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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 4He 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 • Phenomenon is not well-understood
3 2 4 1 Phase Diagram Paths • “Standard cycle”: 1 -> 2 -> 3 -> 4 • “Pressurized Cooldown”: 3 -> 4
Setup Ceramic Feedthrough Upper T Sensor Lower T Sensor Brass HV Sphere PZT wafer Vibration Proof Washers Adjustable Gap Size SS GND
Typical Hysteresis • “Standard pressurization cycle” yields strong hysteretic behavior
Agitated Superfluid • Piezoelectric transducer results not conclusive • Occasional changes in breakdown strength • Brass sphere HV cathode • Resonance ~10.5 kHz He-II He-II
Agitated Superfluid • Resonance ~ 12.7 kHz • SS sphere HV cathode • Brass makes better cathode? He-II
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
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??
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
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
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
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
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
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?
Breakdown Histograms Pressurized cooldown: Brass HV sphere, SS GND, polymer inside gap • Negative polarity: brass sphere cathode clearly performs better Dep. Pres. Cool. Warm + + + - - -
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
Breakdown Histograms Pressurized cooldown: Brass sphere HV, semitron GND • Brass cathode/semitron anode combination better Pres. Cool. Dep. Warm + + + - - -
+ + 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
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