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Laser-triggered RF breakdown experiment with a photo-cathode RF gun at Tsinghua University. Presented on behalf of the collaboration by Jiaru Shi Department of Engineering Physics, Tsinghua University 2013.11.04, CERN. Acknowledgement. Tsinghua University - PowerPoint PPT Presentation
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Laser-triggered RF breakdown experiment with a photo-cathode RF gun at Tsinghua University
Presented on behalf of the collaboration byJiaru Shi
Department of Engineering Physics, Tsinghua University
2013.11.04, CERN
Acknowledgement• Tsinghua University
– Yingchao Du, Jiahang Shao, Lixin Yan, Jianfei Hua, Zhen Zhang, Dan Wang, Jin Yang, Chuanxiang Tang, Huaibi Chen, Wenhui Huang and et. al.
• ANL– Wei Gai, Chunguang Jing
• SLAC– Faya Wang
Content
• Pre-experiment• Experiment setup• Data Analysis• Summary and Plans
Motivation
• RF breakdown dependence on E, B, Sc, ΔTp...– Laser assistant RF breakdown experiment is trying to
isolate some of the contributing effects. Hopefully, a more coherent picture of RF breakdown. [1]
• RF breakdown phenomenon– To better understand detailed RF breakdown progress
and time scale. Quantities like turn on time, breakdown current, explosive emission… [2]
[1] Faya Wang[2] Wei Gai, Chunguang Jing
Pre-Experiment: Laser damage on copper surface
• Shot UV Laser pulse on copper surface.– 10μJ, 20, 30…; 1mm diameter spot size; 1ps pulse length– Microscope image: (30sec@10Hz)
60 μJ 30 μJ
10μJ / 1mm^2 1m J/cm^2 @1ps 1GW/cm^2
oxidization surface damage
SEM image
60μJ
edge
Spot side
Laser energy < ablation threshold 150mJ/cm^2 (248nm UV, 1ps [3])
evidence of melting middle
[3] S. Preuss, et al, Appl. Phys. A 61, 33-37 (1995)
Schematic of the Beamline• laser
– Laser: Ti:Sapphire, 800nm, 400nm and 266nm
– 90 degree incident – Pulse duration: ~1ps– Max Energy: ~500uJ
• 1/3 to cathode from clean room
– Energy jitter: ~5%
RF gun at Tsinghua• RF Source
– 5MW klystron• RF Gun
– 1.6-cell S-band 2856MHz– Solid, demountable Cu
back-plate– Q~6000– 30~50MV/m
Breakdown events with laser-trigger• @ cathode center
– ~55uJ– 52MV/m
• 23 events
0 500 1000 1500 2000 2500 3000 3500 40000
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1Fundamental mode of pickup and reflection
Time (ns)
Breakdown events with laser-trigger
• Fall time– ~ 90 ns
• Energy loss– ~ 1.2 J0 200 400 600 800 1000 1200 1400 1600 1800 2000
0
0.5
1Fundamental mode & Higher order mode (8.6GHz) of pickup, fundamental of reflection
0 200 400 600 800 1000 1200 1400 1600 1800 2000-3
-2
-1
0
1Faraday Cup Signal
Time (ns)
B: pickup Fundmtl’ w/bd
R: pickup HOM w/ bd
Black: reflection w/bd
• Time: Rise time ~15ns, flattop ~35ns, ~5ns (fast) turn-off• Charge: Photo-electron ~100pC, breakdown (collected) ~10nC
“real” breakdown current (by de-convolution)
-50 0 50 100 150 200
-1
-0.8
-0.6
-0.4
-0.2
0
0.2Faraday Cup signal after deconvolution
Time (ns)
0 200 400 600 800 1000 1200 1400 1600 1800 20000
0.5
1
1.5Fundamental mode & Higher order mode (8.6GHz)
0 200 400 600 800 1000 1200 1400 1600 1800 2000-0.1
-0.05
0
0.05
0.1Faraday Cup Signal
Time (ns)
Breakdown events without laser triggerMulti pulses (85% )
τ • τ: time difference between fall of fundamental modeand raise of HOM(evidence for fieldemission)• Random location • Longer fall time
– > 200 ns
Pickup w/o bd
B: pickup Fundmtl’ w/bdR: pickup HOM w/ bd
Black: reflection w/bd
Breakdown events without laser triggerSingle pulse (15%)
0 200 400 600 800 1000 1200 1400 1600 1800 20000
0.5
1
1.5Fundamental mode & Higher order mode (8.6GHz)
0 200 400 600 800 1000 1200 1400 1600 1800 2000-0.1
-0.05
0
0.05
0.1Faraday Cup Signal
Time (ns)
τ
Summary and plan
• Field emission ~100ns prior to the main breakdown event -> short pulse (<50ns), reduce BDR for given gradient
• Laser triggered breakdown: controllable; meaningful to study temporal evolution during breakdown event
• Other controllable breakdown experiment
L-band gun breakdown experiment in ANL
• 0.5 cell high gradient photogun with flat/pin cathode• Study field enhancement, and breakdown dependences• Study the surface treatment techniques -> reducing
conditioning time for high gradient structures