1. Results from the HV Test System Review of current design and assembly J. Long, P. Barnes, J. Boissevain, J. Gomez, S. Lamoreaux, D. Mischke, S. Penttila LANL Amplification and large-gap E-fields Leakage currents Pressure dependence of small-gap breakdown Results with normal state LHe Neutron irradiation Cool-down and heat loads Plans for near future

2. 50 kV/cm 76 mm extrapolation (?) Electric Field: LHe Breakdown vs Gap Figure: J. Gerhold, Cryogenics 38 (1998) 1063 Need 50 kv/cm across 8 cm cell ~ 400 kV Internal amplifier (avoid heat loads, large feedthroughs) Variable capacitor with C >> C cells ~ 50 pf

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

4. Assembly: central volume View of electrodes through side portLeak check of central volume and bellows

5. Assembly: vacuum system LHe vessel in Kevlar sling 20 layers superinsulation Complete system showing HV charger drive rod, linear actuator and motor

6. Cooling and filling of central volume (December 2003) LN 2 transfer tube LHe vessel plug 1 2 3 4 5 Temperature sensor locations 3 days to fill from room temperature start (limited by Cu shield), need ~ 400 liters of LHe

7. Heat loads Estimated sources Kevlar rope suspension10 mW Ground actuator/spider10 mW HV actuator/spider5 mW HV conductor2 mW Unshielded quartz windows30 mW 2585 mW (1555 mW) Supply cryostat neck1800 mW (Neck with 77 K anchor)(770 mW) Temperature sensors1 mW Measurement Average He gas boil-off: (flowmeter near 300 K) 52 liters/min 4 K liquid boil-off: 3.4 liters/hr Total load:2660 mW Estimate after restoration of 77 K anchor: 1600 mW Radiation through SI 725 mW (?)

8. 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

9. 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

10. HV-Charger Capacitance Close HV-G gap Monitor C with bridge on 100 kV feedthrough as increase Charger-HV separation Charger retracted to 4.24 cm where C HC = 1.6 ± 0.2 pF C0Az0C0Az0

11. Pull back G electrode at constant 1inch/min Largest Potentials Attained Expect initial current (first 2.5 mm pullback): ~ 12 nA 2/25/04 11:00, step G from 2.5 to 73 mm, initial potential = 42 kV Shape should track dV/dz : (z 0 ~ 5 cm)

12. Largest Potentials Attained = 862 nC V HG (7.3 cm) = (570 ± 70) kV C HC error 13% SR570 zero drift3% transients2% 2/25/04 11:00, step G from 2.5 to 73 mm, initial potential = 42 kV 2/20/04 22:15, step G from 2.5 to 73 mm, initial potential = -31 kV V HG (7.3 cm) = (-360 ± 60) kV Charging with negative potential: Draws steady current > 50  A below –30 kV Current independent of charger position

13. 2/20/04 23:15, step G out to 7.3 cm initial potential = 29 kV Leakage Current 2/21/04 10:47, return G to 3 mm gap Q HC = 566 nC Q HC = 568 nC C HG = 53 pF (bridge, ± 5%) C HC = (1.6 ±.2) pF  Q HC = (-2 ± 24) nC (3% zero shift) i LEAK = (-2 ± 20) pA  t = 11h 32 min ± 5 min _ (E HG = [52 ± 8] kV/cm)

14. Breakdown vs. Pressure – 3 mm gap Pump LHe bath with roots blower (250 m 3 /hr) ~ 8 psi check valve limits pressure Could reduce to 25 kV/3 mm (300 kV/ 7.3 cm) or worse at low P 2/27 data (open circles): System had 2 additional days at 4 K 5 hr after LHe top-off 30 min. prior HV conditioning

15. Radiation Effects n-flux in gapBreakdown V (kV)CommentsTime (Background)30 ± 1No source11:00 ~10 6 /s, E ~ 1 MeV, 10% ~ 1 keV 34 ± 2Source behind 2 cm plexiglas 11:03 10 6 /s, E ~ 1 MeV36 ± 2Plexiglas removed 11:07 ~ 7 Ci n-source, 50 cm from gap, nearly on-axis Small gap (3 mm), 30 minutes after LHe fill, ~ 5 minutes conditioning, 1 psig Slight improvement (or conditioning) Large gap (maintained for 1 minute, no plexiglas, inward trace shown): V HG (7.3 cm) = (390 ± 60) kV

16. Conclusions Normal State LHe holds 570 kV at 7.3 cm (~ 40% higher than “expected”) 73 mm (570 ± 70) kV Design field at 7.3 cm holds for > 11 hr Max leakage current = 20 pA (~ 3% of tolerable limit) Problem with low-P operation? Small gap breakdown not affected by neutron radiation (10 6 /s, ~MeV) Large wire-seal flanges hold LHe (thermal gradients > 60 K / 60 cm) Superfluid operation (either way…) E-field measurement via Kerr effect (Discussions with UC…) Prototype holding cell (Lucite, coatings…) behind HV electrode Next steps