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 2013.06.04 HG2013, Trieste, Italy
Feb 23, 2016
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
2013.06.04 HG2013, Trieste, Italy
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• Problems 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
Laser damage
10μJ 20μJ 30μJ
40μJ 50μJ 60μJ Surface damage
oxidization
10μJ / 1mm^2 0.1 J/cm^2 @1ps 10GW/cm^2
Schematic of the Beamline• laser
– Laser: Ti:Sapphire, 800nm, 400nm and 266nm
– 90 degree incident – Pulse duration: ~1ps– Max Energy: ~2mJ
• 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
Beamline
1 2 3 4
a=53cmb=85cm
gun laser hole Faraday cup
oscilloscope
Scope: 12GHz Bandwidth / 50GHz sampling rate
Scope signal w/ and w/o breakdown
0 500 1000 1500 2000 2500 3000 3500 4000-2
0
2RF pickup
0 500 1000 1500 2000 2500 3000 3500 4000-1
0
1Reflection
0 500 1000 1500 2000 2500 3000 3500 4000-0.2
0
0.2Photodiode
0 500 1000 1500 2000 2500 3000 3500 4000-5
0
5
Time (ns)
Faraday cup
0 500 1000 1500 2000 2500 3000 3500 4000-2
0
2RF pickup
0 500 1000 1500 2000 2500 3000 3500 4000-1
0
1Reflection
0 500 1000 1500 2000 2500 3000 3500 4000-0.2
0
0.2Photodiode
0 500 1000 1500 2000 2500 3000 3500 4000-5
0
5
Time (ns)
Faraday cup
RF pickup
Reflection
Photo Diode
Faraday Cup
Time aligned ~1ns
ExperimentApr 12 Apr 18 Apr 19
p1 p2 p3 p4 p5 p6 p7Laser Energy @ cathode
(μJ)Scan
up to 135μJ55 150 100 117 128
Input Power (MW)
4.3MW Scan 2-5MW
E field (MW/m)
50MV/m 30-50MV/m
Change location on cathode: (p1, p2, p3)
RF phase fixed 30 degree.
Image after experiment (view from oblique-incident laser window)
90 95 100 105 110 115 120 125 130 135 1400
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
point 1point 2
Laser energy on cathode (uJ)
Brea
kdow
n pr
obab
ility
Breakdown rate v.s. Laser Energy
• E at center of cathode ~50MV/m, note: small statistics
Charge v.s. Laser Energy and E-field
25 30 35 40 45 50 55 6030
40
50
60
70
80
90
100
4.18 55uJ4.18 150uJ4.19 117uJ4.19 128uJ
E on cathode surface (MV/m)
Char
ge (n
C)
Charge strongly-related to E field
Different Laser energy? Maybe: from different location
Timing (cables calib.’ed)
0 1000 2000 3000 4000-1
0
1
2
3
4
t (ns)
Far.CupReflectionDiodeRF pickup
0 1000 2000 3000 4000-1
-0.5
0
0.5
1
1.5
2
t (ns)
Far.CupReflectionDiodeRF pickup
w/o breakdown
w/ breakdown
1180 1200 1220 1240 1260 1280 1300 1320
0
0.5
1
1.5
2
2.5
3
3.5
t (ns)
Far.CupReflectionDiodeRF pickup
after photo-electron: Breakdown current rises ~ns Reflection starts to change ~40ns RF pickup increase because of beam excitation
Take Faraday cup as an LTI system.signal without breakdown can be seen as the impulse response of the system.Try to solve the ‘real’ current signal by deconvolution.
Hδ(t) h(t)
Hδ(t)+b(t) y(t)
δ(t) : impluse functionb(t) : ‘real’ breakdown currenth(t) : Faraday cup signal without breakdowny(t) : Faraday cup signal with breakdown
1160 1180 1200 1220 1240 1260 1280 1300 1320-1
-0.5
0
0.5
1
Time (ns)
Current (A
)
h(t)
1160 1180 1200 1220 1240 1260 1280 1300 1320-2.5
-2
-1.5
-1
-0.5
0
0.5
Time (ns)
y(t)
Current (A
)
Data ProcessingJ. Shao
• Time: Rise time ~15ns, flattop ~35ns, ~5ns (fast) turn-off• Charge: Photo-electron ~100pC, breakdown (collected) ~30nC
0 10 20 30 40 50 60 70 80-0.25
-0.2
-0.15
-0.1
-0.05
0
0.05
0.1solved x^
Time (ns)
“real” breakdown current (by de-convolution)J. Shao
Plans and discussion
• Different laser wavelength• Missing Energy? • Update Faraday Cup eliminate resonance, better
matching, faster time response• Streak Camera Diagnostics of the breakdown current,
micro structure?
Summary and discussion
• Very preliminary data analysis– charge (? get real breakdown current)– (Rough) delay time, turn one time– Laser threshold
• Laser-triggered RF breakdown v.s. self-breakdown, • v.s. DC breakdown?• Suggestions?