squid performance in a hv environment
DESCRIPTION
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) - PowerPoint PPT PresentationTRANSCRIPT
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SQUID Performance in a HV Environment
Chen-Yu LiuCraig Huffer, Maciej Karcz, Josh Long
Indiana University
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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
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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
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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.
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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)
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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→ 1.40
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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
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SQUID ElectronicsD. Drung, Supercond. Sci. Technol. 16 (2003) 1320
Pickup coil
Input coil
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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
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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
SQUID in FC
no FC
• Faraday cage – shields the high frequency
components.– Ensures the large V-
amplitude.
• f3dB~1MHz– Al thickness=85m– 4 layers of 0.0001 in =10 m
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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.
> 4 0
Direct current
Induced emf
SQUID in SC shield~ 0.01 0
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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.
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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)
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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.