Post on 13-Jan-2016
Results from the HV Test System
Review of current design and assembly
J. Long,P. Barnes, J. Boissevain, J. Gomez,
S. Lamoreaux, D. Mischke, S. Penttila
LANL
Amplification and large-gap E-fields
Leakage currents
Pressure dependence of small-gap breakdown
Results with normal state LHe
Neutron irradiation
Cool-down and heat loads
Plans for near future
50 kV/cm
76 mm
extrapolation (?)
Electric Field: LHe Breakdown vs Gap
Figure: J. Gerhold, Cryogenics 38 (1998) 1063
Need 50 kv/cm across 8 cm cell ~ 400 kV
Internal amplifier (avoid heat loads, large feedthroughs)
Variable capacitor with C >> Ccells ~ 50 pf
vacuumchamber
supplycryostat
77 K shield
G-10 foot
linearactuator
air-vacuumHV feedthrough
~2 m
LN2reservoir
Design: vacuum enclosure
Vacuum pump, T- sensor readout attachments
LHe vessel
LHereservoir
Assembly: central volume
View of electrodes through side port Leak check of central volume and bellows
Assembly: vacuum system
LHe vessel in Kevlar sling
20 layers superinsulation
Complete system showing HV charger
drive rod, linear actuator and motor
Cooling and filling of central volume (December 2003)
LN2
transfertube
LHevessel
plug
1
2
3
4
5
Temperature sensor locations
3 days to fill from room temperature start (limited by Cu shield), need ~ 400 liters of LHe
Heat loads
Estimated sources
Kevlar rope suspension 10 mW
Ground actuator/spider 10 mW
HV actuator/spider 5 mW
HV conductor 2 mW
Unshielded quartz windows 30 mW
2585 mW (1555 mW)
Supply cryostat neck 1800 mW
(Neck with 77 K anchor) (770 mW)
Temperature sensors 1 mW
Measurement
Average He gas boil-off:(flowmeter near 300 K)
52 liters/min
4 K liquid boil-off: 3.4 liters/hr
Total load: 2660 mW
Estimate after restorationof 77 K anchor: 1600 mW
Radiation through SI 725 mW (?)
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
cm1.0
cmpF4.6pF43
z
C
Charger retracted to 4.24 cm whereCHC = 1.6 ± 0.2 pF
C0
Az0
Pull back G electrode at constant 1inch/min
Largest Potentials Attained
Expect initial current (first 2.5 mm pullback):
mm5.2
kV42pF6.1
s 60
mm4.25~ 12 nA
2/25/04 11:00, step G from 2.5 to 73 mm, initial potential = 42 kV
dz
dVCv
dz
dQv
dt
dz
dz
dQ
dt
dQi HG
HCHC HG
HF
HGHG
zA
C
Q
C
QV
Shape should track dV/dz :
20
1~
HG
HGzzdz
dV
(z0 ~ 5 cm)
Largest Potentials Attained
= 862 nC
VHG (7.3 cm) = (570 ± 70) kV
CHC error 13%
SR570 zero drift 3%transients 2%
n
nnnnHC iittQ 2/11
2/25/04 11:00, step G from 2.5 to 73 mm, initial potential = 42 kV
2/20/04 22:15, step G from 2.5 to 73 mm, initial potential = -31 kV
VHG (7.3 cm) = (-360 ± 60) kV
Charging with negative potential:
• Draws steady current > 50 A below –30 kV
• Current independent of charger position
2/20/04 23:15, step G out to 7.3 cminitial potential = 29 kV
Leakage Current
2/21/04 10:47, return G to 3 mm gap
t
Q
C
C
t
VC
t
Qi HC
HC
HGHGHG
HGLEAK
QHC = 566 nC QHC = 568 nC
CHG = 53 pF (bridge, ± 5%)
CHC = (1.6 ± .2) pF
QHC = (-2 ± 24) nC (3% zero shift)iLEAK = (-2 ± 20) pA
t = 11h 32 min ± 5 min
_
(EHG = [52 ± 8] kV/cm)
Breakdown vs. Pressure – 3 mm gap
Pump LHe bath with roots blower (250 m3/hr)
~ 8 psi check valve limits pressure
Could reduce to 25 kV/3 mm(300 kV/ 7.3 cm) or worse at low P
2/27 data (open circles):
• System had 2 additional days at 4 K
• 5 hr after LHe top-off
• 30 min. prior HV conditioning
Radiation Effects
n-flux in gap Breakdown V (kV) Comments Time
(Background) 30 ± 1 No source 11:00
~106/s, E ~ 1 MeV,
10% ~ 1 keV
34 ± 2 Source behind
2 cm plexiglas
11:03
106/s, E ~ 1 MeV 36 ± 2 Plexiglas removed
11:07
~ 7 Ci n-source, 50 cm from gap, nearly on-axis
Small gap (3 mm), 30 minutes after LHe fill, ~ 5 minutes conditioning, 1 psig
Slight improvement (or conditioning)
Large gap (maintained for 1 minute, no plexiglas, inward trace shown):
VHG (7.3 cm) = (390 ± 60) kV
Conclusions
Normal State LHe holds 570 kV at 7.3 cm (~ 40% higher than “expected”)
73 mm
(570 ± 70) kV
Design field at 7.3 cm holds for > 11 hr
Max leakage current = 20 pA (~ 3% of tolerable limit)
Problem with low-P operation?
Small gap breakdown not affected by neutron radiation (106/s, ~MeV)
Large wire-seal flanges hold LHe(thermal gradients > 60 K / 60 cm)
Superfluid operation (either way…)
E-field measurement via Kerr effect(Discussions with UC…)
Prototype holding cell (Lucite, coatings…) behind HV electrode
Next steps