1. CLIC Breakdown Workshop – CERN, May 2008 1 / 13 Delay Times in Breakdown Triggering CERN, TS-MME Antoine Descoeudres, Sergio Calatroni, Mauro Taborelli CLIC Breakdown Workshop May 2008

2. CLIC Breakdown Workshop – CERN, May 2008 2 / 13 Time matters Study of delays can give information about the breakdown mechanism t 0 : high voltage is applied between the electrodes no spark… spark ! t > t 0 : Q: how long ?

3. CLIC Breakdown Workshop – CERN, May 2008 3 / 13 DC spark : experimental set-up V 1) 2) power supply (up to 12 kV) 28 nF UHV V HV probe Charged capacitor connected to the electrodes during 2 sec, through a high current mechanical relay Voltage and current measured with probes, connected to a scope  HV probe : 20kV, 75MHz, 1:1000, 100M   current transformer : 500MHz, 1kA peak  scope : 1GHz to scope current probe

4. CLIC Breakdown Workshop – CERN, May 2008 4 / 13 DC spark : experimental set-up NB: slow discharge of the capacitor during the voltage ‘pulse’ due to finite resistance of HV probe (RC circuit) V i 100 M  28 nF resulting ‘pulse’ : circuit time constant :  = RC = 2.8 sec

5. CLIC Breakdown Workshop – CERN, May 2008 5 / 13 DC spark : experimental set-up The moving part of the mechanical relay is bouncing sometimes (loss of contact) …but voltage pulse is ‘flat’ over at least 5 ms (and 5 ms = max. delay observed)

6. CLIC Breakdown Workshop – CERN, May 2008 6 / 13 Timing diagram of a spark Schematic evolution of the voltage and current across the electrode gap t V I ??? HV relay is closed rising time ~ 100 ns spark ~ 2  s delay (< 5 ms)

7. CLIC Breakdown Workshop – CERN, May 2008 7 / 13 Example of a spark delay

8. CLIC Breakdown Workshop – CERN, May 2008 8 / 13 Delay times with Mo electrodes Histogram of delays population #2 ‘delayed’ brkds average : 1.17 ms (  = 0.33 ms) 2 populations, separated by a gap (1 – 100  s) Two different breakdown mechanisms ? population #1 ‘immediate’ brkds average : 129 ns (  = 16 ns) voltage rising time: ~ 100 ns

9. CLIC Breakdown Workshop – CERN, May 2008 9 / 13 Delay times with Mo electrodes Delays during conditioning Delays after conditioning 82% immediate brkds dominate during conditioning delayed brkds dominate after conditioning 24% percentage of delays < 200 ns : gap

10. CLIC Breakdown Workshop – CERN, May 2008 10 / 13 Delay times with stainless steel electrodes Conditioning Same tendency both populations observed at every field, but  immediate breakdowns dominate during conditioning  delayed breakdowns dominate after conditioning ??? pop. #2: delays increase linearly with breakdown field ???

11. CLIC Breakdown Workshop – CERN, May 2008 11 / 13 Delay times with copper electrodes Cu : ‘immediate’ conditioning, lower breakdown field Large majority of immediate breakdowns average : 163 ns (  = 44 ns)

12. CLIC Breakdown Workshop – CERN, May 2008 12 / 13 Delay times with different materials CuTaMoSS R = fraction of delayed breakdowns (excluding conditioning phase) R = 0.07R = 0.29R = 0.76R = 0.83 R increases with average breakdown field E b = 170 MV/mE b = 300 MV/mE b = 430 MV/mE b = 900 MV/m

13. CLIC Breakdown Workshop – CERN, May 2008 13 / 13 Summary Two populations of delays observed :  immediate breakdowns (average ~ 120 ns)  delayed breakdowns (from 0.1 to 5 ms, average ~ 1.3 ms) Both populations observed at every field  during conditioning mostly immediate brkds  after conditioning mostly delayed brkds  delays ~ breakdown field? (SS) Two different breakdown mechanisms?  immediate: associated with surface contamination? gas desorption?  delayed: once the surface is cleaned? Repartition of delayed / immediate breakdowns  depends on the material  the ratio of delayed brkds R increases with the average breakdown field of the material Next : in BDR mode ideas are welcome!

14. CLIC Breakdown Workshop – CERN, May 2008 14 / 13 Thank you !