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 load2 W 1 W 0.5 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 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) 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 s) 4 mm gap, P = 2E-6 torr Time since start of training (min) Breakdo wn voltage (kV) 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 surges Time since start of training (min) Breakdown voltage (kV) 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?

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 Current (nA) vs time (s) Voltage (kV) vs time (s)

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 Current (nA) vs time (s) Voltage (kV) vs time (s) Current (nA) vs V (kV)

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: 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: 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 cm 2, depth ~ 1 mm Residue from bottom of LHe volume (Previous < 1 mm 2, microns depth) Particles with r < 1mm Filter leaks charcoal into open-neck dewar

C HG HVPS 50 kV A C C HC C HP DAQ for 12/06, 02/07 runs A P A G Meters on charger, ground electrode, insulator support plate

C HG HVPS 50 kV A C C HC C HP DAQ for 12/06, 02/07 runs Use SR570 current amplifiers (pA) Readout with ADC at 10 Hz A P A 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) GNDPLATE A P A G CHARGER A C A P A G A C i - i A P A G A C 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: = (276 nC – 212 nC) (60.15 pF/ 0.56 pF) = 6870 nC/ 800s 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: = (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: Expect: Q G = Q HV (C G /C HV ) = 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 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 design Reference 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