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 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)

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

8. 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

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

10. 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

11. 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 in =10 m
SQUID in FC
no FC

12. 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

13. 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.

14. 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)

15. Current progress 3 squids : measured in a probe with a complete Pb can
Star Cryoelectronic magnetometer: 7.17 0/Hz
Quantum Design DC SQUID: 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.