1. Preliminary HV Results in Superfluid Helium Review of HV system design J. Long, J. Boissevain, J. Gomez, S. Lamoreaux, S. Penttila LANL Amplification and large-gap E-fields Leakage currents Pressure dependence of breakdown (includes normal state) Superfluid LHe production Neutron irradiation Possible near future plans (pressurization) HV results Noise issues

2. vacuum chamber supply cryostat 77 K shield G-10 foot linear actuator air-vacuum HV feedthrough ~2 m LN2 reservoir Test System Design Vacuum pump, T- sensor readout attachments LHe vessel LHe reservoir

3. Superfluid Production in HV System 1. Fill HV system with normal state LHe at 4 K from full 500-liter supply dewar (1 hr) 2. Pre-cool LHe in both HV system and supply dewar (3 hrs) HV system: pump bath with roots blower (250 m 3 /hr) to 40 torr (2.2 K, above -point) Supply: pump through vent with scroll pump (15 m 3 /hr) to ~ 230 torr (~3.2 K) 3. Restart LHe transfer, top off HV system with low pressure LHe above -point (1hr) Leave roots blower on system Vapor P in HV system rises to ~ 90 torr (2.6 K) Transfer rate ~ 1 liter/min 4. Stop LHe transfer, leave roots blower on to pump system below -point (3 hrs) Observe -transition (rapid, complete cessation of all turbulence) at 35 torr (2.14 K) Process uses ~ 400 liters of LHe, takes 8 hrs Thanks to John Jarmer (LANSCE-6) for suggesting step 2

4. C HG HVPS 50 kV Q C HC C CC C CF C HF Amplification Measurement: Meter on Charger Use SR570 current amplifier Readout with ADC at 130 Hz First attempted load cell on actuator: P =  0 E 2 /2, Unrepeatable backgrounds at 4 K

5. Readout 10 M  GAMMA 50 kV 1.25 mA HVPS RG8 - BNC SR570-A CURRENT PREAMP TERMINAL STRIP NI-PCI 6024e ADC 64 LabVIEW RG8 7m 500 pF LAKESHORE 218 16 GPIB OMNI- LINK PC RS-232 THOMSON MOTOR 360 W THOMSON DRIVE # CDM010i ~ 4500 N max

6. HV-Charger Capacitance Close HV-G gap Monitor C with bridge on 100 kV feedthrough as increase Charger-HV separation Charger retracted to 5.0 cm where C HC = 1.1 ± 0.1 pF

7. Largest Potentials Attained 6/7/05 17:35, step G from 2.5 to 78 mm, initial V = 13 kV, P = 33.8 torr (T~2.13 K) = 258 nC V HG (7.8 cm) = (259 ± 34) kV C HC error 10% SR570 zero drift3% transients2-13% Truncate sum at each point starting at t = 0 Convert time axis to gap (.085 cm/s) Potential vs gap curve: V HG = (570 ± 70) kV Previous normal state results: E HG (7.8 cm) = (33 ± 4) kV/cm (Design = 50 kV/cm…) E HG = (78 ± 9) kV/cm

8. Largest Potentials Attained: V < 0 6/7/05 17:50, step G from 2.5 to 78 mm, initial V = -14 kV, P = 33.4 torr (T~2.13 K) = -268 nC V HG (7.8 cm) = (-269 ± 35) kV V HG = (-360 ± 60) kV Previous normal state results: E HG (7.8 cm) = (-35 ± 5) kV/cm E HG = (-49 ± 8) kV/cm

9. Radiation Effects n-flux in gapInitial V (kV)CommentsTime (Background)13No source17:23 ~10 6 /s, E ~ 1 MeV, 10% ~ 1 keV ±14Source atop 2 cm plexiglas 17:34 Results just shown (maximum potentials) actually attained with ~ 7 Ci n-source, 50 cm from gap, nearly on-axis Enhancement likely due to larger initial V at small gap (2.5 mm): Slight improvement could have several sources (radiation, conditioning, switch to negative polarity, more transients…) Maximum potential in absence of radiation: V HG (7.8 cm) = (228 ± 30) kV 6/7/05 17:23, initial V = 13 kV, P = 34.7 torr (T~2.14 K)

10. 6/7/05 21:55:50, step G out to 8.0 cm initial V = -6 kV (!), P = 28 torr (T~2.06 K) Leakage Current 6/7/05 22:12:39, return G to 3 mm gap P = 30 torr (T~2.09 K) Q HC = 88.6 nC Q HC = 82.2 nC C HG = 55 pF (bridge, ± 5%) C HC = (1.1 ±.1) pF  Q HC = (6 ± 8) nC (3% zero shift) i LEAK = (-2 ± 20) pA  t = (1009 ± 30) s _ (E HG = [-12 ± 1] kV/cm) i LEAK = (0.40 ± 0.45) nA _ Previous normal state result:

11. Leakage Current - Remarks Attempts before data on last slide: attemptinitial V (kV)time delayfinal V (kV) 1 2 3 5 4 6 8 7 P (torr) Would like to repeat with larger initial V and longer time delay 32 28 25 26 27 12.533 discharge (on pull-out) 12 (discharge) -11.5 1 hr 0 -11 1 hr 0 11 10 -7 (discharge) Stability of HV in SF? Low P?

12. Breakdown vs. Pressure, Temperature -point: pressure reading when SF transition observed in our system 75 te) data below -point are highest attained in ~ 0.05 K bins above 2.05 K Point at 890 torr (4.4 K) is system record: (638 ± 83) kV, (80 ± 10) kV/cm at 8 cm Typical low-pressure normal state OR SF operation: 220 kV, 28 kV/cm at 8cm

13. V = 298 kV SF (34 torr) SF (33 torr) + neutron rad Normal State (322 torr) SF (34 torr) Inward trace (13% increase) Transients Greater effect at low pressure Predominantly positive (negative) when HV positive (negative) Enhanced by neutron radiation ~ 20 ms rise time, ~100 ms FWHM, ~150 ms decay time, ~1-2 nC He gas bubbles? Kerr Effect: E-field measurement less susceptible to this effect?

14. Pressurization Estimates Volume change for  P = 1 atm: Isothermal? Time system spends below 2 K: Need valve in neck above stainless can:  V =  V 0  P = 2.6 l  = compressibility SF LHe = 10 -7 Pa -1 (Keller, 3 He and 4 He) Have 2 spare bellows with  V = 1.4 l each (if initially stretched)  P dV = Mc  T = 130 J   T = 10 mK  Q/Q = 3.2 hr (assumes old 2.7 W load). Leak rate: (1.1 atm – 0.9 atm)/ 3.2 hr X 2.6 l/atm = 160 cc/hr Force on actuator: F = PA = 14 psi X 14 si + typical bellows resistance = 300 lb Actuators in used rated for 1000 lb Open/close while immersed in SF LHe 1” diameter minimum

15. Pressurization Upgrade - Bellows

16. Pressurization Upgrade – Valve, Dewar