Laser-triggered RF breakdown experiment with a photo-cathode RF gun at Tsinghua University Presented on behalf of the collaboration by Jiaru Shi Department.

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– 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 μJ30 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