1. Dielectric Strength of Superfluid Helium Under Pressure Small Pressure-Controlled Cryostat Overview Measurement Cycle Possible Breakdown Mechanisms Dielectric Strength as a function of Pressure, Temperature J. Long Craig Huffer, Maciej Karcz, Young-Jin Kim, C.-Y. Liu Indiana University

2. stainless can aluminum plate wire seal flange G-10 standoff HV plunger control rod Indium seal ceramic standoff HV electrodeground electrode ground control rod Vacuum-LHe HV feedthrough bearings bellows 0.53 m quartz window Review - High voltage system prototype at LANL Vacuum chamber Supply cryostat HV feedthrough Actuator Test proposed amplification method Measure breakdown properties of large volumes of LHe Existing data: 150 kV/cm at 4 K, 1cm gap LHe bath pumping line

3. Review - Results from Large Prototype system at LANL Maximum potentials sustained: 10.5 liters Normal State (4.38 K), 6.4 cm gap: (760 ± 70) kV = (119 ± 11) kV/cm 8.2 liters SF at 2.14 K, 5.0 cm gap: (290 ± 40) kV = (58 ± 8) kV/cm

4. HV = brass sphere, GND = SS plate, Min. gap = 1.3 mm (robust, no parallelism issues) Small Pressure - controlled Cryostat at Indiana V HV GND Inner Volume HV power supply limited to 20 kV (feedthrough rating) Inner volume ~ 0.3 l

5. HV = brass sphere, GND = SS plate, Min. gap = 1.3 mm (robust, no parallelism issues) Small Pressure - controlled Cryostat at Indiana V HV GND Inner Volume HV power supply limited to 20 kV (feedthrough rating) Inner volume ~ 0.3 l

6. HV = brass sphere, GND = SS plate, Min. gap = 1.3 mm (robust, no parallelism issues) Pressure - controlled Cryostat V HV GND Inner Volume Outer volume thermal bath 8 l/s HV power supply limited to 20 kV (feedthrough rating) Inner volume ~ 0.3 l Outer volume ~ 10 l For Cooling

7. HV = brass sphere, GND = SS plate, Min. gap = 1.3 mm (robust, no parallelism issues) Pressure - controlled Cryostat V HV GND Inner Volume Outer volume thermal bath 8 l/s He gas regulator HV power supply limited to 20 kV (feedthrough rating) Inner volume ~ 0.3 l Outer volume ~ 10 l Standard gas cylinder and regulator 1/16” stainless tubing (inside cryostat) Mensor-type pressure gauge (mechanical) “Superfluid- tight” valve For Cooling For Pressurizing

8. HV = brass sphere, GND = SS plate, Min. gap = 1.3 mm (robust, no parallelism issues) Pressure - controlled Cryostat V HV GND Inner Volume Outer volume thermal bath 8 l/s He gas regulator “Superfluid- tight” valve Lid T- sensor Gap T- sensor HV power supply limited to 20 kV (feedthrough rating) Inner volume ~ 0.3 l Outer volume ~ 10 l Standard gas cylinder and regulator 1/16” stainless tubing (inside cryostat) Ruthenium Oxide thermometers Mensor-type pressure gauge (mechanical) For Cooling For Pressurizing

9. Fill with LHe, SF valve open, test HV at 1 atm Measurement Cycle

10. Fill with LHe, SF valve open, test HV at 1 atm Close SF valve, pump outer volume (~10s of minutes), test HV above and below T Measurement Cycle

11. Fill with LHe, SF valve open, test HV at 1 atm Close SF valve, pump outer volume (~10s of minutes), test HV above and below T Pressurize with He gas (~5 minutes) Measurement Cycle

12. Fill with LHe, SF valve open, test HV at 1 atm Close SF valve, pump outer volume (~10s of minutes), test HV above and below T Pressurize with He gas (~5 minutes) Maintain at pressure, let OUTER BATH PUMP cool system below T (~10s of minutes), test HV above and below Measurement Cycle

13. Fill with LHe, SF valve open, test HV at 1 atm Close SF valve, pump outer volume (~10s of minutes), test HV above and below T Pressurize with He gas (~5 minutes) Maintain at pressure, let OUTER BATH PUMP cool system below T (~10s of minutes), test HV above and below Decompress with regulator, test HV at several pressures Measurement Cycle

14. Fill with LHe, SF valve open, test HV at 1 atm Close SF valve, pump outer volume (~10s of minutes), test HV above and below T Pressurize with He gas (~5 minutes) Maintain at pressure, let OUTER ABTH PUMP cool system below T (~10s of minutes), test HV above and below Decompress with regulator, test HV at several pressures Measurement Cycle Notes: 1.Not necessary to warm up, cool down, and re-pressurize system after a breakdown – just continue around path to next point in phase diagram 2.Maximum voltage at each point in phase diagram is usually the best of 2-3 measurements

15. Maximum Voltage Data – Cool Down Lid and Gap temperature sensors track each other as expected Degradation of dielectric strength observed as in large prototype system Degradation might be expected during pump-down due to increased boil- off/turbulence; this is problematic below lambda point…

16. Maximum Voltage Data – Pressurization Initial GAP temperature sensor reading: Just on vapor pressure curve Subsequent readings approach closer to - point Initial LID temperature sensor reading: Well into vapor region, consistent with gas influx Subsequent readings approach closer to vapor pressure curve Full 20 kV applicable to 1.3 mm gap independent of gap temperature

17. Maximum Voltage Data – Decompression Regulator adjusted to provide pressures of 600, 400, and 200 torr (breakdown voltage measured at each), before regulator shut off whereby inner volume pressure determined by temperature 600 torr: Lid temperature catches up with gap sensor (re-liquification of He near lid) DIELECTRIC STRENGTH REMAINS HIGH DOWN TO 1.9 K AND 20 TORR

18. Second Run Test basic repeatability of effect Chose slightly different path in phase diagram: Cool down to colder temperature Pressurize to lower maximum pressure

19. Maximum Voltage Data – Cool Down (Path 2) Cooled to lower temperature (1.7 K) Further degradation of dielectric strength observed below 2 K

20. Maximum Voltage Data – Pressurization (Path 2) Pressurized only to 200 torr Initial GAP temperature sensor reading: Just on vapor pressure curve Subsequent readings approach closer to - point Initial LID temperature sensor reading: Well into vapor region, consistent with gas influx Subsequent readings continued to increase for several minutes Full 20 kV applicable to 1.3 mm gap independent of gap temperature as before

21. Maximum Voltage Data – Decompression (Path 2) Regulator adjusted to provide pressures of 100 and 50 torr (breakdown voltage measured at each), before regulator shut off whereby inner volume pressure determined by temperature ~20 torr: Lid temperature catches up with gap sensor (re-liquification of He near lid) DIELECTRIC STRENGTH REMAINS HIGH DOWN TO 1.7 K AND 10 TORR (some degradation at 5.5 torr)

22. Possible Breakdown Mechanism in He Vapor bubble (cavitation) –Breakdown voltage < 5kV/cm at 20 Torr helium vapor –Requires negative pressure (P vapor above liq. <P bubble ) –Energy barrier is pressure dependent. –Only exist in He I ?? Two fluid model: ~55% He I at 2K –Formation probability decreases as T drops Electron bubbles –Field emission of electrons into He bath –Size of bubble = 19 A (at P=0) –Bubble Explosion is also pressure dependent –Drift under the influence of electric field (external field + space charge field) –Formation is T indep. Mobility increases rapidly as T drops → leakage current Vortices –Electrons move inside the vortex tube easily –Electron bubbles attached to the vortex tube, & explode (cavitate) –Formation of vortices are highly dependent on the degree of turbulence in liquid (hysteretic)

23. Vortices Formation in He II “Superfluid turbulence manifests itself as a tangle of vortex lines and can be generated in many ways.” 2.172K, 14 s 2.148K, 83 s 2.125K, 102 s 2.160K, 37 s Counterflow Heat flux exceeds a critical value, Convection from boiling Co-flow Rotating cylinder Ultrasounds, Spinning discs, Propellers n > 1000/cm 2 G. Bewley, D. Lathrop, K Screenivasan Concentration:of lines n ~ 100 /cm 2

24. Cooling path Initial cooldownCooldown under pressure

25. Electron Bubbles A Ghosh, H. Maris, PRL 95, 265301 (2005)

26. Summary Degradation of dielectric strength with pumpdown-cooling observed as in large HV system Dielectric strength recovers when pressurized Need not pressurize all the way to 1 atm for full recovery Recovery is (mostly) maintained upon subsequent cooling at constant pressure, AND on subsequent de-compression at ~ constant temperature Different paths in phase diagram: De-compress with system above -point (vortex-free regime) to test vortex mechanism; cool-down away from vapor pressure curve. Test with electrodes of different composition (both stainless, Torlon, Semitron, aluminum): different surfaces = different hysteresis effects? Higher breakdown? Next Steps (?)