Statistics of breakdown and conditioning in pulsed dc and rf systems

Transcription

1 Statistics of breakdown and conditioning in pulsed dc and rf systems Anders Korsback, Jorge Giner Navaro, Robin Rajamaki and Walter Wuensch mini-mevarc, 21 March 2016

2 Motivation Statistics Physics The statistical properties of breakdown may give us insight into the evolution of the surface under pulses, the underlying trigger mechanism and what happens to the surface after breakdown. Practical We try to operate the structures ever closer the gradient limit but we demand reliability and life-time. The statistical properties may give the essential life functions. Conditioning Physics Determining what exactly gets better as a structure conditions could give us insight into etc. Practical Conditioning is long, months at 50 Hz, and consequently expensive. How can we shorten this process or replace it with another one? mini-mevarc, 21 March 2016

3 Breakdown statistics: RF and DC Anders Korsbäck CERN / University of Helsinki Robin Rajamäki CERN / Aalto University Jorge Giner Navarro CERN / University of Valencia Walter Wuensch CERN WW note: The first time I saw a BD interval plot was by W. Farabolini

4 What is breakdown statistics? The operational history of an accelerating structure tested in Xbox-1 is shown. Instead of accumulating breakdowns at a constant rate, it shows a staircase structure on many scales in a self-similar way. Hence, a single (overall) breakdown rate, i.e. n breakdowns /n pulses, is clearly insufficient to describe what s going on. It doesn t say anything about when breakdowns happen in relation to the overall history in relation to each other

5 Two-Rate Statistics Or, to visualize what was just explained, let s return to the operational history vector and to number of pulses to breakdown: : non-breakdown pulse, : primary BD, : follow-up BD Red numbers are values for nr of pulses to BD for primary BDs, blue for follow/up BDs. Red and blue are individually Poissonian, giving a two-exponential probability density when put together 2, 3, 1, 2, 1, 1, 2 7, 18, 9

6 Comparison of rf and dc rf Data Long-term BDR= 2.59e-005 Short-term BDR= 2.07e-003 Two-exponential fit 500 Cumulative BDs KEK Probability density Cumulative pulses x Number of pulses before breakdown x x 104 dc Cumulative nr of breakdowns Cumulative nr of pulses x 10 8 mini-mevarc, 21 March 2016

7 Breakdown positioning in CLIC prototype RF accelerating structures CLIC workshop 2016 R. Rajamäki*, W. Farabolini, J. Giner Navarro, T. Argyropoulos, B. Woolley, W. Wuensch *Aalto university / CERN

8 Introduction What? Localize BDs in RF accelerating structures Why? Structure diagnostics Breakdown studies How? RF power and phase Directional coupler Structure vibrations Accelerometer Electron emission Faraday cup Spectrometer Photons PMT/ camera X-ray

9 Structure diagnostics (1/2) TD26CC y-axis projection BD cell map x-axis projection

10 Method comparison (3/3) τ d,edge τ d,corr2d 3. Observations: 1. Methods are generally in agreement 2. Non-symmetric spread 3. Peak of correlation method What about possible expanations?

11 Breakdown migration? i. ii. Downstream migration Mainly upstream migration iii. Possible migration scenario Courtesy of W. Farabolini Upstream migration

12 Spatio-temporal correlations i. iii. ii. Δτ d (n) = τ d (n) τd (n 1) Vertical lines = artefacts of conditioning algorithm Breakdowns arriving shortly after each other occur close to each other.

13 Newest pulsed dc data Long pulsed dc run with electrodes prepared with same procedure as rf structures. mini-mevarc, 21 March 2016

14 Heat treatment and joining KEK/SLAC Tsinghua U. mini-mevarc, SINAP 21 March 2016 CERN

15 BDR as a function of field mini-mevarc, 21 March 2016

16 And corresponding distributions mini-mevarc, 21 March 2016

17 Performance summary at CLIC specifications BDR E 30 τ 5 mini-mevarc, 21 March 2016

18 Conditioning Accelerating structures do not run right away at full specification pulse length and gradient need to be gradually increased while pulsing. Typical behaviour looks like this: Pulse length steps BDR falls during flat E run mini-mevarc, 21 March million pulses per day at 50 Hz

19 Comparing conditioning Scaled gradient vs cumulative number of PULSES Pulses BDR E 30 τ 5 Scaled gradient vs cumulative number of BREAKDOWNS Breakdowns mini-mevarc, 21 March 2016

20 Newest pulsed dc data Long pulsed dc run with electrodes prepared with same procedure as rf structures. mini-mevarc, 21 March 2016

21 Longer term operation 7 50 Hz rf pulsed dc 30 1 khz mini-mevarc, 21 March 2016

22 Long term evolution of BDR pulsed dc rf Pulses mini-mevarc, 21 March 2016

23 Effect of venting system Normalized BDR ( (MV/m) -30 ns -6 ) Test vent of dc system, 3 days Normalized BDR ( (MV/m) -30 ns -6 ) 1. Feedback phase, gradient Constant voltages phase, gradient Stepped voltage phase, gradient Reconditioning after 3 day vent, gradient Before venting After venting gradient gradient Cumulative nr of pulses mini-mevarc, March Cumulative nr of pulses

24 Dynamic Vacuum Meter is at CERN Antti Meriläinen 1,2, Robin Rajamäki 3, Ivan Kassamakov 1,2, Walter Wuensch 3, Kenneth Österberg 1,2 and Edward Hæggström 1 1) Department of Physics, University of Helsinki 2) Helsinki Institute of Physics 3) CERN

25 Dynamic Vacuum Meter Vacuum tube or AS-element Breakdown p(t) R QCM = 6.5 mm 50 cm x = 5 ± 0.5 cm R Beam = 5 mm Quartz crystal microbalance (QCM) as reference Dr. Walter Wuensch, Introduction to CLIC, Collaboration meeting at HIP

26 Electrode Design R Design For 0.1 mm gap, r 0.3 mm R Design = 0.5 mm 0.3 mm

27 Cad Design

28 Optics and Electrodes

29 DVM & DC Spark Estimated signal for conditioning process Cu atoms release Estimated signal for Breakdown Cu atoms release Breakdown Spark

30 New HV pulser 15 kv, 1 khz Marx generator from ISEL, Lisbon. Fast rise and fall time! mini-mevarc, 21 March 2016

31 Electrode pipeline Running Hard copper, with acoustic sensors - running Available Cu, CuAg, stainless steel - SLAC Nb 3-D printed Ti Small ridge for optical access About 10 pairs of diamond machined Cu Ceramic spacers Under preparation Voids - Helsinki Diamond like coating - PSI mini-mevarc, 21 March 2016

32 Nb electrode made in central workshop. We propose to build this cool-able to cryogenic temperatures. mini-mevarc, 21 March 2016