high voltage test system results, 03-04/07 j. long, indiana university system overview and...

High Voltage Test System Results, 03-04/07

J. Long, Indiana University

System Overview and Modifications

Readout system

Breakdown tests of acrylic

System performance away from breakdown

Direct monitoring of resting current

stainless canaluminumplate

wire sealflange

G-10standoff

HV plungercontrol rod

Indium seal

ceramicstandoff

HV electrodeground electrode

groundcontrol rod

Vacuum-LHeHV feedthrough

bearingsbellows

0.53 m

quartz window

High voltage system prototype at LANL

Vacuumchamber

Supplycryostat

HVfeedthrough

Actuator

Test proposed amplification method

Measure breakdown properties of large volumes of LHe

Existing data: 150 kV/cm at 4 K, 1cm gap

LHe bath pumping line

Previous results from prototype system

Maximum leakage currents under these conditions (95% C. L.) :SF (2.07 K): 733 pANormal State (3.98 K): 169 pA

Short-duration breakdown not affected by neutron radiation (106/s, ~MeV)

Small commercial HV feedthrough exceeded maximum rating in air (40 kV) by 25% when immersed in SF

Maximum potentials sustained:11.8 liters Normal State (4.38 K), 7.2 cm gap:

(96 ± 7) kV/cm

12.8 liters SF at 2.14 K, 7.8 cm gap:

(31 ± 3) kV/cmPossible further degradation below 1.9 K

Bubble formation (common breakdown culprit) observed when charging above 10 kV, likely coincident with noise

HV System – Previous modifications

2005-2006: “Common sense” improvements

Polished electrodes, plasma discharge cleaner (for Hydrocarbons), dry pumps only, LN2 traps on pumps, pre-cool with cold He gas, basic LHe filtering, attempt electrode conditioning

Results: no improvement (pitting, contamination, mechanical failure, discharges behind HV electrode)

Acrylic Breakdown Tests

Replaced ceramic standoffs behind HV electrode with acrylic

Advantages

Poor match of actual perimeter of reference cell designReference cell perimeter = 2 (50 cm + 10 cm) = 120 cm

Total perimeter of standoffs = 44 cm

More expensive than slab between electrodes (?)

Disadvantages

Hollow construction possible

Leakage current monitoring comparable to baseline (ceramic)

Avoids problems associated with holding objects between electrodes

Strong fields (60 kV/cm) available at gaps comparable to reference gap

Mimic electrode recesses without modifying electrodes

Field behind HV electrode less uniform

Modified Standoff Design for Acrylic Breakdown Tests

6” long, 2” diameter acrylic tube replaces ceramic

Steel end pieces same dimensions as on ceramic, mimic recesses

John Ramsey, LANL

Modified Standoff Design for Acrylic Breakdown Tests

Spring-loaded retaining ring holds acrylic annulus in place against slipping from thermal contraction

John Ramsey, LANL

Condition of HV electrode – 12/06 and 02/07 runs

Fill-holes and scratches

http://www.jlab.org/~tajima/edm/EDM_HV.html

Shim with sharp edges

Installation of acrylic – 04/07 runshttp://www.jlab.org/~tajima/edm/EDM_HV.html

Holes patched and polished (insulators have through-holes)

No shims necessary

Capacitances Relevant to Voltage, Currents

CHG

CHC

CHP

Capacitances Relevant to Voltage, Currents (cont.)

CHF

CHG

HVPS

50 kV

A C

CHC

CHP

Basic Charging and Readout Circuit

A PA G

CHF

SR570 current amplifier (pA)

Readout - details

Amplification Measurement

CHG CHP

A PA G

CHC

A C

- - - - - - - - - - - -

++ ++++ ++++ ++

A PA G A C

- -

- - - - - - - - - - - -

+++ ++++++ ++++ +

+ i - i - i

HC

HC

HP

HP

HG

HG

C

Q

C

Q

C

QV 0

HC

HC

HP

HP

HG

HGf C

Q

C

Q

C

QV


CHG CHP

A PA G

CHC

A C

- - - - - - - - - - - -

++ ++++ ++++ ++

A PA G A C

- -

- - - - - - - - - - - -

+++ ++++++ ++++ +

+ i - i - i

HC

HC

HP

HP

HG

HG

C

Q

C

Q

C

QV 0

HC

HC

HP

HP

HG

HGf C

Q

C

Q

C

QV

HC

HC

HC

HC

HC

HCf C

Q

C

Q

C

QVV

0

Easiest (stays fixed, large gap,smaller errors)


CHG CHP

A PA G

CHC

A C

- - - - - - - - - - - -

++ ++++ ++++ ++

A PA G A C

- -

- - - - - - - - - - - -

+++ ++++++ ++++ +

+ i - i - i

HC

HC

HP

HP

HG

HG

C

Q

C

Q

C

QV 0

HC

HC

HP

HP

HG

HGf C

Q

C

Q

C

QV

HC

HC

HC

HC

HC

HCf C

Q

C

Q

C

QVV

0

0VC

idtV

HC

Cf

Easiest (stays fixed,large gap,smaller errors)

Maximum voltages attained – 04/07

Maximum potentials sustained:10.5 liters Normal State (4.38 K), 6.4 cm gap:

(760 ± 70) kV = (119 ± 11) kV/cm

8.2 liters SF at 2.14 K, 5.0 cm gap:

(290 ± 40) kV = (58 ± 8) kV/cm

Ranges Comparison with previous results (means)

~20% improvement with extrapolation to 7.5 cm gap

Improved initial gap capacitance

Capacitance of HV electrode at small electrode gap:

Can now charge larger ( ~ 25%) initial capacitance to same initial voltage (35-40 kV): expect ~ 25% improvement

Range of start gaps, 4/07

Previous start gap

Likely the result of careful shimming of gap before 4/07 tests

V0 > 0.95 V0 MAX V0 ~ 0.8 V0 MAX V0 ~ 0.5 V0 MAX

4.4 K (18 tests) 6/6 8/8 4/4

2.1 K (14 tests) 0/4 2/4 2/6

V0T

Charging “statistics:”Established initial gap breakdown of V0 MAX

# successes

# attempts to charge at some fraction of V0 MAX

System performance at and below maximum voltage

Recorded:

V0 > 0.95 V0 MAX V0 ~ 0.8 V0 MAX V0 ~ 0.5 V0 MAX

4.4 K (18 tests) 2/6(?) 0/8 0/4

2.1 K (4 tests) 0/0 1/2 0/2

V0T

Number of breakdowns during amplification:

Leakage Current 04/07 – Traditional Method

t

CQCQC

t

VC

t

Qi HCINHCHCOUTHCHVHVTOTALHVTOTALHVLEAK

)]/()/[( )()()()(

QHV = (0.3 ± 2.4) nC

iLEAK ≤ 220 pA (95% C. L.)_

AVERAGE leakage current along ALL paths during holding time t between amplification (moving out) and attenuation (moving in):

QHV = (-0.4 ± 16) nC

iLEAK ≤ 5 nA (95% C. L.)_

4.4 K (t ~ 7 hr):

2.1 K (t ~ 11 min):

0 100 200 300 400

2 1013

0

2 1013

4 1013

6 1013currentAvs times

0 100 200 300 400

2 10131 1013

0

1 10132 10133 10134 1013

currentAvs times

0 100 200 300 400

4 1010

3 1010

2 1010

1 1010

0

currentAvs times

0 100 200 300 400

0

1 1010

2 1010

3 1010

4 1010


0 100 200 300 400

1.5 1011

1 1011

5 1012

0

5 1012

1 1011

1.5 1011

currentAvs times

0 100 200 300 400 5000

1 1010

2 1010

3 1010

4 1010


0 100 200 300 400 500

1 1011

5 1012

0

5 1012

1 1011

currentAvs times

Resting Current 04/07 – Direct monitoringLeave system at maximum gap

Set current amplifiers to maximum possible sensitivity (resolution ~ 0.01 pA [?])

Take data at 500 Hz (input filter: 100 Hz, 12 dB/octave) for ~ 500 s

Insulators GND electrode Charger

0 kV

150 kV

290 kV

2.1 K data:

N/A (bad connection)?

N/A (bad connection)?

0 kV

150 kV

290 kV

~ 1s time slices of Insulator current data

(60 Hz)

(no obvious scaling)

Insulator Ground Charger

0(offset) 0.11 0.11 -3.4

150 0.20 NA 67.4

289 0.57 NA 61.7

chV

Resting Current 04/07 – Direct monitoring

Average current (pA), 2.1 K dataAvg Insulator current vs voltage

Insulator Ground Charger

0(offset) 0.13 0.19 -8.2

-375 -0.15 0.44 -50.0

-525 -0.18 0.59 -14.0

-749 -0.13 NA -59.0

Resting Current 04/07 – Direct monitoring

Average current (pA), 4.4 K data Avg Insulator current vs voltage

Resting current spectra – 4K

Sample rate = 500 Hz

Amplifier input filter 100 Hz, 12 dB/octave

Insulators:

Ground electrode:

Resting current spectra – 4K


Amplifier input filter 100 Hz, 12 dB/octave Charger:

Resting current spectra – 4K mean values

dzdt

dz

dz

dCV

dt

dCVi 210~ at ~ 50 Hz

(picometer oscillations cause 100 fA currents??)A/m

Transducer effect?

Resting current spectra – 4K and 2K



Insulator data only

Factor ~ 10 greater noise below 10 Hz

Mean amplitude decreases with applied voltage

Beating of 2 additional pumps?

(switching off pumps does NOT improve breakdown)

Resting current spectra – 4K, pumps off



Insulator data shown

LHe boil-off increases significantly (“white” effect?) over course of measurement

Resting current spectra – 4K, HV fixed vs floating

Insulators:

Ground electrode:

Set gap to 2.3 cm(ground connection OK)

Applied 40 kV with charger fixed to HV

Took ~ 5 min. data

Retracted charger

Took ~ 5 min. data

Time-averaged insulator currents:

0.05 pA (HV floating)

0.18 pA (HV fixed)

Conclusions

Initial capacitance, smooth electrode features crucial factors in system performance

No obvious breakdown / leakage current problems with (uncoated) acrylic

Stability/performance of system below 2.2 K (35 torr) still serious concern

Some trends in dc and ac resting current with applied voltage and fixed vs floating HV electrode, but no serious degradation

Readout needs improvement for sensitivity to small signals

V0 > 0.95 V0 MAX V0 > 0.8 V0 MAX V0 > 0.5 V0 MAX

4.4 K (18 tests) 6/6 14/14 18/18

2.1 K (14 tests) 0/4 2/8 4/14

V0T

Charging “statistics:”Established initial gap breakdown of V0 MAX

# successes

# attempts to charge above some fraction of V0 MAX

System performance at and below maximum voltage

Recorded:

V0 > 0.95 V0 MAX V0 > 0.8 V0 MAX V0 > 0.5 V0 MAX

4.4 K (18 tests) 2/6 2/14 2/18

2.1 K (4 tests) 0/0 1/2 1/4

V0T

Number of breakdowns during amplification:

John Ramsey, LANL

Design finished this week

Procurement by end June

Need technician to assemble and install

New inner (4K) shield

0 100 200 300 4001.4

1.2

1

0.8

0.6

0.4

0.2

0integrated currentnCvs ts

0 100 200 300 4000

0.01

0.02

0.03

0.04

integrated currentnCvs ts

0 100 200 300 4000

0.01

0.02

0.03

0.04


0 100 200 300 400

0

0.02

0.04

0.06

0.08

0.1integrated currentnCvs ts

0 100 200 300 4000

5

10

15

20

25

30


0 100 200 300 400 5000

5

10

15

20

25


0 100 200 300 400 5000

0.05

0.1

0.15

0.2

0.25


0 100 200 300 4005 1013

2.5 10130

2.5 10135 1013

7.5 10131 1012

currentAvs times

0 100 200 300 400

0

2 1013

4 1013

6 1013

currentAvs times

0 100 200 300 4005 1010

4 1010

3 1010

2 1010

1 1010

0

currentAvs times

0 200 400 600

4 10123 10122 10121 1012

0

1 10122 1012

currentAvs times

0 200 400 600

6 10124 10122 1012

0

2 10124 10126 10128 1012

currentAvs times

0 200 400 6005 1010

4 1010

3 1010

2 1010

1 1010

0

currentAvs times

0 200 400 600 800

4 1012

2 1012

0

2 1012


0 200 400 600 8001.5 1011

1 1011

5 1012

0

5 1012

1 1011

1.5 1011currentAvs times

0 200 400 600 8005 1010

4 1010

3 1010

2 1010

1 1010

0

currentAvs times

0 200 400 6006 1012

4 1012

2 1012

0

2 1012

4 1012


0 200 400 6003 1010

2.5 1010

2 1010

1.5 1010

1 1010

5 1011

currentAvs times

0 200 400 6005 1010

4 1010

3 1010

2 1010

1 1010

0

currentAvs times

Leakage Current 12/06 – Traditional Method

t

CQCQC

t

VC

t

Qi HCINHCHCOUTHCHVHVTOTALHVTOTALHVLEAK

)]/()/[( )()()()(

QHV = (6.8 ± 2.6) C

iLEAK = (8.6 ± 3.3) nA_

AVERAGE leakage current along ALL paths during holding time t between amplification (moving out) and attenuation (moving in):

Leakage current 12/06 – Direct Monitoring

Output of current amplifier on plate (insulators):

0 200 400 600 8000

2.5 1095 109

7.5 1091 108

1.25 1081.5 108


15

10

5

00 200 400 600 800

Current (nA) vs time (s)

0 200 400 600 8000

1000

2000

3000

4000

integrated currentnCvs ts3

2

1

00 200 400 600 800

Integrated Current (C) vs time (s)

4


QP = 4.4 ± 0.1 C (zero drift only)


0 200 400 600 8000

2.5 1095 109

7.5 1091 108

1.25 1081.5 108


15

10

5

00 200 400 600 800


0 200 400 600 8000

1000

2000

3000

4000


2

1

00 200 400 600 800


4

iLEAK = (5.5 ± 0.2) nA_


QP = 4.4 ± 0.1 C (zero drift only)


0 200 400 600 8000

2.5 1095 109

7.5 1091 108

1.25 1081.5 108


15

10

5

00 200 400 600 800


0 200 400 600 8000

1000

2000

3000

4000


2

1

00 200 400 600 800


4

iLEAK = (5.5 ± 0.2) nA_

QHV = (6.8 ± 2.6) C

iLEAK = (8.6 ± 3.3) nA_

Consistent with traditional result but room for several nA leakage elsewhere

0 200 400 600 800

2000

1500

1000

500



QG = 2.21 ± 0.07 C (zero drift only)

Output of current amplifier on ground electrode:

-0.5

-1.0

-1.5

-2.0

0 200 400 600 800

Integrated Current (C) vs time (s)0.0

0 200 400 600 800

8 109

6 109

4 109

2 109

0currentAvs times

0

-2

-4

-6

0 200 400 600 800


-8

0 200 400 600 800

2000

1500

1000

500



QG = 2.21 ± 0.07 C (zero drift only)

Output of current amplifier on ground electrode:

-0.5

-1.0

-1.5

-2.0

0 200 400 600 800

Integrated Current (C) vs time (s)0.0

0 200 400 600 800

8 109

6 109

4 109

2 109

0currentAvs times

0

-2

-4

-6

0 200 400 600 800


-8

A PA G A C

+

++ ++

- - - --

- i

+ i

- i

A PA G A C

+ +

+++ +++ +++ +++

- - - - - - - - - - - -- -

Expectation if +6.8 nC leaked off HV via path other than CHG:

high voltage test system results, 03-04/07 j. long, indiana university system overview and...

Documents