1. Studies of Voltage Breakdown in Superfluid 4 He May 20, 2008 Maciej Karcz, Craig Huffer, Young Jin Kim, Chen-Yu Liu, Josh Long 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. Phase Diagram Paths “Standard cycle”: 1 -> 2 -> 3 -> 4 “Pressurized Cooldown”: 3 -> 4 1 2 3 4

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

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

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 He-II Brass anode appears to have better HV performance Neither recovers on warm up?? SS HV anodeBrass HV anode

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 SS Brass Vapor Curve Cooldown Vapor Curve Cooldown Pressurization Depressurization Warm up 78 86

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

13. Breakdown Collectives 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” 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 our investigations saturated liquid subcooled liquid gas

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

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

17. Semitron Tests 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 Semitron cathodeSemitron anode

18. Breakdown Histograms Brass cathode/semitron anode combination better Pressurized cooldown: Brass sphere HV, semitron GND 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