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 canaluminumplate

wire sealflange

G-10standoff

HV plungercontrol rod

Indium seal

ceramicstandoff

HV electrodeground electrode

groundcontrol rod

Vacuum-LHeHV feedthrough

bearingsbellows

0.53 m

quartz window

High voltage system prototype at LANL

Vacuumchamber

Supplycryostat

HVfeedthrough

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 pANormal State (3.98 K): 169 pA

Short-duration breakdown not affected by neutron radiation (106/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/0603231

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) 432-439 ]

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: iLEAK = C (dV/dt)

Recent HV test system operation

March 2006DAQ 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 2006Cleaner 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?