Breakdown Testing on RF Antennas for Fusion Plasma

1

• Breakdown Testing on RF Antennas for Fusion
  Plasma
• Aaron Virshup
• Mentor John Caughman
• ORNL Fusion Energy Division
• December 11, 2002

2
Background

• Fusion reaction
• Ingredients kept in ionized state (plasma)
• Plasma confined by magnetic fields
• Energy pumped into plasma with RF antennas

3
Breakdown

• If areas of RF antennas exceed a certain voltage,
  an arc of current flows from high to low voltage
  areas
• Breakdown voltage determined by geometry of
  antenna and its environment
• Breakdown limits antenna output, damages antenna
  and other equipment

4
Experimental Setup Mini-RFTF

• Mini-Radio Frequency Test Facility1
• Over 100 kiloWatt RF transmitter
• Magnet coils generate up to 600 Gauss fields
• Setup enclosed in 10-6 Torr vacuum chamber - can
  be filled with gasses and plasma

High Voltage Transmission Line
magnetic coils
turbo-pump
High Voltage Electrode
vacuum chamber
5
High Voltage Tester Circuit
Vac. Cap. To Ground C65.44 pF L31.0 nH
High Voltage Line Z088 ? Loss 0.122 ?
Vacuum Feedthrough Z055 ? Loss 0.101 ?
Feeds to Junction Z050 ? Loss0.014 ?
RF Emitter R106 ? C5.2 pF
1.0 m
1.78 m
1.0 m
Feed to Input Z050 ?
6
Circuit Tuning

• Changing the capacitance of the vacuum capacitor
  changes the resonant frequency of the antenna
• At best resonance, circuit achieves better than
  -30 dB match gt 99.999 power transfer

C56.3 pF 63.16 MHz
C76.4 pF 61.51 MHz
C65.44 pF 62.63 MHz
7
High Voltage Electrode

• Highest voltage point on antenna
• Breakdown localized to this area
• Asymmetric inner conductor localizes breakdown to
  ends closest to outer conductor

8
Circuit Models

• Created computational model of circuit parameters
  to predict non-breakdown behavior

Voltage Magnitude Along Transmission Line
Input Impedance Magnitude vs.
1
60
Measured
RF Emitter Voltage
Predicted
V(x) / Vout
Zin (?)
.5
30
Input voltage
0
0
6
62.0
62.8
62.4
0
12
Distance (m)
(MHz)
9
Experimental Model Predictions
Electrode Voltage (V)
Input power (W)
10
HV Electrode Voltage Calibration
Vector Voltage Probe

• Capacitor probe mounted underneath HV electrode
  measures its voltage
• Calibrate cap probe to HV electrode voltage

High Voltage Port
Capacitor Probe
determine V(x)
11
Capacitor Probe Calibration
12
Magnetic Deflection
Electron Paths

• Magnetic fields deflect electrons away from outer
  conductor
• Increase in path length ? increase in voltage
  standoff

13
Magnetic Fields in Mini-RFTF

• Insulation effect observed in NSTX test reactor -
  has not yet been quantified

14
Breakdown Testing

• Plot breakdown voltage vs. gas pressure
• Gas provides additional electrons to breakdown
  arcs and should decrease breakdown voltage
• 10 ms pulses through antenna every 10 s

15
Preliminary Results
Breakdown Voltage vs. Pressure
Breakdown Voltage (V)
16
Summary

• Created precision model of antenna that gives
  impedance and voltage distribution
• Correctly predicts impedance vs. frequency
• Predicts voltages as high as 60 kV for 100 kW
  input power
• Calibrated capacitor probe to HV electrode as
  function of gap distance
• Model predicts strong significant positive
  dependence of breakdown voltage vs. magnetic
  field strength
• Preliminary results qualititatively agree with
  previous results
• Hope to test magnetic field dependence in next
  two days

17
Acknowledgements/References

• The ERULF program and fusion energy research at
  ORNL are supported by the US Department of
  Energy.
• The ORSS program is directed by Dr. Ron Winters,
  and is administered by the ACM and GLCA
  consortiums.
• John Caughman, Mentor
• David Rasmussen, Plasma Technology Group Leader
• Andy Fadnek
• Rick Goulding
• Dennis Sparks
• References
• 1 F. W. Baity, D.W. Swain et al. The
  ORNL/ASDEX Upgrade RF Breakdown TesterResults
  and Plans. Presented at 14th Topical Conference
  on Radio Frequency Power in Plasmas, Oxnard, CA.
  2001.