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

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

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

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

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

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

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 (?)

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

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 GPIB OMNI- LINK PC RS-232 THOMSON MOTOR 360 W THOMSON DRIVE # CDM010i ~ 4500 N max

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

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)

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

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)

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

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: /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

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