1. Breakdown position analysis
Alberto DEGIOVANNI
W. Farabolini, J. Giner-Navarro, L. Navarro, R. Rajamaki, B. Woolley
High Gradient Workshop 2015
Tsinghua University
Beijing, June 2015

2. A. Degiovanni - HG2015 workshop - Tsinghua University, China
Outline
Motivation
BD cell location methods:
Transmitted and Reflected signals delay
Incident and Reflected signals delay
Shock wave propagation (accelerometers)
Data analysis T24 dogleg
Results from Crab cavity
Conclusion
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A. Degiovanni - HG2015 workshop - Tsinghua University, China

3. A. Degiovanni - HG2015 workshop - Tsinghua University, China
Motivation and goals
BD position analysis is important for:
better understanding structures performance
monitoring progress in conditioning process
extract information to study more in details BD phenomena (temporal-spatial correlation of subsequent BD)
GOAL:
Reconstruct position of BD events inside the structure
Study evolution of BD
Long term (accumulated BD in structure)
During pulse (BD onset time and drifting)
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A. Degiovanni - HG2015 workshop - Tsinghua University, China

4. Methods for BD cell location based on RF signals
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A. Degiovanni - HG2015 workshop - Tsinghua University, China

5. BD Location based on RF signals
Reflected rising edge
Transmitted falling edge
NOTE: the 1st method detects the BD location at its onset, the 2nd method at the end of the RF pulse (meanwhile the BD can moved).
Pros: accurate determination by signals correlation (no thresholds used for edges determination).
Cons: requires a time structure at the end of the incident pulse (not always present).
1st method:
EDGE detection
By Dt between Reflected rising edge and Transmitted falling edge
(BD start)
Incident power
falling edge
Reflected falling edge
2nd method:
CORRELATION method
By Dt between Input falling edge and Reflected falling edge (echo)
(BD end)
W. Farabolini
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A. Degiovanni - HG2015 workshop - Tsinghua University, China

6. BD Location based on RF signals
1. Delay between rising edge of REF signal and falling edge of TRA signal
2. Delay that maximizes correlation between INC and REF signal in the tail
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A. Degiovanni - HG2015 workshop - Tsinghua University, China

7. BD Location based on RF signals
1. Delay between rising edge of REF signal (t1REF) and falling edge of TRA signal (t2TRA) 2. Delay that maximizes correlation between INC and REF signal in the tail (t2REF - t2INC)
t1REF
t2TRA
tfill
ΔtREF = t2REF -t2INC
t2INC
t2REF
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A. Degiovanni - HG2015 workshop - Tsinghua University, China

8. BD Location based on RF signals
1. Delay between rising edge of REF signal (t1REF) and falling edge of TRA signal (t2TRA)
2. Delay that maximizes correlation between INC and REF signal in the tail (t2REF - t2INC)
Time delays produced by both estimators can be made comparable:
t1REF
t2TRA
tfill
ΔtREF = t2REF -t2INC
t2INC
t2REF
t2TRA – tfill = t1REF – ΔtREF
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A. Degiovanni - HG2015 workshop - Tsinghua University, China

9. A. Degiovanni - HG2015 workshop - Tsinghua University, China
Comment
Time delays produced by correlation and edge detection methods can be directly compared
tfill
x ΔtREF
o t2TRA – t1REF
o t1REF – t2TRA
t2TRA – tfill = t1REF – ΔtREF
t2TRA – t1REF = tfill – ΔtREF
t1REF – t2TRA = ΔtREF – tfill
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A. Degiovanni - HG2015 workshop - Tsinghua University, China

10. A. Degiovanni - HG2015 workshop - Tsinghua University, China
Edge detection
Edge detection in two stages:
1. threshold detection t2TRA: 90%of flat top avg. t1REF: 10% of flat top avg.
2. detection of peaks in the derivative of the signals (with a 5 samples moving average to avoid spikes detection)
Comparison with previous pulse also considered: TRA(n)/TRA(n-1) REF(n-1)/REF(n)
Power
INC
TRA
REF
Log detectors signals
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A. Degiovanni - HG2015 workshop - Tsinghua University, China

11. Correlation method details
A time slot of the signals is determined after the end of the klystron pulse.
Replicas of the REF signal with various delays and amplitude are correlated with the INC signal.
Correlation peak (delay and amplitude) is determined, with confidence estimators (the sharpness of the peak) and integrated error between the superposed signals.
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A. Degiovanni - HG2015 workshop - Tsinghua University, China

12. Data from T24 dogleg experiment
BD index
t1REF – t2TRA [ns]
Filter for good BD in range -100 ns to 400 ns
Good BDs: / 15835
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A. Degiovanni - HG2015 workshop - Tsinghua University, China

13. Data from T24 dogleg experiment
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A. Degiovanni - HG2015 workshop - Tsinghua University, China

14. Full history - time delays
Edge detection method
Correlation method (evaluated in the tail of the pulse)
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A. Degiovanni - HG2015 workshop - Tsinghua University, China

15. From measured delays to cell location
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A. Degiovanni - HG2015 workshop - Tsinghua University, China

16. From measured delays to cell location
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A. Degiovanni - HG2015 workshop - Tsinghua University, China

17. Full history – BD cell location evolution8
0 ns
120
136
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18. A. Degiovanni - HG2015 workshop - Tsinghua University, China
Aug 2014 (unloaded)
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19. A. Degiovanni - HG2015 workshop - Tsinghua University, China
Apr 2015 (unloaded)
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A. Degiovanni - HG2015 workshop - Tsinghua University, China

20. A. Degiovanni - HG2015 workshop - Tsinghua University, China
BD drifting
Phase signal jumps
Pattern in the tail of the pulse moving
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A. Degiovanni - HG2015 workshop - Tsinghua University, China

21. Early BD in structure + BD in waveguide
TRA signal drops
REF rises
INC signal drops
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A. Degiovanni - HG2015 workshop - Tsinghua University, China

22. Possible explanation of the BD drift
TRA
BD ignition. Plasma development. Wave absorption . TRA falls 1.
REF
Plasma density sufficient to reflect the INC wave.
REF raises 2.
REF
Another BD appears upstream. The second reflected wave combines with the first one in power and phase and can cause destructive interference. 3.
REF
The upstream BD plasma density is sufficient to reflect the INC wave. REF raises again but with a different phase and higher amplitude. 4.
Power is likely to be absorbed before it is reflected. This is impairing the accuracy of BD location methods based on REFup – TRAdown thresholds
W. Farabolini
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A. Degiovanni - HG2015 workshop - Tsinghua University, China

23. Shock wave propagation
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A. Degiovanni - HG2015 workshop - Tsinghua University, China

24. Measurement setup on crab cavity in Xbox2
RF in
RF out
N. Catalan
Accelerometer
6 accelerometers with dynamic range ± 500 g
#1 cell deg. (top of the structure)
#2 cell deg.
#3 cell deg. (cooling block)
#4 cell deg.
#5 cell deg.
#6 cell deg.
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A. Degiovanni - HG2015 workshop - Tsinghua University, China

25. A. Degiovanni - HG2015 workshop - Tsinghua University, China
1. BD at RF input
Strong shock on the cell #3 accelerometers (300 g)
Same phase for all polar angle position (circle inflates)
Slight delay on accelerometer #3 due to cooling block thickness
Shock propagates to accelerometers #5 and then #6
Lower signal than regular RF pulse on downstream accelerometers since the BD has interrupted a part of the RF travelling wave
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A. Degiovanni - HG2015 workshop - Tsinghua University, China

26. A. Degiovanni - HG2015 workshop - Tsinghua University, China
2. BD at RF output
Strong shock on the cell #10 accelerometer (200 g)
Shock propagates towards upstream accelerometers
Higher signal than regular RF pulse on upstream accelerometers since the BD has reflected a part of the RF travelling wave
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A. Degiovanni - HG2015 workshop - Tsinghua University, China

27. 3. BD in the middle of the structure
Shock first and larger on the cell #7
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A. Degiovanni - HG2015 workshop - Tsinghua University, China

28. 4. BD before the structure
All accelerometers amplitudes lower than regular RF pulse
Same waveform shape as for regular RF pulse
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A. Degiovanni - HG2015 workshop - Tsinghua University, China

29. A. Degiovanni - HG2015 workshop - Tsinghua University, China
Spectrum analysis
There is a common frequency line for accelerometers at 45.3 kHz
Otherwise the spectrums of both accelerometers are slightly different (Eigen-modes ?)
Mechanical characterisation of the structure should be done (mode analysis, measure with mechanical excitation)
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A. Degiovanni - HG2015 workshop - Tsinghua University, China

30. A. Degiovanni - HG2015 workshop - Tsinghua University, China
Summary
BD position analysis gives a better picture of BD phenomena in high gradient structures and help in understanding the evolution
Different methods have been presented based both on RF signals and on accelerometers signals
Accelerometers can deliver a clear signature of BD, and delays between array of accelerometers can be used to locate BD position (eventually 1 sensor per cell ?)
There are hints of BD drifting through the structure during BD pulse
Further analysis of the data is needed
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A. Degiovanni - HG2015 workshop - Tsinghua University, China

31. THANK YOU FOR YOUR ATTENTION !
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A. Degiovanni - HG2015 workshop - Tsinghua University, China

32. A. Degiovanni - HG2015 workshop - Tsinghua University, China
EXTRA SLIDES
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A. Degiovanni - HG2015 workshop - Tsinghua University, China

33. A. Degiovanni - HG2015 workshop - Tsinghua University, China
BD INTERLOCKS
REF BD
KREF BD
TRA en
REF en
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35. A. Degiovanni - HG2015 workshop - Tsinghua University, China
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36. A. Degiovanni - HG2015 workshop - Tsinghua University, China
3 in a row
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A. Degiovanni - HG2015 workshop - Tsinghua University, China