Comparative studies of high- gradient rf and dc breakdowns CLIC talk Jan W. Kovermann

Outline

High-gradient rf structures Extreme conditions in high-gradient structures running at 100MV/m: Surface fields up to 300 MV/m Current densities of 10’s of kA/m 2 10 J per rf pulse 240 ns pulse length 10’s of K pulsed surface heating 50 pulses/s for 10y  pulses Assuming luminosity loss in case of a breakdown: Max. BDR /pulse/structure (to stay above 99% of nom. luminosity) 11.4 GHz C10vg1.35 structure used for breakdown MW

Introduction to rf breakdowns When the surface cannot withstand these conditions: Rf breakdown (occurring randomly with constant rate  statistic process) Effects of a breakdown: P in P ref P tra e-e- P ref  P in P tra  0 e-e- γ γ Spectroscopy Faraday cup ADC RF DAQ Instrumentation used in the presented experiments:

Experimental facilities and setups

Typical performance plot of a rf structure:  Conditioning necessary  Why is the breakdown rate ~ E 30 ?  Why are we limited below these values? Standard rf test areas and structure performance measurements CTF2, CERN, 30 GHz ASTA, SLAC, 11.4 GHz Low noise, 0.8Hz 70ns, difficult access for instrumentation High noise, 60 Hz 240 ns, better access for instrumentation BDR ~ E 30 CERN T18 Coming soon: The CERN 12GHz klystron power source

Dc tests as a complement Structure tests take ~ 1y from first design to tests results, limited availability of structure test facilities, huge costs and a lot of ideas to test!  Use high-gradient dc fields to tests materials and surface machining! d=20 μm Cu ~kV 20 μm gap, 5 kV  250 MV/m 20 nF, 5 kV  0.25 J Instrumentation: spectrograph, camera, electric properties Difference to rf: micron gap, mm 2 surface probed, no B field. Is the physics behind the dc breakdown the same as in rf? Similar to rf! Comparison necessary!

The breakdown model

The breakdown model 1/3 βE ≈ 8 GV/m = 0.8 V/Å ≈ cohesive strength of Cu atom β: Field enhancement factor Φ: Work function OTR, electric Spectroscopy, electric ~ μA ‘Electron Emission in Intense Electric Fields’, R.H. Fowler and L. Nordheim, Proc. R. Soc. Lond. A, 119:173–181, May 1928 ‘trigger’ ‘run-away’

The breakdown model 2/3 PIC plasma simulation: ‘A One-Dimensional Particle-in-Cell Model of Plasma Build-up in Vacuum Arcs’, H. Timko, K. Matyash, R. Schneider, F. Djurabekova, K. Nordlund, A. Hansen, A. Descoeudres, J. Kovermann, A. Grudiev, W. Wuensch, S. Calatroni, M. Taborelli, Contrib. Plasma Phys., Accepted for publication (2010) Plasma sheath Spectroscopy, electric Spectroscopy, electric ~ 100 A nm to 10’s of μm, μA to 10’s of A  breakdown physics covers huge ranges!

The breakdown model 3/3 Formation of new field emitters by pulsed heating, diffusion, sputtering and field-assisted tip growth Model origin: ‘Surface Erosion by Electrical Arcs’, R. Behrisch, NATO ASI Series, Series B: Physics 1986, 131, p Multi-scale simulations and cratering: ‘Mechanism of surface modification in the plasma-surface interaction in electrical arcs’, H.Timko et.al., Phys. Rev. B, May 2010, vol. 81 nr. 18, p Spectroscopy, electric Electric

Experimental results comparing rf and dc breakdowns

Electric breakdown signals in rf No breakdown  ~ 50% ohmic losses Data from 30 GHz TM02 structure tests in CTF at CERN, taken during breakdown spectroscopy experiments Breakdown ‘Missing E’  E mis = E in - E tra - E ref  Flat distribution of E mis Random effect! delay

Electric breakdown signals in dc  Immediate U drop and I rise  High I, low U  low P? What is systematic and what breakdown? Measurement Simulation Development of a circuit model for the dc spark setup:  Waveform fully reproducible with fitted breakdown parameters Breakdown resistivity ~ 1 Ω

Experimental results from integrating optical spectroscopy of breakdowns in dc Integrated over one breakdown in dc, 8kV, Cu electrodes Andor 303 monochromator, Idus CCD camera, fiber based light collection Excitation, not ionized Double ionized (20 eV) Single ionized (7.7 eV) Continuous background, 70% of total intensity

Experimental results from integrating optical spectroscopy of breakdowns in rf 40 accumulated breakdowns in the C10 structure. P in = 40 MW, t p = 200 ns, same instrumentation than in dc. Reflectance modulation Raw data saturated Strong CuI lines ~ 60% cont. background In situ diagnostics during rf structure tests!

Experimental results from integrating optical spectroscopy of breakdowns in dc Ion temperature estimation using relative line intensities: Assumes LTE  Maxwell-Boltzmann distribution! 1. Two-line-method:  T = 0.3 eV to 1.3 eV 2. Boltzmann plot: DC, Cu  T = 0.52 eV (0.33 eV to 1.25 eV) Huge spread for LTE plasma!  No LTE plasma!  Geometry effects

Experimental results from integrating optical spectroscopy of breakdowns in dc and rf Reproducibility of breakdown spectra MeanStd. dev. Average of 499 breakdowns in dc, 410 MV/m, Cu Breakdown spectra are reproducible under equal conditions! First deterministic effect seen for breakdowns! MeanStd. dev. Average of 17 breakdowns at 37 MW, 200 ns, C10 structure ‘Missing E’

Experimental results from time-resolved optical spectroscopy of breakdowns in dc and rf Instrumentation: Monochromator with adjustable Δλ, fast spectroscopy PMT DC, Cu  Light emission time ~ μs  Two emission peaks DC, Cu, zoom RF, C10  Light without rf power? (full spectrum intensity, Δλ = 600 nm)

Experimental results from time-resolved optical spectroscopy of breakdowns in dc 10 BDs averaged per Δλ = 1 nm, 300 BDs in total Sum of each line, CCD spectrum for comparison Σ PMT CCD DC, Cu Strong continuum background, has to be explained by breakdown plasma model and simulation Exc. CuI decay times ~ 14 ns

Optical transition radiation from the dc spark gap and high-gradient rf structures Light emission from spark gap was observed with electric field but no breakdown d=20 μm Cu ~kV MV/m acc. grad. ≈ 200 MV/m surf. field 70 ns, 5 Hz  12.6 ms MV/m surf. field RF DC  e - hitting tip create visible OTR  OTR intensity ~ γ. I FN  Spectrum modulated by Cu interband transitions, pol.  Similar spectrum in rf and dc

Optical transition radiation from the dc spark gap as diagnostic tool Due to OTR intensity ~ γ. I FN  β measurement close to breakdown threshold possible (inaccessible for Amp. meters due to low dynamic range and possible destruction) OTR intensity rises with voltage  No fixed breakdown gradient  β values compatible with I FN current measurements at low voltages Fowler-Nordheim plot

Further theoretical and experimental results and research (only short overview!!!)

Experiments in the dc setup (TE-VSC group)  Ranking of materials according to their breakdown field Depending on lattice structure?  Conditioning also necessary in dc, but different behavior to rf (structures need 100’s of h)  BDR vs. gradient behavior very similar in rf and dc, both show strong exponential correlation ~ E 30  Low BDR region accessible soon with new HV, high rep.rate power supply

Experiments in the dc setup (TE-VSC group) One of the very basic questions: Does the applied field change the surface? If yes, in which way?  Measurement of the field enhancement factor β Does β rise when an E field is applied (until a breakdown resets it…)?

Theoretical approach to breakdowns (Helsinki Institute of Physics and CERN) Goal: A multi-scale physics model for breakdowns Stage 1: Charge distribution at the surface Method: DFT with external electric field Stage 4: Plasma evolution, burning of arc Method: Particle-in-Cell (PIC) Stage 5: Surface damage due to the intense ion bombardment from plasma Method: Arc MD ~few fs ~few ns ~ sec/hours ~10s ns ~ sec/min ~100s ns PLASMAONSETPLASMAONSET Stage 2: Atomic motion & evaporation ; Joule heating (electron dynamics) Method: Hybrid ED&MD model (includes Laplace and heat equation solutions) Stage 3b: Evolution of surface morphology due to the given charge distribution Method: Kinetic Monte Carlo Stage 3a: Onset of tip growth; Dislocation mechanism Method: MD, analysis of dislocations H.Timko IWLC2010 See future CLIC talk by Helga Timko!

The breakdown movie

Summary  100 MV/m at BDR is a feasibility issue for CLIC  Complex, multi-physics phenomenon  Studies in rf and dc, similar physics?  Spectroscopic and electrical measurements: Strong similarities between rf and dc breakdown physics  Experiments: Input for breakdown modeling and simulation, benchmarking theory

Thank you! And even more thanks to all people working for CLIC, CTF and TE-VSC who helped and supported me during my doctoral thesis!