Laser-triggered RF breakdown experiment with a photo-cathode RF gun at Tsinghua University

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?