Plans for HV R&D at LANL

Vacuumchamber

Supplycryostat

Josh Long, Indiana U.

Tests in progress

Control purity, contaminants, attempt T < 1.8 K

Near Future

Breakdown studies of test cell materials

2007

Large-volume HV studies at DR temperatures, high pressure

Previous results from prototype system

Maximum leakage currents under these conditions (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)

Small commercial HV feedthrough exceeded maximum rating in air (40 kV) by 25% when immersed 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

Pressure effects common in literature, e.g.:

LHe breakdown at 2 K (at 100 m gap) : 500 kV/cm (25 torr), 800 kV/ cm (700 torr) [Suehiro, et al., 12th Intl. Conf. Conduction and Breakdown in Dielectric Liq., Rome, 1996, 320.]Possible further degradation below 1.9 K

Bubble formation (common breakdown culprit) observed when charging above 10 kV, likely coincident with noise

Better performance (wide T, P range) without pressurizing?

Reduced surface roughness (64 -inch to 16 -inch)

Expect ~2x improvement in breakdown [Gerhold, IEEE Trans. on Dielec. and Elec. Insul., 1 (1994) 432-439 ]

No observed improvement at 1.9 K or 4 K

Results uncertain due to excessive contamination (oil, air backstreaming from pump)

Attempt to improve purity of LHe and reduce surface contaminants

Principal concerns (from literature):

Hydrocarbons

LHe contaminated with oil “useless” as insulator [Gerhold, Cryogenics (October 1972) 370]

Conducting contaminants

Oxygen

Existing (“accidental”) observations

4 K breakdown strength unchanged after many hours pumping with oil pump

Small gap breakdown at 2 K and 4 K unchanged after backstreaming event

Unchanged after frozen contaminants left to evaporate and system replenished with LHe passed through 10 mm pore filter

Current tests in progress

Control purity and surface contaminants systematically

Complete solvent cleaning of interior

RF Plasma discharge cleaner (for Hydrocarbons)

Dry pumps only for all applications

No LN2 pre-cooling: attempt using cold, filtered He gas from LHe supply dewar(can filter LN2 if not practical)

LN2 trap on LHe bath pumping line

LHe (and gas) filter on transfer line outlet

Charcoal + low porosity materials, Quantum Technologies

RGA monitoring

Further reduced surface roughness

Electrodes polished to ~ 8 -inch finish

Current tests in progress

Attempt operation below 1.8K

Colder LHe transfers

New transfer line operates with HV system below lambda point

Heat load reductions (also important for eventual operation with DR):

Tie central volume lateral support posts to 77 K

Tie supply cryostat upper neck to 77 K shield

Remove all unnecessary (conducting) instrumentation from supply cryostat upper neck

Cover open viewport holes in 77 K shield with quartz windows

Tie actuator rods to 77 K shield

Previous heat load 2 W

1 W

0.5 W

0.1 W

0.1 W

0.02 W

Other

Video monitoring of gap

Improved level sensing

R&D questions (initial test)

Breakdown study of test cell materials

Assess if low breakdown strength

Modify system as little as possible to compare to established baseline (initial test)

Acrylic: breakdown strength = 200 kV/cm in air at 300 K

Note: G-10 standoffs = 400 kV/cm

Assess if high leakage current (especially across straight, smooth surface)

Use existing HV test system

Later phase: refine test cells to reflect final geometry, coatings (or as necessary to attain higher breakdown if problems)

Plan/ technical assumptions

~65% of field in maximum HV-Ground gap

Good uniformity

Single 14 x 9 x 2 cm piece (rounded edges)

Equidistant between standoffs (back view):

Breakdown study of test cell materials

(ignoring G10 standoffs [ ~ 5])

Possible phase II: add 5 more at different angular positions to match edge length of actual cell (120 cm)

electrode

standoff

Breakdown study of test cell materials

Complications:

Acrylic thermal expansion ~ .02 from 2-300 K (?)

Cell shrinks 2 mm relative to 14 cm G-10 standoffs

-> Spring-loaded base to attach to isolation plate (?)

Cell shrinks 0.5 mm relative to adjacent points on electrode

-> Issues for mounting in recesses:

electrode

cell

Task Est Cost Est Duration Start Finishdesign cell (LANL engineer) 592 1 10/12/2006 10/13/2006design mount/electrode mods (LANL engineer) 2960 5 10/16/2006 10/20/2006procure cell 500 15 10/20/2006 11/3/2006procure mounts/electrode mods 2000 15 10/20/2006 11/3/2006install electrodes and cells (LANL tech) 2240 5 11/6/2006 11/10/2006procure LHe 4500 5 11/6/2006 11/10/2006run tests (LANL tech) 4480 14 11/13/2006 11/27/2006contingency (20%) 3454

Initial test total and dates 20726 10/12/2006 11/27/2006

procure remaining cells 2500 15 12/1/2006 12/14/2006procure remaining mounts/electrode mods 4000 15 12/1/2006 12/14/2006install electrodes and cells 2240 5 12/18/2006 12/22/2006procure LHe 4500 5 1/8/2007 1/12/2007run tests 4480 14 1/15/2007 1/29/2007contingency (20%) 3544

Full test total and dates 21264 12/1/2006 1/29/2007

Acrylic test total and dates 41990 10/10/2006 1/21/2007

Breakdown study of test cell materials

R&D questions

HV tests in upper cryostat prototype

What happens to breakdown strength and leakage currents in large LHe volumes/electrode gaps at 500 mK?

System should be dis-mountable within ~ 1 week to run w/o DR if necessary

Couple existing HV test system to DR via dedicated insert in upper cryostat prototype

Plan/ technical assumptions

No interference with injection/relaxation insert (though separate experiments will have to run in series)

Heat loads in existing test system can be controlled

How much improvement can be attained with pressurized LHe (~1 atm) at these temperatures?

Incorporate best candidate materials for electrodes and cell prototypes

Address remaining questions about purity by condensing LHe from filtered gas (if time)

Pump and flush HV volume

Backfill with P > 1atm He gas

Pre-cool with LN2 through coiled line

Flush coiled line

Pre-cool to 4.2 K with cold He/ LHe through coiled line

Evacuate and seal coiled line

Open LHe valve to fill HV volume from LHe supply

Close LHe valve

OPTION: Pressurize LHe volume with He gas to P ~ 1.4 atm

Push down plastic heat stopper

Engage thermal link(s) to DR

Cool to 1.5 K (0.5 K)

Design from D. Haase, 8/06

Design from D. Haase, 8/06

Significant design work remaining

Operation contingent on reducing heat loads in HV volume

DR ~ 50 mW @ 300 mK (?)

Current HV system heat load: 2W

Target: < 1 W

Additional coiled pre-cooling line around copper shield possible (10 K?)

Heat load through SI: ~ 0.5 W

~20” Revised Spool

Modified Adapter

Proposed location for HV 1K frig

Location for future purifier

Procurement begun for adaptor parts

Task Est Cost Est Duration Start Finishdesign insert (LANL engineer) 8880 21 10/9/2006 10/30/2006procure pre-cool lines (HV tank, shield) 500 7 10/31/2006 11/6/2006procure pre-cool lines (DR volume) 500 7 10/31/2006 11/6/2006procure LHe valve, f ilter 2000 30 10/31/2006 11/30/2006procure safety valve, line 2000 30 10/31/2006 11/30/2006procure thermal sw itch(es) 5000 30 10/31/2006 11/30/2006procure LHe volume 5000 50 10/31/2006 12/19/2006procure heat exchanger(s) 5000 50 10/31/2006 12/19/2006

install LHe volume, valve, f ilter (LANL tech) 6720 21 1/8/2007 1/28/2007install heat exchangers, sw itches (LANL tech) 6720 21 1/29/2007 2/17/2007Modify copper shield (pre-cool line, LANL tech) 2240 7 2/18/2007 2/24/2007Install HV volume pre-cool line (LANL tech) 2240 7 2/25/2007 3/2/2007

contingency (50%) 23400

assembly total and dates 70200 10/9/2006 3/2/2007

Initial test (debug, low -T data):procure LHe 6000 5 3/5/2007 3/9/2007run tests (LANL tech) 13440 40 3/3/2007 4/15/2007

operations contingency (50%) 9720

initial operation total and dates 29160 3/5/2007 4/15/2007total 99360

HV tests with DR (guesses; independent of cell tests)

Subsequent tests (pressurization, new electrodes, etc: 3-4 weeks each with dedicated use of DR ~ 2x as long to assemble (Summer 2007) if no work in parallel with cell tests ?

R&D questions

Other HV issues

Possible in principle via Kerr effect (accuracy? Effect of acrylic?)

Either can be incorporated whether or not system coupled to DR

Plan/ technical assumptions

Can we assess field uniformity in test system?

Will need additional windows for scanning large regions of gap or scanning behind HV electrode (Kerr)

Can we bring 350 kV directly into system?

Figure: Karamath, Lepton Moments conference (2006)

Can already bring 50 kV into present LANL test system at 1.9 K

(1.5 < gap < 8) cm

Electrical breakdown in a full cell

an extensive literature deriving from interest in insulating high power superconducting apparatus

suggested possibilities 1) impurities 2) motion of individual electrons – localized or delocalized 3) field emission

field emission from the cathode is the most likely possibility based on what is known about helium

highly localized dissipation of energy by electrons creates hot spotliquid vaporizes, creates macroscopic bubble; avalanche and breakdown

ensues

FROM G. SEIDEL, LANL, 9/27/06 (NOT PRESENTED)

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