High Voltage Studies, 10/06 – 02/07

J. Long, Indiana University

System Modifications

Conditioning tests

Amplification, leakage current (preliminary)

Future plans

HV System - Changes Made for 10/06 run

Control purity and surface contaminants systematically

Complete solvent cleaning of interior

RF Plasma discharge cleaner (for Hydrocarbons)

Dry pumps only for all applications

No LN2 pre-cooling: use cold, filtered He gas from LHe supply dewar

LN2 trap on LHe bath pumping line

LHe (and gas) filter on transfer line outlet

Charcoal backed by glass filter paper, Quantum Technologies

RGA monitoring

Further reduced surface roughness

Electrodes polished to ~ 8 -inch finish

Attempt conditioning of electrodes at small gaps in vacuum

Changes made for 10/06 run

Attempt operation below 1.8K

Colder LHe transfers

New transfer line operates with HV system below lambda point

Heat load reductions (also important for eventual operation with DR):

Tie central volume lateral support posts to 77 K

Tie supply cryostat upper neck to 77 K shield

Remove all unnecessary (conducting) instrumentation from supply cryostat upper neck

Cover open viewport holes in 77 K shield with quartz windows

Tie actuator rods to 77 K shield

Previous heat load 2 W

1 W

0.5 W

0.1 W

0.1 W

0.02 W

Other

Video monitoring of gap

Improved level sensing

Changes made for 12/06, 02/07 runs

Replaced LHe filter with simpler home-made model

5/16” OD tube on end of stinger packed with copper wool

No copper residue observed in leak tests

Did not attempt conditioning in vacuum

Reduces LHe flowrate by factor ~2 relative to open stinger

Replaced “temporary” smooth G-10 insulators with custom ribbed ceramic

10/06 results likely limited by severe pitting on electrodes

Also slight contamination of charcoal leaking from filter

Dismounted, re-polished electrodes to #16 finish

12/06 data limited to single amplification/ leakage current test after moveable ground electrode broke off its support

Combination of thermal shrinkage, embrittlement, slight anti-parallelism and misalignment of electrodes – shear force on screws during electrode contact enough to break (?)

Plastic screws holding electrode to support broke at heads

Cryogenic performance

Heat load (all runs)

Unchanged

Copper “77 K” radiation shield rarely cools below 120 K

Pre-cooling (all runs)

(boil-off still 50 l/min He gas at 18 C, or 0.1 g/s and 2.5 W)

Cooling below lambda point (10/06 run only)

System cools to 4 K at steady ~ 15 K/hr with 400 l/minute of cold gas flow

400-500 l LHe used

Insufficient flow rate for low-pressure LHe top-off

Reduced conductance of stinger with filter

System reached 3.4 torr (1.5 K) with existing pumps while 40% full of LHe

Vacuum Conditioning tests - 10/06

Time since start of

training (min)

Breakdown voltage

(kV)

5 -31.0

10 -30.6

15 -31.9

20 -27.0

44 -33.27

50 -33.00

74 -33.41

116 -33.61

129 -33.47

145 -33.55

151 -33.28

193 -33.96

Ramped ~ 10V/s for final 2-3 kV

Monitored current across gap and insulators (by eye)

Waited for current surges (~ microamps) to subside before proceeding higher (usually 10-60 s)

4 mm gap, P = 2E-6 torr

Time since start of training

(min)

Breakdown

voltage (kV)

10 28.6

20 32.3

24 27.2

26 25.2

32 28.0

54 31.3

79 28.5

81 26.0

92 28.2

Damage during final runs with V < 0 ?

Vacuum Conditioning tests - 10/06

Ramped in 10V steps

Recorded all current surges across gap and insulators

Waited for current surges to return to (within few % of) resting current

4 mm gap, P = 2E-6 torr

Ground electrode surges Insulator surges

No obvious breakdown precursor event

LHe Conditioning tests - 10/06

Ramped in 10 V steps for final 2-3 kV

Recorded all current surges across gap and insulators AND resting current

Waited for current surges to return to (within few % of) resting current

3 mm gap, P = 890 torr (could charge 4mm to ~ 50 kV)

Ground electrode surgesTime since

start of training (min)

Breakdown voltage

(kV)

10 33.2

20 36.7

70 34.7

270 34.4

Absence of trend unusual

Maxima plotted only, “surges” usually several minutes

LHe Conditioning tests - 10/06

Trend in ground electrode resting current – possible breakdown precursor?

0 1000 2000 3000 4000

6 1010

5 1010

4 1010

3 1010

2 1010

1 1010

currentAvs ts

LHe Conditioning tests - 02/07

PC-based DAQ ready

Stop ramp at first clear sign of resting current increase; resume only after recovery of several minutes

0 1000 2000 3000 4000

34

33

32

31

mean VkVvs ts, with cuts

0.0

-0.2

-0.4

-0.6

Current (nA) vs time (s)

-30

-32

-34

Voltage (kV) vs time (s)

0 1 2 3 43 1010

2.5 1010

2 1010

1.5 1010

1 1010

5 1011

currentAvs VkV

0 1000 2000 3000 4000

6 1010

5 1010

4 1010

3 1010

2 1010

1 1010

currentAvs ts

LHe Conditioning tests - 02/07

PC-based DAQ ready

Stop ramp at first clear sign of resting current increase; resume only after recovery of several minutes

0 1000 2000 3000 4000

34

33

32

31

mean VkVvs ts, with cuts

0.0

-0.2

-0.4

-0.6

Current (nA) vs time (s)

-30

-32

-34

Voltage (kV) vs time (s)

0.0

-0.1

-0.2

-0.3

Current (nA) vs V (kV)

-30 -31 -32 -33 -34

Amplification tests – 10/06

Six runs with valid data

Breakdown at ~ 2cm gap

Previous record at 890 torr

(Followed by single test with V > 0: + 510 kV)

Could not charge initial gap any further

Mean voltage (~15% error)

V < 0, P = 890 torr, max gap = 7.8 cm

Amplification tests – 10/06

Breakdown at ~ 2cm gap

Previous record at 890 torr

(Followed by single test with V > 0: + 510 kV)

Could not charge initial gap any further

Mean voltage (~12% error) Best mean voltage vs gap

Six runs with valid data

V < 0, P = 890 torr, max gap = 7.8 cm

Pitting and Contamination – 10/06

Ground electrode surface

Area ~ 1 cm2, depth ~ 1 mm

Residue from bottom of LHe volume

(Previous < 1 mm2, microns depth)

Particles with r < 1mm

Filter leaks charcoal into open-neck dewar

CHG

HVPS

50 kV

A C

CHC

CHP

DAQ for 12/06, 02/07 runs

A PA G

Meters on charger, ground electrode, insulator support plate

CHG

HVPS

50 kV

A C

CHC

CHP

HC

HCHG C

QV

DAQ for 12/06, 02/07 runs

• Use SR570 current amplifiers (pA)

• Readout with ADC at 10 Hz

)( dtiQ HCHC

A PA G

Meters on charger, ground electrode, insulator support plate

Amplification tests – 12/06, 02/07

12/06: single run only (broken ground electrode), 4 K

V = (520 ± 60) kV, 5.7 cm gap (ceramic insulators longer, shimmed)

20 30 40 50 60 70 80 90

80

60

40

20

0

20SR570 Output GND PLATE

A PA G

CHARGER

A C

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

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

A PA G A C

- -

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

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

+ i - i - i

A PA G A C

-

- - - -

++ +++

+ i + i

- i

02/07: 15 runs, 4 K:

Happens on all runs when V exceeds ~ ± 150 kV

Current (nA) vs time (s)

Amplification tests – 02/07

Single test without breakdown:

Initial voltage: -7 kV at 5mm gap

Result: V = 121 kV, 5.7 cm gap (amplification factor = 17)

Maximum initial voltage in 02/07 runs: ± 34.5 kV

Theoretical maximum in absence of breakdown: 596 kV (4 K, 5.7 cm)

Record 4K test from 2004, cut off at 5.7 cm: 670 kV

HV – Ground capacitance vs. minimum gap

Ground-HV capacitance saturates below 3 mm

Hair-line gap visible when electrodes in electrical contact

HV or Ground electrode likely skewed ~ 0.5 degrees

Capacitance of initial gap, amplification smaller

Leakage current tests – 12/06

Traditional method: difference between charge as measured from outward and inward strokes, divided by time:

t

Q

C

Ci HC

HC

HVLEAK

= (276 nC – 212 nC) (60.15 pF/ 0.56 pF) = 6870 nC/ 800s

0 200 400 600 8000

2.5 1095 109

7.5 1091 108

1.25 1081.5 108

1.75 108currentAvs times

0 200 400 600 8000

1000

2000

3000

4000

integrated currentnCvs ts

Q = 4370 nC after 800s

Direct monitoring of current through insulators (plate):

Leakage current tests – 12/06

Traditional method: difference between charge as measured from outward and inward strokes, divided by time:

t

Q

C

Ci HC

HC

HVLEAK

= (276 nC – 212 nC) (60.15 pF/ 0.56 pF) = 6870 nC/ 800s

Q = 2206 nC after 800s

Direct monitoring of current through ground electrode:

0 200 400 600 800

8 109

6 109

4 109

2 109

0currentAvs times

0 200 400 600 800

2000

1500

1000

500

0integrated currentnCvs ts

Expect: QG = QHV (CG/CHV)

= 6870 nC (22 pF/60 pF) = 2519 nC

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

Modified Standoff Design for Acrylic Breakdown Tests

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

Schedule

February 19

Acrylic insulators delivered

Coat insulators (?)

Shim electrodes for parallelism

Install insulators

February 19 - March 12

March 12 - 18

Recover 7.8 mm gap

Open system; inspect HV electrode and charger, polish

March 19 -

Next tests

~ July 1

HV insert for dual-use cryostat completed

Expect ~ 1 month dedicated use for initial debug/test

~ 2-3 weeks for any additional test

Acrylic Breakdown Tests

Method 1: Acrylic slab between electrodes

Advantages

Closely match actual perimeter of reference cell design

Reference cell perimeter = 2 (50 cm + 10 cm) = 120 cm

HV test system electrode diameter (flat portion) = 112 cm

Uniform fields > 50 kV/cm possible (for thickness < 1cm)

Inexpensive

Disadvantages

Leakage current monitoring comparable to baseline (pure LHe)

Inexpensive method (clamp and hold slab between electrodes during assembly, operation) not straightforward; long intervention if slab falls

Thickness limited to ~ 1 cm if desire fields > 50 kV/cm

Hollow construction not practical

Acrylic Breakdown Tests

Method 2: Replace 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 (?)

Disadvantages

Hollow construction possible

Leakage current monitoring comparable to baseline (ceramic)

Avoids problems associated with holding objects between electrodes

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

Mimic electrode recesses without modifying electrodes

Field behind HV electrode less uniform