High voltage test system update Summary of 2005 results Results from March, May 2006 Possible remaining R&D projects Polished electrodes, clean(er) LHe Limiting factor for breakdown still unresolved HV tests on reference design materials (or close approximations) J. Long, Indiana U. 26 May 2006

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 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

First results from prototype system Maximum leakage currents (95% C. L.) : SF (2.07 K): 733 pA Normal State (3.98 K): 169 pA Short-duration breakdown not affected by neutron radiation (10 6 /s, ~MeV) Large wire-seal flanges hold SF LHe (thermal gradients > 60 K / 60 cm) Small HV feedthrough exceeded maximum rating in air (40 kV) by 25% when immersed in SF Standard bearings (steel, plastic- encased) and welded bellows OK for electrode position control in SF Maximum potentials sustained: 11.8 liters Normal State (4.38 K), 7.2 cm gap: (96 ± 7) kV/cm 12.8 liters SF at 2.14 K, 7.8 cm gap: (31 ± 3) kV/cm arxiv: physics/

E-field specification (M. Cooper 5/25/06) Optimal performance: saturated LHe not limiting factor if T > 3.1 K (P > 200 torr) Minimal performance: saturated LHe not the limiting factor if T > 2.0 K ( P > 25 torr) After modifications of March and May 2006: cannot make a stronger statement

Recent Modifications New readout with amplifier connected to oscillating ground electrode Expect ~ 2x breakdown field improvement for ~ 5x surface roughness reduction (?) [Gerhold, IEEE Trans. on Dielec. and Elec. Insul., 1 (1994) ] Electrodes (Aluminum) polished Microamp signals (previous: nA) ~ pA leakage currents detectable in a few hours Operation with DR pump on LHe bath (~30% more speed, cooling power) all surfaces 16  -inch finish (previously 64  -inch) January-March 2006 April-May 2006 Reliable LHe level sensing Filter on transfer line (~ 10  m pores) Short t scales: i = V (dC/dt) Long t scales: i LEAK = C (dV/dt)

Recent HV test system operation March 2006 DAQ too cumbersome to be used for more than 3 leakage current runs - Need ~ 1 wk more programming “Hybrid” method (ground electrode as DC probe of HV) inconclusive - Analysis still in progress Trip-off and backstreaming of DR pump -> mm-sized frozen contaminants coating electrode surfaces before any data obtained New leakage current results: (5 ± 1) pA [1.8 K, 3 kV on 8 cm gap] (10 ± 2) pA [4.0 K, 70 kV on 8 cm gap] - Correct to within overall scale factor ~2 (capacitance mis-measurement) May 2006 Cleaner LHe (10  m pore filter), no visible contaminants (except last run) Careful measurement of maximum V for 0.2 – 2.0 cm gap at 1.8 and 4.0 K

Recent small-gap results Extrapolates to 96 kV/cm at 8 cm Extrapolates to 31 kV/cm at 8 cm closed points = SF open points = normal state Blue = 2005 results Black = filtered LHe Red = heavy contaminants Power supply saturated Electrode finish, LHe purity (in range from poor to ghastly) not limiting factors in the HV test system Degradation below 1.9 K: LHe level possibly below tops of electrodes

Remaining R&D projects ~ simplest to most complex More careful purification tests (still limiting factor?) Charcoal or other filter on LHe delivery line LN2 trap on LHe bath pumping line (in absence of dry system) - Few x $1000, few weeks depending on lead times Breakdown tests on prototype holding cell (surprises…) Material (acrylic, pyrex?, d-polystyrene layer?, d-TPB coating?) Dimensions (10 x 10 x 8 cm possible, want hollow?, mounted in recesses in electrode?) More breakdown diagnostics? Replace the aluminum electrodes (more surprises…) Material (acrylic coated with graphite, diamond?, how apply coat?) - Total assuming 2 x cost and delivery of existing electrodes ~ $10 k, 3 mo. - Hollow coated box with no electrode modification: $10 k (incl. design, LHe), 2 mo.

Remaining R&D projects ~ simplest to most complex Kerr measurement ~ $30 k, 4 months, depending on optical system element lead times Assumes no provision for background subtraction scheme HV tests in pressurized SF LHe Useful to de-couple P and T effects on breakdown, even to 1.5 K If not completely intractable for actual EDM experiment (valves, heat loads), at least get engineering drawings for upgrade of HV test system D. Haase has developed design, vetted by S. Penttila (heat loads, thermal expansions, pressure measurement) ~ $30 k, 6 months HV tests at 0.5 K Install HV test system central volume in 3He R&D cryostat Need ~ 1 dedicated month for initial tests, ~2 week dedicated slots for other tests Sacrifice test system until installed (~ 3 months?) Smaller pair of fixed electrodes installed in a DR?