1. The CERN dc Spark System (and a little bit of theory)

2. Introduction
Over the past years CERN has built and operated a series of small high- voltage systems in parallel to our main high-gradient rf testing program for CLIC.
The main reasons are to:
Complement when relevant expensive and time consuming rf tests with simplified, cheap tests. Compare materials, surface preparation, try out conditioning strategies etc.
Provide a platform to make experiments which test basic ideas about material dynamics under high surface fields. Simplified experimental conditions, direct benchmarking of simulation tools etc.

3. First an overview of the hardware of our dc systems

4. Hardware status and evolution: plane cathode, tip anode
Plane cathode, typically 12 mm diameter disk sample. Tip anode, 1 mm radius hemispherical tip. Moveable anode with capacitive gap-height control. Gaps typically μm.

5. Hardware status and evolution:
Large area electrodes
62 mm diameter electrodes separated by precision ceramic spacer, gaps between 10 and 60 μm. Very large surface both compared to breakdown crater size and high field region in rf cavities allows study of effects of production (machining, heat treatment, chemistry) and operation (conditioning, breakdown statistics) related issues.

6. Vacuum chamber

7. Mechanical switch based
high voltage pulser
Initial system based on mechanical switches. Limited to 1 Hz repetition rate so becoming obsolete. However still used for field emission measurements due to high impedance of switches.

8. High repetition rate, high-voltage pulser
We now use a MOSFET-based commercial switch, which allows us to pulse up to 1 KHz with pulse lengths from 1 to around 8 μs (followed by exponential decay).

9. Same conditioning algorithm in rf and dc
Part of dc conditioning interface.
Part of rf conditioning interface.
Both implemented in National Instruments PXI/Labview.

10. Highlights of recent results and capabilities
A high-priority for us has been to first show that the high-rep rate and large-electrode system behaves similarly to rf.
I will show you data which indicates that this is accurate.
The process of exploiting the new hardware this is thus just beginning.

11. Most important empirical dependencies
For a fixed pulse length
For a fixed BDR

12. Gradient dependence of BDRdc rf
rf: breakdown rate as a function of field .
The same dependence is seen in with dc. This data was take with the anode-tip system.

13. Gradient dependence of BDR
Further data taken with large electrodes and high-rep rate pulser.

14. Pulse length dependence of BDR
Pulse length varied by adjusting switching time and bleed resistor.
Preliminary results compared to τ6 dependence typically seen in rf. Will be repeated, especially with Marx generator.

15. Conditioning in rf and dc
From first fully heat treated electrode pair! Unfortunately the electrode surface was in contact with ceramic. Conditions anyway. To be repeated.
rf data from CLIC damped structures

16. Long-term evolution of breakdown rate
Power law fit:
rf structures range between -6.8 and -9.2
dc system, -7.8

17. Effect of conditioning algorithm
Preliminary investigation of effect of conditioning algorithm.

18. Effect of venting system
Test vent of dc system, 3 days

19. Breakdown statistics rf
KEK dc

20. Theoretical studies

21. Dislocation dynamics and criticality – Hebrew University of Jerusalem

22. Atomistic simulations – University of Helsinki

23. FEM simulations and connection to KMC – University of Tartu

24. Advanced microscopy – Hebrew University of Jerusalem and CERN

25. V. Dolgashev, EAAC2015

26. We didn’t measure breakdown rate and quote “maximum
We didn’t measure breakdown rate and quote “maximum.” From memory was probably around 10-2

28. Outlook – scientific program
optimization of production – multi-sample program for machining, chemistry and heat treatment.
Optimization of conditioning strategy
Electrodes for INFN to optimize chemical treatment of non-brazed rf photoinector
Investigate high electric field behaviour of Ti 3-D printed electrodes to support printed rf component development.
Re-heat of conditioned cathodes to determine mechanism of conditioning.
Time structure of field emission.
Nb electrodes
Integrate dynamic vacuum measurement
Surface microscopy

29. Outlook – hardware development
Marx generator for fast rise and fall time. Good for pulse length dependence and comparison to rf.
2nd large electrode chamber
Cool-able, 4.2 ⁰K, system: To test high-peak power processing for superconducting cavities, high-field material dependence (Cu is FCC, Nb is BCC, field emission and BDR as a function of temperature.

30. Conclusions
The large-electrode pulsed dc system shows fundamental behaviour similar to rf structures, so its validity as a test bed has been validated.
Ready to exploit for rf structure development, CLIC and beyond.
Steady advance in the quantitative understanding of high-gradient phenomena. This too starts to feed back on rf structure development.
However severe lack of people-power in the lab. Please help!

31. More information
And a workshop dedicated to vacuum arcs

32. Acknowledgements
I have the luxury of reporting on the hard work of others. Names in roughly the order of appearance in this presentation:
S. Calatroni, F. Djurabekova, A. Descoudres, N. Shipman, D. Godkov, A. Solodko, A. Olyunin, J. Koverman, M. Barnes, I. Profotalova, T. Murananka, B. Woolly, A. Degiovanni, J. Giner, A. Grudiev, T. Higo, A Korsback, Y. Ashkenasi, T. Muranaka, I. Profatilova, F. Djurabekova, S. Parviainen, V. Jaanson, V. Zhadin, V. Dolgashev