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DC breakdown experiments

DC breakdown experiments M.Taborelli , S.Calatroni, A.Descoeudres, Y.Levinsen, J.Kovermann, W.Wuensch CERN. Ranking of materials Cathode mechanism Field emission Residual gas effects Time delay Breakdown rate. Motivation for DC experiment:

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DC breakdown experiments

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  1. DC breakdown experiments M.Taborelli, S.Calatroni, A.Descoeudres, Y.Levinsen, J.Kovermann, W.Wuensch CERN Ranking of materials Cathode mechanism Field emission Residual gas effects Time delay Breakdown rate

  2. Motivation for DC experiment: • -understanding breakdown mechanism in simpler system and simpler infrastructure than RF: many tests • reproducibility check • various materials • change parameters • ……. • what can be transferred to RF? • see from the results if the mechanisms are plausible also for RF • …obviously no B-field here

  3. Eb Q remaining after 2s Eb Q initial nb of breakdown DC breakdown setup DC spark test in UHV Hemispherical tip (2.3 mm diam) and flat sample, same material for both In UHV (10-9 mbar), baked system Max voltage 12KV, typical gap 20-30 μm, and spark energy 1J Charge applied on capacitor step by step until breakdown occurs. Breakdown detected with current pulse or/and with charge remaining on the capacitor C 28nF HV Q I probe

  4. Conditioning curves of various metals OFE graphite

  5. Other materials : Alloys Tungsten carbide composite 316LN Al15 C15000 • Others : • Cu + 500µm Cr coating (≈ Cr) • Mo + 2µm DLC coating (low Eb)

  6. Ranking of materials with respect to breakdown field • Also to be considered: • conditioning speed (depends on material treatment, here all were just cleaned by detergents/solvents as for UHV parts) • ranking of Cu, W, Mo is as in RF (at high breakdown rate, 30GHz) material “erosion”: for Ti, V and Cr the gap must be often readjusted

  7. W Ti W Ti Cathode limited breakdown field Exchanging the materials of tip (anode) and sample (cathode) shows that the breakdown field is cathode limited

  8. Evidence for field emission current before sparking Here the capacitor is discharged through field emission current from the sample Measured FE (far from breakdown field) (field enhancement β=17) ln [I/E2] 200MV/m NB: at higher fields emission from hot tips can be thermo-ionic

  9. Degassing during and before spark Gradual increase of pressure burst (H2 and CO mainly) with increasing field before sparking Consistent with increasing FE current for increasing field

  10. Emitters -Typical measured β are in the range 10-100. for cylinder: β = (h/r) + 2 -No sharp features seen in SEM images of DC samples, either the tips are very small, or they are there only present when the field is applied, or the apparent β is not due to geometry β=20

  11. RF, Mo structure 30GHz estimated beta, from geometry 100 μm Cones observed after high power RF tests on Mo and Ti, not on Cu

  12. Simulated Cu tip evolution tip evolution on Cu in 2ns, 800K Time for “diffusion smoothing” of the tip down to a flat monolayer on the surface (K.Nordlund et al,J.Phys: Cond. Matt. 16, 2995, 2004) Simulation with applied field is in progress. (K.Nordlund, Uni Helsinki, Finland within the CLIC collaboration)

  13. dz R Surface migration, macroscopic approach for a solid tip Without field the tip “dulls” dz/dt ~ D0exp(-Q/kT) R3 = surface energy Q= activation energy of surface diffusion W The field stabilizes the tip dz/dt ~ (1-k ε0R E2) dz/dt  E E=0 k0.5-1 =(900 MV/m)-2 for W, R=50nm =(650 MV/m)-2 for Cu, R=50nm Barbour et al. Phys. Rev. 117, 1452 (1960) Dyke et al. J.Appl.Phys. 31, 790, (1960)

  14. Effect of various gases Molybdenum -for inert gases (Ar) there is no effect at least up to 10-5 mbar -for reactive gases (air, O2, CO ) the breakdown field is lowered for prolonged exposure and sparking -no effect for Cu in the above studied pressure range for air and CO

  15. Needs 10-2 mbar air pressure to have an effect gap 0.13mm V [KV] Indeed it was already known… R.Hackman et al, J.Appl.Phys. 46, 629, 1975  it needs 10-2 mbar of gas to favor breakdown for small gap geometry

  16. Which mechanism could provide the gas to initiate breakdown (and form a plasma)? Copper, in less than 100 ns, with 20 μm electrode distance • To get 10-2 mbar in the tip-plane space : 106-108 atoms of Cu • Vapor pressure of hot tip of 100 μm2 surface (Pvap and conductance through the a spot) at Tm: 103 atoms of Cu  melting temperature is not sufficient • Thermal desorption of 1ML of adsorbates: 109 molecules • Electron stimulated desorption (from anode), with 1mA FE: 108 molecules the last two would be less relevant after conditioning • Sublimation of a cylinder tip of 100 nm diameter and β=30: • 109 Cu atoms • …how? • Field enhancement on the tip by ionized gas in front and field induced atom desorption ? Particle in cell calculations by R.Schneider Max-Planck Inst. Greifswald, Germany and Uni Helsinki Finland in progress within CLIC collaboration

  17. UHV 2) 1) 28 nF V power supply (up to 12 kV) V HV probe current probe Time delay for breakdown delay

  18. Distribution of delays Histogram of delays Immediate breakdowns Mo “avg.” 119 ns, but resolution is of the order of 100ns Delayed breakdowns avg.1.17 ms Similar to RF pulse length range Much slower than usual RF case 2 mechanisms of breakdown

  19. Delay times for different materials Cu Ta Mo SS Eb = 170 MV/m Eb = 300 MV/m Eb = 430 MV/m Eb = 900 MV/m R = 0.07 R = 0.29 R = 0.76 R = 0.83 fraction R of delayed breakdowns (excluding conditioning phase) increases with the average breakdown field

  20. Measurement of the breakdown rate (BDR) • It is important to know when it breaks down, but also at which field it can be safely used • measured by applying/removing the field and monitoring y/n breakdown with voltage probe no breakdown often grouped breakdown The present setup is limited to a breakdown probability of about 10-4, for reasonable measuring times

  21. Breakdown rate vs field : RF (30 GHz) With power law (BDR=0 @E=0) different materials give different exponents With exponential law different materials give different slopes Cu 70ns Mo 80ns from S. Doebert BDR ~ E30 for Cu for Mo BDR ~ E20

  22. Breakdown rate vs field : DC NB: RF data are displayed vs surface field Ranking of slopes of BDR opposite to RF case BDR ~ E10-15 for Cu for Mo BDR ~ E30-35

  23. Breakdown rate vs normalized field Idea of the normalization : ‘how many decades of BDR do we gain if we decrease the max. field by X%’

  24. Conclusions -cathode limited breakdown resistance -field emission as precursor -time lags indicate two mechanisms -time lags compatible with RF

  25. Acknowledgments G.Arnau-Izquierdo, S.Calatroni, S.Heikkinen, H.Neupert, T.Ramsvik, S.Sgobba, CLIC study team

  26. Gas effect, chemical: oxygen or air exposure of Mo during breakdown 10-6 air A prolonged exposure to 10-6 mbar range produced surface oxidation and lowers the breakdown field: similarly part of the initial conditioning process is also removal of the oxide

  27. X-ray Photo Emission Spectroscopy After prolonged breakdown in O2 10-6 mbar oxidized again Region of spark metallic initial state oxidized sputter cleaned + 1h ambient air Conditioning of Mo is removal of oxide

  28. Fast conditioning: heat-treated Mo 600 500 400 300 200 100 0 Eb[MV/m 0 20 40 60 80 100 120 Number of sparks In UHV oven , ex situ treatment ~ 60 sparks ~ 15 sparks ~ 12 sparks ~ 10 sparks (to reach 400 MV/m) and e-beam ex situ heating:  immediate conditioning No significant change of saturated breakdown field !

  29. Breakdown initiated by field emission Ebreakdown [MV/m] E [MV/m] for 10-7 A FE current Consistent with the cathode dominated scenario The precursor to breakdown is possibly FE current reaching a threshold value (which can be field dependent)

  30. Which tip size can melt in such a short time through FE currents? Select β E>Erunaway IFE tmelting Calculation as in Williams et al J.Phys D5, 280 (1972) Tips which can heat so fast are very small, below 50 nm diam

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