Walter Wuensch, CERNCLIC Project meeting, 29 September2015

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

Walter Wuensch, CERNCLIC Project meeting, 29 September2015

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.

Introduction

Walter Wuensch, CERNCLIC Project meeting, 29 September2015

First an overview of the hardware of our dc systems

Walter Wuensch, CERNCLIC Project meeting, 29 September2015

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 10-50 μm.

Walter Wuensch, CERNCLIC Project meeting, 29 September2015

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.

Walter Wuensch, CERNCLIC Project meeting, 29 September2015

Vacuum chamber

Walter Wuensch, CERNCLIC Project meeting, 29 September2015

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.

Mechanical switch based high voltage pulser

Walter Wuensch, CERNCLIC Project meeting, 29 September2015

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).

Walter Wuensch, CERNCLIC Project meeting, 29 September2015

Same conditioning algorithm in rf and dc

Part of rf conditioning interface.Part of dc conditioning interface.

Both implemented in National Instruments PXI/Labview.

Walter Wuensch, CERNCLIC Project meeting, 29 September2015

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.

Highlights of recent results and capabilities

Walter Wuensch, CERNCLIC Project meeting, 29 September2015

consttE pa 6/130~ aEBDR

For a fixed pulse length For a fixed BDR

constBDR

tE pa 530

Most important empirical dependencies

Walter Wuensch, CERNCLIC Project meeting, 29 September2015

Gradient dependence of BDR

rf

dc

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.

Walter Wuensch, CERNCLIC Project meeting, 29 September2015

Gradient dependence of BDR

Further data taken with large electrodes and high-rep rate pulser.

Walter Wuensch, CERNCLIC Project meeting, 29 September2015

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.

Walter Wuensch, CERNCLIC Project meeting, 29 September2015

From first fully heat treated electrode pair! Unfortunately the electrode surface was in contact with ceramic. Conditions anyway. To be repeated.

Conditioning in rf and dc

rf data from CLIC damped structures

Walter Wuensch, CERNCLIC Project meeting, 29 September2015

Power law fit:• rf structures range between -6.8 and -9.2 • dc system, -7.8

Long-term evolution of breakdown rate

Walter Wuensch, CERNCLIC Project meeting, 29 September2015

108

109

1010

10-210

10-205

10-200

Cumulative nr of pulses

Nor

mal

ized

bre

akdo

wn

rate

2. Constant voltages phase,gradient -2.59

3. Steppedvoltagephase,gradient-3.12

1. Feedback phase,gradient -7.87

Preliminary investigation of effect of conditioning algorithm.

Effect of conditioning algorithm

Walter Wuensch, CERNCLIC Project meeting, 29 September2015108

109

1010

10-84

10-82

10-80

10-78

10-76

10-74

Cumulative nr of pulses

Nor

mal

ized

BD

R (

(M

V/m

)-30 n

s-6 )

2

3

4

1. Feedback phase, gradient -7.872. Constant voltages phase, gradient -2.593. Stepped voltage phase, gradient -3.154. Reconditioning after 3 day vent, gradient -28.12

1 109.8

109.9

10-86

10-85

10-84

10-83

10-82

Cumulative nr of pulses

Nor

mal

ized

BD

R (

(M

V/m

)-30 n

s-6 )

Before ventingAfter ventinggradient -3.146gradient -28.14

Effect of venting system

Test vent of dc system, 3 days

Walter Wuensch, CERNCLIC Project meeting, 29 September2015

Breakdown statistics

0 2 4 6 8

x 108

0

0.5

1

1.5

2

2.5

3

3.5x 10

4

Cumulative nr of pulses

Cum

ulat

ive

nr o

f bre

akdo

wns

0 1 2 3 4 5 6

x 107

0

100

200

300

400

500

600

700

Cumulative pulses

Cum

ulat

ive

BD

s

0 1 2 3 4 5 6 7 8

x 104

10-6

10-5

10-4

10-3

Number of pulses before breakdown

Pro

babi

lity

dens

ity

Data

Long-term BDR= 2.59e-005Short-term BDR= 2.07e-003

Two-exponential fitrf

dc

KEK

Walter Wuensch, CERNCLIC Project meeting, 29 September2015

Theoretical studies

Walter Wuensch, CERNCLIC Project meeting, 29 September2015

Dislocation dynamics and criticality – Hebrew University of Jerusalem

Walter Wuensch, CERNCLIC Project meeting, 29 September2015

Atomistic simulations – University of Helsinki

Walter Wuensch, CERNCLIC Project meeting, 29 September2015

FEM simulations and connection to KMC – University of Tartu

Walter Wuensch, CERNCLIC Project meeting, 29 September2015

Advanced microscopy – Hebrew University of Jerusalem and CERN

Walter Wuensch, CERNCLIC Project meeting, 29 September2015

V. Dolgashev, EAAC2015

Walter Wuensch, CERNCLIC Project meeting, 29 September2015

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

Walter Wuensch, CERNCLIC Project meeting, 29 September2015

Walter Wuensch, CERNCLIC Project meeting, 29 September2015

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

Walter Wuensch, CERNCLIC Project meeting, 29 September2015

• 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.

Outlook – hardware development

Walter Wuensch, CERNCLIC Project meeting, 29 September2015

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!

Walter Wuensch, CERNCLIC Project meeting, 29 September2015

More information

And a workshop dedicated to vacuum arcs https://indico.cern.ch/event/354854/.

Walter Wuensch, CERNCLIC Project meeting, 29 September2015

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