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: -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

DC breakdown setup DC spark test in UHV HV C 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 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 28nF I probe Q Q initial EbEb Q remaining after 2s nb of breakdown EbEb

Conditioning curves of various metals OFE graphite

Other materials : Alloys Others : – Cu + 500µm Cr coating (≈ Cr) – Mo + 2µm DLC coating (low E b ) Al15 316LN Tungsten carbide composite C15000

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

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

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) 200MV/m ln [I/E 2 ] NB: at higher fields emission from hot tips can be thermo-ionic

Degassing during and before spark Gradual increase of pressure burst (H 2 and CO mainly) with increasing field before sparking Consistent with increasing FE current for increasing field

Emitters -Typical measured β are in the range 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

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

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 Simulation with applied field is in progress. (K.Nordlund, Uni Helsinki, Finland within the CLIC collaboration) (K.Nordlund et al,J.Phys: Cond. Matt. 16, 2995, 2004)

Surface migration, macroscopic approach for a solid tip Barbour et al. Phys. Rev. 117, 1452 (1960) R dz Without field the tip “dulls” dz/dt ~ D 0 exp(-Q/kT) R3R3 The field stabilizes the tip dz/dt ~ (1-k ε 0 R E 2 ) dz/dt E=0 E   = surface energy Q= activation energy of surface diffusion k0.5-1 =(900 MV/m)- 2 for W, R=50nm =(650 MV/m)- 2 for Cu, R=50nm Dyke et al. J.Appl.Phys. 31, 790, (1960) W

Effect of various gases -for inert gases (Ar) there is no effect at least up to mbar -for reactive gases (air, O 2, 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 Molybdenum

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

Which mechanism could provide the gas to initiate breakdown (and form a plasma)?  To get mbar in the tip-plane space : atoms of Cu  Vapor pressure of hot tip of 100 μm 2 surface (Pvap and conductance through the a spot) at Tm: 10 3 atoms of Cu  melting temperature is not sufficient  Thermal desorption of 1ML of adsorbates: 10 9 molecules  Electron stimulated desorption (from anode), with 1mA FE: 10 8 molecules  t he last two would be less relevant after conditioning  Sublimation of a cylinder tip of 100 nm diameter and β=30: 10 9 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 Copper, in less than 100 ns, with 20 μm electrode distance

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

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

Delay times for different materials CuTaMoSS fraction R of delayed breakdowns (excluding conditioning phase) increases with the average breakdown field R = 0.07R = 0.29R = 0.76R = 0.83 E b = 170 MV/mE b = 300 MV/mE b = 430 MV/mE b = 900 MV/m

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 breakdown Measurement of the breakdown rate (BDR) The present setup is limited to a breakdown probability of about 10 -4, for reasonable measuring times often grouped

Breakdown rate vs field : RF (30 GHz) different materials give different slopes from S. Doebert Cu 70ns Mo 80ns for Cu for Mo BDR ~ E 30 BDR ~ E 20 With exponential law With power law different materials give different exponents

Breakdown rate vs field : DC NB: RF data are displayed vs surface field Ranking of slopes of BDR opposite to RF case for Cu for Mo BDR ~ E BDR ~ E 30-35

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%’ DCRF Cu Mo

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

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

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

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

Fast conditioning: heat-treated Mo (to reach 400 MV/m) ~ 60 sparks~ 15 sparks~ 12 sparks ~ 10 sparks In UHV oven, ex situ treatment and e-beam ex situ heating:  immediate conditioning E b [MV/m Number of sparks No significant change of saturated breakdown field !

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

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