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SQUID Performance in a HV Environment

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)

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SQUID Performance in a HV Environment

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

  2. 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

  3. 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

  4. 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.

  5. SQUID Noise Spectrumin 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)

  6. 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

  7. 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

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

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

  10. Radio-Frequency Interference 1k • 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. • 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

  11. 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

  12. 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.

  13. Summary • Destroyed one SQUID sensor in breakdown • 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)

  14. 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.

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