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