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On the energy dependence of DC sparks

On the energy dependence of DC sparks. A. Hansen, A. Descoeudres and H. Timkó CERN. Outline. DC spark setup Energy dependence of breakdown properties for Cu and Mo How do breakdown field and other quantities change? A collection of other results Evolution of ß without breakdowns

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On the energy dependence of DC sparks

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  1. On the energy dependence of DC sparks A. Hansen, A. Descoeudres and H. Timkó CERN

  2. Outline • DC spark setup • Energy dependence of breakdown properties for Cu and Mo • How do breakdown field and other quantities change? • A collection of other results • Evolution of ß without breakdowns • The effect of an oxide • Can lattice structure influence EBRD? CLIC workshop 2009

  3. Exploring DC sparks Why DC sparks? • Allows to study breakdowns on a fundamental level • Simple and fast testing of materials, surface treatments etc. CLIC workshop 2009

  4. Why energy dependence is interesting... • The DC setup has been designed to match the energy lost in RF structures during breakdown (we store ≈1J in a capacitor) • However, it can be that only a fraction of this energy is consumed by the breakdown itself • Also, if we know the breakdown behaviour of materials as a function of energy, we can optimise our structures • Lowering the energy available ↔ changing the capacitor CLIC workshop 2009

  5. EBRD, ß and ELOC for Cu as a function of energy • EBRDincreasing with decreasing energy (less deconditioning is possible) • ß and ELOC remain constant • EBRD=190MV/m,ß=62, • ELOC=10.4 GV/m CLIC workshop 2009

  6. EBRD, ß and ELOC for Moas a function of energy • EBRDseems to decrease with decreasing energy (less conditioning possible) • ß and ELOC remain constant • EBRD=350MV/m,ß=34, • ELOC=11.3GV/m CLIC workshop 2009

  7. How conditioning effects EBRD • Conditioning time remains the same, however, • With less energy available for the breakdown, less (de-)conditioning can be achieved EBRDchanges • This was confirmed by preconditioning tests • By preconditioning with higher energy, higher EBRD can be reached (for Mo) High E Low E CLIC workshop 2009

  8. Energy scaling of the spot size • Also the diameter of the damaged area depends on the energy available • Area mostly determined by the conditioning phase • Decreases with decreasing energy; saturates below a given threshold Mo Cu CLIC workshop 2009

  9. Saturation of spot size • Below a given threshold, the energy is not sufficient to create deep craters (a heat spike) Cu CLIC workshop 2009

  10. Scaling of single craters in Cu • Depth to width ratio is independent of dose • Both experiments and simulations gave the same aspect ratios 10 μm 50 nm CLIC workshop 2009

  11. Other breakdown phenomena CLIC workshop 2009

  12. The evolution of ß during breakdown rate measurements • 200 MV/m: BDR = 4×10-3 • 225 MV/m: BDR = 0.11 • ß increases during the breakdown-free periods Breakdown Cu CLIC workshop 2009

  13. The oxide layer of Cu • An oxide layer has been grown on Cu, which was thicker than the natural oxide layer • Higher initial EBRD and conditioning last longer • Has also a different ELOCas the naturally oxidised Cu 125°C, 48h ~ 15nm layer • During conditioning, EBRD=350-500 MV/m in both cases • This lasts only for 15-20 sparks (left case) or 20-40 sparks (right case) 200°C, 72h even thicker layer CLIC workshop 2009

  14. Can reoxidation recover EBRD? • A sparked (damaged) surface was reoxidised by heating and was sparked then again • Was not able to recover the initial high EBRD • Oxidised, smooth surface  high EBRD • Oxidised, sparked surface  no improvement • Connection to the oxidation process? CLIC workshop 2009

  15. What reoxidation does 1st oxidation 150 sparks 2nd oxidation 180 sparks O element% = 3.8 O element% = 1.0 O element% = 3.5-3.7 1st oxidation 30 sparks 2nd oxidation O element% = 1.6 O element% = 0.9-2.1 O element%=1.3-1.7 O element% = 0.3 CLIC workshop 2009

  16. The breakdown field of Co • Crystal structure can influence the formation of field emitters and therefore, breakdown properties • A EBRD ranking of materials by crystal structure was suggested by F. Djurabekova (HIP) • We tested Co, which has HCP lattice, and expected a high EBRD • Experiments confirmed this CLIC workshop 2009

  17. Ranking materials by crystal structure? CLIC workshop 2009

  18. Summary • The energy dependence of breakdown properties is governed by (de)conditioning properties of the material (oxide layer, etc.) • ß steadily grows when a field is applied (without breakdown events) • A thick oxide layer on Cu can improve EBRDfor a while • The crystal structure might influence the triggering of breakdowns CLIC workshop 2009

  19. Future plans • Growth of spot size during conditioning • Oxidised Cu: effect of surface treatments • Investigation of differently oriented single crystals • Thermal effects: heating and cooling • Other materials... CLIC workshop 2009

  20. Thank you! CLIC workshop 2009

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