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Studies of Voltage Breakdown in Superfluid 4 He

Studies of Voltage Breakdown in Superfluid 4 He. May 20, 2008 Maciej Karcz, Craig Huffer, Young Jin Kim, Chen-Yu Liu, Josh Long. Outline. Possible to sustain strong electric field in depressurized He-II by exploiting hysteretic phenomenon Results suggest some caveats

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Studies of Voltage Breakdown in Superfluid 4 He

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  1. Studies of Voltage Breakdown in Superfluid 4He May 20, 2008 Maciej Karcz, Craig Huffer, Young Jin Kim, Chen-Yu Liu, Josh Long

  2. Outline • Possible to sustain strong electric field in depressurized He-II by exploiting hysteretic phenomenon • Results suggest some caveats • Phenomenon is not well-understood

  3. 3 2 4 1 Phase Diagram Paths • “Standard cycle”: 1 -> 2 -> 3 -> 4 • “Pressurized Cooldown”: 3 -> 4

  4. Setup Ceramic Feedthrough Upper T Sensor Lower T Sensor Brass HV Sphere PZT wafer Vibration Proof Washers Adjustable Gap Size SS GND

  5. Typical Hysteresis • “Standard pressurization cycle” yields strong hysteretic behavior

  6. Agitated Superfluid • Piezoelectric transducer results not conclusive • Occasional changes in breakdown strength • Brass sphere HV cathode • Resonance ~10.5 kHz He-II He-II

  7. Agitated Superfluid • Resonance ~ 12.7 kHz • SS sphere HV cathode • Brass makes better cathode? He-II

  8. Pressurized Cooldown • SS sphere HV anode • Sharp decline in dielectric strength across lambda transition • No recovery on warm up?? • LHe level on warm up ~ 3” He-II

  9. Pressurized Cooldown SS HV anode Brass HV anode He-II He-II • Brass anode appears to have better HV performance • Neither recovers on warm up??

  10. Standard Cycle • Attempt Std Cycle with brass sphere HV anode • SF transition marks large decline in breakdown strength • With HV sphere anode, on depress. tend to recover only ~50% of max. dielectric strength (with spherical cathode: 100% recovery) He-II

  11. Breakdown Histograms Standard pressurization cycles: SS and brass sphere HV anodes, SS GND cathodes Depressurization Pressurization Warm up Vapor Curve Cooldown SS 78 Depressurization Pressurization Warm up Vapor Curve Cooldown 86 Brass

  12. Breakdown Histograms Pressurized cooldowns: SS and brass HV anodes, SS GNDs • SF transition marks significant change Warm Pres. Cool. Dep. 72 SS Warm Pres. Cool. Dep. 91 Brass

  13. Breakdown Collectives Breakdown probability density Breakdown in low-stress collective J. Gerhold, “Helium Breakdown Near The Critical State”, IEEE Transcations On Electrical Insulation, Vol. 23, Issue 4, 765-768, 1988 • Breakdown histograms in literature suggest two collectives: low-stress depends on pressure, high-stress on liquid density • “low-stress collective tendency is consistent with the ideas of bubble breakdown mechanisms” saturated liquid gas subcooled liquid our investigations

  14. Plastic Polymer: Non-metallic Mirror • Reflect scintillation light in actual nEDM exp., improve light collection • Folded into thirds, held by spring force Brass HV SS GND Polymer

  15. Polymer Test Brass anode, SS cathode Brass cathode, SS anode • Anode draws current at ~150kV/cm • Adding polymer does not affect overall qualitative behavior • Smaller, spherical cathode better: less field emission?

  16. Breakdown Histograms Pressurized cooldown: Brass HV sphere, SS GND, polymer inside gap • Negative polarity: brass sphere cathode clearly performs better Dep. Pres. Cool. Warm + + + - - -

  17. Semitron Tests Semitron cathode Semitron anode • Electrode material study • Carbon-loaded plastic GND electrode • Pressurized cooldowns with semitron GND and brass HV sphere • Semitron cathode cannot sustain max field very long: leakage current? • Semitron makes poor cathode, good anode

  18. Breakdown Histograms Pressurized cooldown: Brass sphere HV, semitron GND • Brass cathode/semitron anode combination better Pres. Cool. Dep. Warm + + + - - -

  19. + + Remarks - - + - + + - - - - - + + - - + + • Pressurized cooldown: positive polarity runs show significant decline in dielectric strength during and after SF transition, negative polarity tend to perform better (surface area effect?) • Problem depressurizing during pressurized cooldowns: pressure does not respond to gas cylinder regulator, need to open SF valve to depressurize...possibly condensing too much He gas in the process of cooldown

  20. Currently • Surface wetting • Electropolished SS electrodes ready • Float glass, doped silicon wafers • Fiber optic setup: study HV-induced background • detect all wavelengths during breakdown • search for microdischarges before breakdown

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