Preliminary HV Results in Superfluid Helium Review of HV system design J. Long, J. Boissevain, J....

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