High voltage test system update
description
Transcript of High voltage test system update
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?