studies of voltage breakdown in superfluid 4 he may 20, 2008 maciej karcz, craig huffer, young jin...
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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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