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Preliminary HV Results in Superfluid Helium
Review of HV system design
J. Long, J. Boissevain, J. Gomez, S. Lamoreaux, S. Penttila
LANL
Amplification and large-gap E-fields
Leakage currentsPressure dependence of breakdown (includes normal state)
Superfluid LHe production
Neutron irradiation
Possible near future plans (pressurization)
HV results
Noise issues
vacuumchamber
supplycryostat
77 K shield
G-10 foot
linearactuator
air-vacuumHV feedthrough
~2 m
LN2reservoir
Test System Design
Vacuum pump, T- sensor readout attachments
LHe vessel
LHereservoir
Superfluid Production in HV System
1. Fill HV system with normal state LHe at 4 K from full 500-liter supply dewar (1 hr)
2. Pre-cool LHe in both HV system and supply dewar (3 hrs)
HV system: pump bath with roots blower (250 m3/hr) to 40 torr (2.2 K, above -point)
Supply: pump through vent with scroll pump (15 m3/hr) to ~ 230 torr (~3.2 K)
3. Restart LHe transfer, top off HV system with low pressure LHe above -point (1hr)
Leave roots blower on system
Vapor P in HV system rises to ~ 90 torr (2.6 K)
Transfer rate ~ 1 liter/min
4. Stop LHe transfer, leave roots blower on to pump system below -point (3 hrs)
Observe -transition (rapid, complete cessation of all turbulence) at 35 torr (2.14 K)
Process uses ~ 400 liters of LHe, takes 8 hrs
Thanks to John Jarmer (LANSCE-6) for suggesting step 2
CHG
HVPS
50 kV
Q
CHC
CCCCCF
CHF
HC
HC
CCCFHCHCHG C
Q
CCCQV
11
Amplification Measurement: Meter on Charger
• Use SR570 current amplifier
• Readout with ADC at 130 Hz
)( dtiQ HCHC
First attempted load cell on actuator: P = 0E2/2, Unrepeatable backgrounds at 4 K
Readout
10 M
GAMMA 50 kV 1.25 mA
HVPS
RG8 - BNC SR570-ACURRENTPREAMP
TERMINALSTRIP
NI-PCI6024eADC
64
LabVIEW
RG87m500 pF
LAKESHORE218
16GPIB
OMNI-LINK
PCRS-232
THOMSONMOTOR360 W
THOMSONDRIVE
# CDM010i
~ 4500 N max
HV-Charger Capacitance
Close HV-G gap
Monitor C with bridge on100 kV feedthrough as increaseCharger-HV separation
cmz
cmpFpFC
1.0
1.5132
Charger retracted to 5.0 cm whereCHC = 1.1 ± 0.1 pF
Largest Potentials Attained6/7/05 17:35, step G from 2.5 to 78 mm, initial V = 13 kV, P = 33.8 torr (T~2.13 K)
= 258 nC
VHG (7.8 cm) = (259 ± 34) kV
CHC error 10%
SR570 zero drift 3%transients 2-13%
n
nnnnHC iittQ 2/11
Truncate sum at each point starting at t = 0
Convert time axis to gap (.085 cm/s)
Potential vs gap curve:
VHG = (570 ± 70) kV
Previous normal state results:
EHG (7.8 cm) = (33 ± 4) kV/cm
(Design = 50 kV/cm…)
EHG = (78 ± 9) kV/cm
Largest Potentials Attained: V < 06/7/05 17:50, step G from 2.5 to 78 mm, initial V = -14 kV, P = 33.4 torr (T~2.13 K)
= -268 nC
VHG (7.8 cm) = (-269 ± 35) kV
n
nnnnHC iittQ 2/11
VHG = (-360 ± 60) kV
Previous normal state results:
EHG (7.8 cm) = (-35 ± 5) kV/cmEHG = (-49 ± 8) kV/cm
Radiation Effects
n-flux in gap Initial V (kV) Comments Time
(Background) 13 No source 17:23
~106/s, E ~ 1 MeV,
10% ~ 1 keV
±14 Source atop
2 cm plexiglas
17:34
• Results just shown (maximum potentials) actually attained with ~ 7 Ci n-source, 50 cm from gap, nearly on-axis
• Enhancement likely due to larger initial V at small gap (2.5 mm):
• Slight improvement could have several sources (radiation, conditioning, switch tonegative polarity, more transients…)
Maximum potential in absence of radiation:
VHG (7.8 cm) = (228 ± 30) kV
6/7/05 17:23, initial V = 13 kV, P = 34.7 torr (T~2.14 K)
6/7/05 21:55:50, step G out to 8.0 cminitial V = -6 kV (!), P = 28 torr (T~2.06 K)
Leakage Current6/7/05 22:12:39, return G to 3 mm gapP = 30 torr (T~2.09 K)
t
Q
C
C
t
VC
t
Qi HC
HC
HGHGHG
HGLEAK
QHC = 88.6 nC QHC = 82.2 nC
CHG = 55 pF (bridge, ± 5%)
CHC = (1.1 ± .1) pF
QHC = (6 ± 8) nC (3% zero shift)
iLEAK = (-2 ± 20) pAt = (1009 ± 30) s
_
(EHG = [-12 ± 1] kV/cm)
iLEAK = (0.40 ± 0.45) nA
_Previous normal state result:
Leakage Current - Remarks
• Attempts before data on last slide:
attempt initial V (kV) time delay final V (kV)
1
2
3
5
4
6
8
7
P (torr)
• Would like to repeat with larger initial V and longer time delay
32
32
28
25
25
26
27
12.533 discharge (on pull-out)
12 (discharge)
-11.5 1 hr 0
-11 1 hr 0
11
10
-7
-7
(discharge)
(discharge)
(discharge)
(discharge)
• Stability of HV in SF? Low P?
Breakdown vs. Pressure, Temperature
• -point: pressure reading when SF transition observed in our system
7575
te)
• data below -point are highest attained in ~ 0.05 K bins above 2.05 K
• Point at 890 torr (4.4 K) is system record: (638 ± 83) kV, (80 ± 10) kV/cm at 8 cm
• Typical low-pressure normal state OR SF operation: 220 kV, 28 kV/cm at 8cm
V = 298 kV
SF (34 torr) SF (33 torr) + neutron rad
Normal State(322 torr)
SF (34 torr)Inward trace
(13% increase)
Transients
• Greater effect at low pressure
• Predominantly positive (negative) when HV positive (negative)
• Enhanced by neutron radiation
• ~ 20 ms rise time, ~100 ms FWHM, ~150 ms decay time, ~1-2 nC
• He gas bubbles?
• Kerr Effect: E-field measurement less susceptible to this effect?
Pressurization EstimatesVolume change for P = 1 atm:
Isothermal?
Time system spends below 2 K:
Need valve in neck above stainless can:
V = V0P = 2.6 l
= compressibility SF LHe = 10-7 Pa-1 (Keller, 3He and 4He)
Have 2 spare bellows with V = 1.4 l each (if initially stretched)
P dV = McT = 130 J T = 10 mK
Q/Q = 3.2 hr (assumes old 2.7 W load) .
Leak rate: (1.1 atm – 0.9 atm)/ 3.2 hr X 2.6 l/atm = 160 cc/hr
Force on actuator:
F = PA = 14 psi X 14 si + typical bellows resistance = 300 lbActuators in used rated for 1000 lb
Open/close while immersed in SF LHe1” diameter minimum
Pressurization Upgrade - Bellows
Pressurization Upgrade – Valve, Dewar