SQUID Performance in a HV Environment Chen-Yu Liu Craig Huffer, Maciej Karcz, Josh Long Indiana University

Scenarios to study HV Breakdown Induce HV from sparks (ESD) could produce current exceeding the current limit of the Josephson junctions, destroy SQUID (remedy: SQUID in a Faraday Cage) Induced current could drive superconductor over its critical field, cause flux trap, increase noise (no remedy required, might need to heat up SQUID periodically) Radio Frequency Interference (RFI) minor: Increase the SQUID noise moderate: flux jumps serious: unable to lock SQUID RF Source: micro-discharge, ground loop, switching mode power supply Remedy: SQUID in Faraday Cage, low pass filtered PS, proper RF shield, proper ground

Experimental Setup Disk electrodes: 1.25” diameter, 0.25” thick Pb S.C. shield HV feedthrough (ceramic) rated for 20kV. Star Cryoelectronic magnetometer on chip

SQUID Noise Spectrum Star Cryoelectronics magnetometer prototype. 8x8mm2 pick-up coil built in on the SQUID chip. 0.64 nT/0 Intrinsic noise < 5/Hz no HV, SQUID sensor is placed in a faraday cage (4 layers of Al coated mylar super-insulation) Measurements: Noise ~ 30 0/Hz S.C. Shielding should be improved. HV should also be better shielded.

SQUID Noise Spectrum in HV environments Noise floor does not increase significantly with HV. Jumps add to 1/f noise and white noise. E > 28 kV/cm (parallel plates) E > 72 kV/cm (spherical HV electrode)

SQUID Response Under Large Current To simulate a large current during breakdown Amplitude Modulated Sinusoidal Signal (1kHz) into a current loop (15.3 ) Current loop is directly on top of the SQUID sensor I=65 A→ 1.40

Observations Largest applied current I=10Vpp/15.3 = 0.65A SQUID recovers to working condition right after the current is off. In nEDM system, assuming the discharge time is ~ micro-seconds, the spark current is about 23 A (~ 35 times bigger than the small system) However, the SQUID is further away from the high field region

SQUID Electronics Input coil Pickup coil D. Drung, Supercond. Sci. Technol. 16 (2003) 1320 SQUID Electronics Input coil Pickup coil

Radio-Frequency Interference SQUID in flux lock mode (feedback circuit is on). Apply 50mVpp Sinusoidal Waveform into the current loop with 1k resistor in series. BW = 40kHz 1k

Radio-Frequency Interference SQUID in TUNE mode Measure the amplitude of the V- curve. Apply 50mVpp Sinusoidal waveform into the current loop with 1 k resistor in series. 1k Faraday cage shields the high frequency components. Ensures the large V- amplitude. f3dB~1MHz Al thickness=85m 4 layers of 0.0001 in =10 m SQUID in FC no FC

Micro-discharge vs Spark Use a spherical HV electrode to ensure the breakdown occurs in the field gap. (E up to 364 kV/cm) Monitor the micro-discharge and spark currents Direct monitor on the ground electrode (through 1 in series). Induced emf in the current loop. Direct current Induced emf SQUID in SC shield ~ 0.01 0 > 4 0

Frequency Spectrum of direct current measurement Major frequencies: 30MHz, 85MHz, 145MHz Corresponds to 6.6m, 2.35m, 1.37m System dimensions: HV conductor: 0.66m HV cable: 1.21m Due to impedance mismatch at various transitions.

Summary Destroyed one SQUID sensor in breakdown Micro-discharge Field = 15kV / 0.55mm = 273 kV/cm Instantaneous spark current > 80A Micro-discharge E> 7kV / 2.5mm = 28kV/cm (disk electrode) E> 4kV / 0.55mm = 72kV/cm (spherical electrode) I ~ 20 mA (4000 times smaller than the spark current) SQUID jumps Increases the 1/f noise, corner ~ 200Hz Starts at a lower field than the HV breakdown fields. Continuing study of effective RF shielding Micro-discharge. HV power supply (Glassman HV, series EH)

Current progress 3 squids : measured in a probe with a complete Pb can Star Cryoelectronic magnetometer: 7.17 0/Hz Quantum Design DC SQUID: 12.34 0/Hz Supracon Blue2CE: 8.64 0/Hz Additional RF shielding (Al cage) around the high voltage input feedthrough. After the HV study in pressurized He, we are ready to carry out more RFI studies on these SQUID sensors.