Studies of Voltage Breakdown in Superfluid 4He

Studies of Voltage Breakdown in Superfluid 4He

May 20, 2008Maciej Karcz, Craig Huffer, Young Jin

Kim, Chen-Yu Liu, Josh Long

May 20, 2008Maciej Karcz, Craig Huffer, Young Jin

Kim, Chen-Yu Liu, Josh Long

OutlineOutline

Possible to sustain strong electric field in depressurized He-II by exploiting hysteretic phenomenon

Results suggest some caveats

Phenomenon is not well-understood

Phase Diagram Paths

Phase Diagram Paths

“Standard cycle”: 1 -> 2 -> 3 -> 4

“Pressurized Cooldown”: 3 -> 4

1

2

3

4

SetupSetup

PZTwafer

SSGND

Brass HVSphere

UpperT Sensor

CeramicFeedthrough

LowerT Sensor

VibrationProof

Washers

AdjustableGap Size

Typical HysteresisTypical Hysteresis

“Standard pressurization cycle” yields strong hysteretic behavior

Agitated SuperfluidAgitated Superfluid

Piezoelectric transducer results not conclusive

Occasional changes in breakdown strength

Brass sphere HV cathode

Resonance ~10.5 kHz

He-IIHe-II

Agitated SuperfluidAgitated Superfluid

Resonance ~ 12.7 kHz

SS sphere HV cathode

Brass makes better cathode?

He-II

Pressurized Cooldown

Pressurized Cooldown

SS sphere HV anode

Sharp decline in dielectric strength across lambda transition

No recovery on warm up??

LHe level on warm up ~ 3”

He-II

Pressurized CooldownPressurized Cooldown

He-II

He-II

Brass anode appears to have better HV performance

Neither recovers on warm up??

SS HV anode Brass HV anode

Standard CycleStandard CycleAttempt Std Cycle with brass sphere HV anode

SF transition marks large decline in breakdown strength

With HV sphere anode, on depress. tend to recover only ~50% of max. dielectric strength (with spherical cathode: 100% recovery)

He-II

Breakdown HistogramsBreakdown HistogramsStandard pressurization cycles: SS and brass

sphere HV anodes, SS GND cathodes

SS

Bra

ss

Vapor CurveCooldown

Vapor CurveCooldown

Pressurization

Pressurization

Depressurization

Depressurization

Warm up

Warm up

78

86

Breakdown HistogramsBreakdown Histograms

SF transition marks significant change

Pressurized cooldowns: SS and brass HV anodes, SS GNDs

SS

Bra

ss

Pres.Cool.

Pres.Cool.

Dep.

Dep.

Warm

Warm

72

91

Breakdown CollectivesBreakdown Collectives

Breakdown histograms in literature suggest two collectives: low-stress depends on pressure, high-stress on liquid density

“low-stress collective tendency is consistent with the ideas of bubble breakdown mechanisms”

Breakdown probability densityBreakdown in low-stress collective

J. Gerhold, “Helium Breakdown Near The Critical State”, IEEE

Transcations On Electrical Insulation, Vol. 23, Issue 4, 765-768, 1988

our investigations

saturated liquid

subcooled liquid

gas

Plastic Polymer: Non-metallic Mirror

Plastic Polymer: Non-metallic Mirror

Reflect scintillation light in actual nEDM exp., improve light collection

Folded into thirds, held by spring force

BrassHV

SSGND Polymer

Polymer TestPolymer Test

Anode draws current at ~150kV/cm

Adding polymer does not affect overall qualitative behavior

Smaller, spherical cathode better: less field emission?

Brass anode, SS cathodeBrass cathode, SS anode

Breakdown HistogramsBreakdown Histograms

Negative polarity: brass sphere cathode clearly performs better

Pressurized cooldown: Brass HV sphere, SS GND, polymer inside gap

Pres.Cool.

Dep. Warm

-+

-+

-+

Semitron TestsSemitron TestsElectrode material study

Carbon-loaded plastic GND electrode

Pressurized cooldowns with semitron GND and brass HV sphere

Semitron cathode cannot sustain max field very long: leakage current?

Semitron makes poor cathode, good anode

Semitron cathode Semitron anode

Breakdown HistogramsBreakdown Histograms

Brass cathode/semitron anode combination better

Pressurized cooldown: Brass sphere HV, semitron GND

Pres.Cool.

Dep. Warm

-+

-+

-+

RemarksRemarksPressurized cooldown: positive polarity runs show significant decline in dielectric strength during and after SF transition, negative polarity tend to perform better (surface area effect?)

Problem depressurizing during pressurized cooldowns: pressure does not respond to gas cylinder regulator, need to open SF valve to depressurize...possibly condensing too much He gas in the process of cooldown

++++

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- --- -+

CurrentlyCurrentlySurface wetting

Electropolished SS electrodes ready

Float glass, doped silicon wafers

Fiber optic setup: study HV-induced background

detect all wavelengths during breakdown

search for microdischarges before breakdown