure collaboration Meeting, CERN

  7. Dark current evolutionTD18_Quad#5 Nextef • Once it went up in the middle of the testing, it went down but did not back to • its initial value. • Dark current of Quad structure is larger than that of Disk at least one, probably • a few order of magnitude. (Notice that the field Eacc is about half.) 44th X-band structure collaboration Meeting, CERN

  8. Dark Current at the very final stage of the test: TD18_Quad#5 FC-UP b=76 FC-Mid b =50 44th X-band structure collaboration Meeting, CERN

  9. Vacuum characteristics • Vacuum total pressure • Base pressure at <10-6Pa • Typically processing <10-5Pa • Increases every time at few to 5MW range if after RF-OFF for more than several hours • Mass spectrum • M=2, 28 and 44 increase with RF-ON, but not M=18 • Especially when reaching power limit • M=2 becomes dominant residual gas after an hour or so run • M=27 and 28 change in a similar manner as time, indicating hydrocarbon-origin surface contamination 44th X-band structure collaboration Meeting, CERN

  10. Summary • RF on hours amounts to nearly 1000hrs. • ACC-related breakdowns counted more than 7000 times. • Power is limited by 19MW (~58MV/m) at 51 ns pulse. • Further increase of field is not expected from the trend pattern. • Long-pulse operation was tried at 113, 173 and 293nsec. • NEG application was tried to see the effect of hydrogen contents. The hydrogen pressure decreased about half but the attainable gradient did not changed(improved). • Q: Does 50microns Chamfer at the iris help to improve the high gradient performance? -> Probably NOT unfortunately. 44th X-band structure collaboration Meeting, CERN

  11. KEK’s version: 50 micron chamfer Made of CuZr without heat treatment. 50 micron rounding: shape with angles and bumps. Reference planes were formed by milling in a few micron level without re-chucking for shaping cells. Assembly was done within ten micron level. 44th X-band structure collaboration Meeting, CERN

  12. Electric field enhancement in a shallow channel with round chamfer Calculation done by T. Abe by CST MS. Waveguide field. 44th X-band structure collaboration Meeting, CERN

  13. 44th X-band structure collaboration Meeting, CERN

  14. No.18 Q2-2 to match Q1-1 Yasuo Higashi 44th X-band structure collaboration Meeting, CERN

  15. Surface inspection summary • Optical inspection with long-distance focus microscope • Still not easy to see the arcing spots • Will go to CERN SEM for full inspection 44th X-band structure collaboration Meeting, CERN

  16. Breakdown pulse analysis 44th X-band structure collaboration Meeting, CERN

  17. Timing distribution for change>2000 44th X-band structure collaboration Meeting, CERN

  18. Breakdown cell distribution >2000 50ns higher target run Mostly downstream half. Simply increasing toward output end. Indication of BD following some field gradient. 534 events were analyzed out of 1919 INTLK. 44th X-band structure collaboration Meeting, CERN

  19. TD18_Disk#2 44th X-band structure collaboration Meeting, CERN

  20. Test of TD18_Disk2 at Nextef Dec. 2009: Delivery to KEK made and installation work started. Jan. 2010: Testing of the structure started. Slow conditioning and gradient restricted (about 80MV/m). Frequent small reflection from Left-side output line were observed. Decided to replace whole left output line. Mar. 2010 Restart test with new output line. Reached 100MV/m. Operable with 60MW input (100MV/m), RF pulse width 250ns. Apr. 2010 Measurement of Breakdown rate started. We are now in the final stage of the testing. 44th X-band structure collaboration Meeting, CERN

  21. Nextef Load changed The Eacc values during 1300~1500 may not be right due to system change. (Some ADCs of data acquisition system were over-ranged .) 44th X-band structure collaboration Meeting, CERN

  22. Changing Load • Acc input power was restricted within about 40MW (but this does not look a “hard limit”), 51ns, operating time was about 1000Hr. • Frequent small reflection signals from the left –side load line are observed at most of the breakdown events. • We have decided to change the whole line in the end of Feb. • Performance of the “new” line had been checked at KT-1 previously. • The small reflections disappeared after the change. The system looks normal after the change. • We could reach to 100MV/m gradient as well as we could increase the pulse width. • The “bad” line was tested at KT-1, but the behavior observed in Nextef did not reproduce. 44th X-band structure collaboration Meeting, CERN

  23. BDR evaluation planTD18_Disk#2 Nextef 44th X-band structure collaboration Meeting, CERN

  24. TD18_Disk_#2 Breakdown rate 44th X-band structure collaboration Meeting, CERN

  25. BDR Pulse Width Dependence/NLCTA TD18_Disk 10^-3 Faya Wang Nextef 250ns 100MV/m BDR=7e-7 (250ns,100MV/m) for T18 at Nextef Nextef 90ns 100MV/m 10^-6 Our data looks consistent nicely with NLCTA one. 44th X-band structure collaboration Meeting, CERN

  26. Breakdown rate measurement252nsCheck statistic of the data Run 37 (50MW) 12 BD’s BDR [10^-6 /pulse/m] Run 34 (60MW) 80 BD’s BDR [10^-6 /pulse/m] Run 35+36 (55MW) 41 BD’s BDR [10^-6 /pulse/m] Instant BDR(10^-6) Elapsed time (Hour) It is naturally expected to be converged to the “final” BDR value, and actually we see this behavior. 44th X-band structure collaboration Meeting, CERN

  27. Timing statistics for Rs rise and Tr fall Rs: 100—350ns for 252nsec pulse Most of Rs change sit around 360+-50nsec Tr: 150—400ns for 252nsec pulse Most frequent when approaching to the end, from 300 to 370ns. There is a small bump at 170 nsec, at the beginning of pulse. 44th X-band structure collaboration Meeting, CERN

  28. Search for the hot cells: identification of the breakdown location 44th X-band structure collaboration Meeting, CERN

  29. Some pulse: Run34 #38 and #39 Rs Ra Tr F This case was not analyzed reasonably. FC-UP FC-Mid I_Rs Q_Rs Mostly analyzed well as these events. Difficulty in automatic analysis is remaining! Should be revised! 44th X-band structure collaboration Meeting, CERN

  30. FC-Mid 51nsRF 44th X-band structure collaboration Meeting, CERN

  31. FN Plot 44th X-band structure collaboration Meeting, CERN

  32. FN Beta History: FC-Mid T18@Nextef Beta=38 1950hrs 44th X-band structure collaboration Meeting, CERN

  33. UP Mid DN Still going up 44th X-band structure collaboration Meeting, CERN

  34. Summaryof TD18_Disk#2 testing • Finally it reached over than 100MV/m at 50ns width • But it took much more hours and breakdowns than T18, 5000 BD’s and 1300 hours • Widening pulse was easily established at 100MV/m level • With only 1000 BD’s and 200 hours • Dark current was initially reduced but recently has been saturated. • Breakdown rates were and is being measured. It is 10-4/pulse/m at 100MV/m at 252nsec. • Pulse shape analysis and phase information are still to be developed. 44th X-band structure collaboration Meeting, CERN

  35. Conclusions 44th X-band structure collaboration Meeting, CERN

  36. Conclusions: TD18_Quad#5 • TD18Quad#5 test was done. • Attained gradient was as low as some 60MV/m with short pulse width 51ns. • Remarkable outgas activity was seen during the operation. Base pressure of the structure chamber was 10^-6Pa . • Large amount of dark current observed. The beta value of FN plots (for FC-Mid, beta=50) is slightly larger than that of TD18Disk (now about 40). • Follow-up works, such as SEM observations on the surface of the structure, are ongoing. 44th X-band structure collaboration Meeting, CERN

  37. Conclusions: TD18_Disk#2 • Testing of TD18_Disk#2 structure is at its final stage. • 100MV/m with 250ns has been reached. • BDR has been measured and the values agree with NLCTA data. (Although it looks still decreasing) it may be fair to conclude that BDR of TD18_Disk#2 is greater than that of T18 by about two order of magnitude. • The amount of dark current has been decreasing by a few order of magnitude during the test (over some 2000hrs) and became very similar to that of T18. • The field enhancement factor, beta, has decreased by factor of 2. The final value is some 40. 44th X-band structure collaboration Meeting, CERN

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