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