Download dc system final

The CERN dc Spark System
(and a little bit of theory)
CLIC Project meeting, 29 September2015
Walter Wuensch, CERN
Introduction
Over the past years CERN has built and operated a series of small highvoltage 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.
CLIC Project meeting, 29 September2015
Walter Wuensch, CERN
First an overview of the hardware of our
dc systems
CLIC Project meeting, 29 September2015
Walter Wuensch, CERN
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.
CLIC Project meeting, 29 September2015
Walter Wuensch, CERN
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.
CLIC Project meeting, 29 September2015
Walter Wuensch, CERN
Vacuum chamber
CLIC Project meeting, 29 September2015
Walter Wuensch, CERN
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.
CLIC Project meeting, 29 September2015
Walter Wuensch, CERN
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).
CLIC Project meeting, 29 September2015
Walter Wuensch, CERN
Same conditioning algorithm in rf and dc
Part of rf conditioning interface.
Part of dc conditioning
interface.
Both implemented in National Instruments PXI/Labview.
CLIC Project meeting, 29 September2015
Walter Wuensch, CERN
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.
CLIC Project meeting, 29 September2015
Walter Wuensch, CERN
Most important empirical dependencies
For a fixed BDR
For a fixed pulse length
BDR ~ E
Ea  t1p/ 6  const
30
a
Ea30  t 5p
BDR
CLIC Project meeting, 29 September2015
 const
Walter Wuensch, CERN
Gradient dependence of BDR
dc
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.
CLIC Project meeting, 29 September2015
Walter Wuensch, CERN
Gradient dependence of BDR
Further data taken with large electrodes and high-rep rate
pulser.
CLIC Project meeting, 29 September2015
Walter Wuensch, CERN
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.
CLIC Project meeting, 29 September2015
Walter Wuensch, CERN
Conditioning in rf and dc
rf data from CLIC damped structures
CLIC 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.
Walter Wuensch, CERN
Long-term evolution of breakdown rate
Power law fit:
• rf structures range between -6.8 and -9.2
• dc system, -7.8
CLIC Project meeting, 29 September2015
Walter Wuensch, CERN
Effect of conditioning algorithm
-200
Normalized breakdown rate
10
-205
10
1. Feedback phase,
gradient -7.87
3. Stepped
voltage
phase,
gradient
-3.12
-210
10
2. Constant voltages phase,
gradient -2.59
8
10
9
10
Cumulative nr of pulses
10
10
Preliminary investigation of effect of conditioning algorithm.
CLIC Project meeting, 29 September2015
Walter Wuensch, CERN
Effect of venting system
-82
10
Normalized BDR ( (MV/m)
-74
10
Normalized BDR ( (MV/m) -30 ns-6 )
-83
10
-30
-6
ns )
Test vent of dc
system, 3 days
Before venting
After venting
gradient -3.146
gradient -28.14
-84
10
-85
10
-76
10
-86
10
-78
10
9.8
1
9.9
10
10
Cumulative nr of pulses
-80
10
4
-82
10
2
1. Feedback phase, gradient -7.87
2. Constant voltages phase, gradient -2.59
3. Stepped voltage phase, gradient -3.15
4. Reconditioning after 3 day vent, gradient -28.12
-84
10
8
9
10
CLIC10Project meeting, 29 September2015
Cumulative nr of pulses
3
10
10
Walter Wuensch, CERN
Breakdown statistics
700
Data
Long-term BDR= 2.59e-005
Short-term BDR= 2.07e-003
Two-exponential fit
-3
10
rf
600
400
300
KEK
Probability density
Cumulative BDs
500
-4
10
-5
10
200
100
-6
0
10
0
dc
Cumulative nr of breakdowns
3.5
1
x 10
2
3
Cumulative pulses
4
5
6
7
x 10
0
1
2
3
4
5
6
Number of pulses before breakdown
7
8
4
x 10
4
3
2.5
2
1.5
1
0.5
0
0
2
4
6
8
CLIC Project meeting,
29nr September2015
Cumulative
of pulses
x 10
8
Walter Wuensch, CERN
Theoretical studies
CLIC Project meeting, 29 September2015
Walter Wuensch, CERN
Dislocation dynamics and criticality –
Hebrew University of Jerusalem
CLIC Project meeting, 29 September2015
Walter Wuensch, CERN
Atomistic simulations – University of Helsinki
CLIC Project meeting, 29 September2015
Walter Wuensch, CERN
FEM simulations and connection to KMC –
University of Tartu
CLIC Project meeting, 29 September2015
Walter Wuensch, CERN
Advanced microscopy – Hebrew University of
Jerusalem and CERN
CLIC Project meeting, 29 September2015
Walter Wuensch, CERN
V. Dolgashev, EAAC2015
CLIC Project meeting, 29 September2015
Walter Wuensch, CERN
We didn’t measure breakdown rate
and quote “maximum.” From
memory was probably around 10-2
CLIC Project meeting, 29 September2015
Walter Wuensch, CERN
CLIC Project meeting, 29 September2015
Walter Wuensch, CERN
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
CLIC Project meeting, 29 September2015
Walter Wuensch, CERN
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.
CLIC Project meeting, 29 September2015
Walter Wuensch, CERN
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!
CLIC Project meeting, 29 September2015
Walter Wuensch, CERN
More information
And a workshop dedicated to vacuum arcs
https://indico.cern.ch/event/354854/.
CLIC Project meeting, 29 September2015
Walter Wuensch, CERN
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
CLIC Project meeting, 29 September2015
Walter Wuensch, CERN