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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 by Jiaru Shi Department of Engineering Physics, Tsinghua University 2013.11.04, CERN. Acknowledgement. Tsinghua University

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

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  1. 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

  2. Acknowledgement • Tsinghua University • Yingchao Du, JiahangShao, Lixin Yan, JianfeiHua, Zhen Zhang, Dan Wang, Jin Yang, Chuanxiang Tang, Huaibi Chen, Wenhui Huang and et. al. • ANL • Wei Gai, Chunguang Jing • SLAC • Faya Wang

  3. Content • Pre-experiment • Experiment setup • Data Analysis • Summary and Plans

  4. 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

  5. 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) oxidization surface damage 30 μJ 60 μJ 10μJ / 1mm^2  1m J/cm^2 @1ps 1GW/cm^2

  6. SEM image 60μJ edge Laser energy < ablation threshold 150mJ/cm^2 (248nm UV, 1ps [3]) evidence of melting Spot side middle [3] S. Preuss, et al, Appl. Phys. A 61, 33-37 (1995)

  7. Schematic of the Beamline • laser • Laser: Ti:Sapphire, 800nm, 400nmand 266nm • 90 degree incident • Pulse duration: ~1ps • Max Energy：~500uJ • 1/3 to cathode from clean room • Energy jitter: ~5%

  8. 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

  9. Breakdown events with laser-trigger • @ cathode center • ~55uJ • 52MV/m • 23 events

  10. Breakdown events with laser-trigger • Fall time • ~ 90 ns • Energy loss • ~ 1.2 J Black: reflection w/bd B: pickup Fundmtl’ w/bd R: pickup HOM w/ bd

  11. “real” breakdown current (by de-convolution) • Time: Rise time ~15ns, flattop ~35ns, ~5ns (fast) turn-off • Charge: Photo-electron ~100pC, breakdown (collected) ~10nC

  12. Breakdown events without laser trigger Multi pulses (85% ) • τ: time difference between fall of fundamental mode and raise of HOM (evidence for field emission) • Random location • Longer fall time • > 200 ns τ Pickup w/o bd Black: reflection w/bd B: pickup Fundmtl’ w/bd R: pickup HOM w/ bd

  13. Breakdown events without laser trigger Single pulse (15%) τ

  14. 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

  15. L-band gun breakdown experiment in ANL • 0.5 cell high gradient photogunwith flat/pin cathode • Study field enhancement, and breakdown dependences • Study the surface treatment techniques -> reducing conditioning time for high gradient structures

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