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Spectroscopy of breakdowns

This workshop led by J. Kovermann presents advanced techniques in spectroscopy for analyzing breakdown phenomena in high-power applications. It compares DC and RF breakdowns, highlighting experimental setups and diagnostics at CERN. Utilizing time-integrated and time-resolved spectroscopy, the study emphasizes the reproducibility of results and the correlation between plasma parameters and breakdown characteristics. Key findings include insights into light emissions, line ratios, and energy distributions critical for enhancing simulation and design processes for CLIC studies and RF systems.

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Spectroscopy of breakdowns

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  1. Spectroscopy of breakdowns Breakdown physics workshop J.Kovermann 6.5.2010

  2. DC measurements High-power scaling laws Breakdown diagnostics How it fits into the CLIC study: RF measurements Comprehensive RF design Breakdown simulation New experimental techniques complementary to RF tests

  3. Breakdown diagnostics Plasma composition simulation and mat.sci. input Plasma size/position  simulation and design input Plasma parameters simulation input Spectroscopy, Integrated and time-resolved BREAKDOWN DIAGNOSTICS RF measurements, FC, XRAY  simulation and design input Missing energy ? OTR in nominal pulses  simulation and machine parameter input SEM  simulation and design input

  4. Spectroscopy in rf and dc Subject of my thesis: Comparative studies of rf and dc breakdowns by optical spectroscopy Task: Show validity of dc breakdown experiments for rf simulation and design Approach: Time integrated and time-resolved spectroscopy of dc and rf breakdowns Dc experiments are faster and easier, e.g. few CHF per sample, up to kHz rep.-rate with new power supply (end 2010) Two dc setups available at CERN, others under construction Less scheduling issues

  5. Int. ratio lines/continuum = 1/4 Time-integrated spectroscopySpectrograph and CCD camera Dc breakdown, 400MV/m, 0.93J

  6. Fast failure diagnostics! Time-integrated spectroscopySpectrograph and CCD camera Rf breakdown, 40MW, 200ns, 8J, SLAC C10

  7. Rf, 37MW, 200ns, C10, 17 BDs DC, 8kV, 0.98J, Cu, 499 BDs Time-integrated spectroscopyReproducibility and comparison  Remarkably reproducible after normalization to total intensity !

  8. Time-integrated spectroscopyReproducibility of line-ratios • Line ratios are constant over many breakdowns in rf and dc • TLM applicable? (dc: 4280K±9K, rf: 5366K±61K) • TLM not consistent for all line pairs!

  9. Time-integrated spectroscopyLine-ratios and total intensity • Line intensity / total intensity is spreading towards shorter wavelengths • Line ratios are only weakly connected to dissipated energy

  10. Light present when electric field is applied • Shoulder at 2.1eV (interband transition) •  OTR light present in dc and 30GHz rf, not found in X-band rf… Dc, 4.25kV, Cu, 600s int. Time-integrated spectroscopyNo breakdown  OTR Rf 30GHz SBS, 66MV/m 1h int.

  11. The Mo OTR mystery: • Why only these two lines? 694.7nm MoI Time-integrated spectroscopyNo breakdown  OTR 693.4nm MoI

  12. New way of measuring β close to the breakdown threshold (impossible with current dc setup) Time-integrated spectroscopyNo breakdown  OTR  field enhancement factor

  13. Dc Time-resolved spectroscopyPower and light waveforms Rf • Dc: Max. light emission after max. power • Rf: Light lasts longer than input power

  14. Acquired time-resolved with PMT, integrated by computer afterwards (20BDs averaged per bin) Time-resolved spectroscopyConsistency with integrated spectroscopy Integrated by CCD camera (single BD) • Time-resolved (PMT) and integrated (CCD) spectroscopy • show consistent results, even though the method and number of BDs are totally different

  15. continuum lines Time-resolved spectroscopyContinuum and lines • Dc: Spectrum consists of continuum and lines

  16. 522nm 516nm Time-resolved spectroscopyReproducibility of emission waveforms 518nm (background) • Dc: Waveform reproducible, lines emit longer than continuum

  17. Spectra of rf and dc breakdowns show Cu ions up two CuIII • Other elements were not identified (but still one unidentified broad line) • Main emission intensity originates from continuum • Continuum emission ends before line emission and is weaker • Total intensity of spectrum scales with line intensity and vice versa • Non-LTE plasma, temperature calculations are inconsistent • OTR emission seen in rf and dc • Cu OTR spectrum modulated by Cu reflectivity • OTR light is linear proportional to current (and rel. factor) • Can be used to get field enhancement factor close to breakdown limit • High electric noise environment, very low light levels, fast processes and (yet) unpredictable position (rf structures!!) complicates experiments a lot! Breakdown spectroscopyConclusion Thankyou!

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