Plans for HV R&D at LANL Vacuum chamber Supply cryostat 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 pA Normal State (3.98 K): 169 pA Short-duration breakdown not affected by neutron radiation (10 6 /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., 12 th 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) ] 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 load2 W 1 W 0.5 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 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

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

Significant design work remaining Operation contingent on reducing heat loads in HV volume DR ~ 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

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 spot liquid vaporizes, creates macroscopic bubble; avalanche and breakdown ensues FROM G. SEIDEL, LANL, 9/27/06 (NOT PRESENTED)

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