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SQUID Performance in a HV Environment. Young Jin Kim, Chen-Yu Liu. May 21, 2008. SQUIDs pretest. Cryoelectronics magnetometer. Quantum Design. Superacon. Cast Pb can. Inserted the probe into the He supply dewar. 1. Superacon SQUID Noise Signal. FFT. @ 1kHz. White Noise baseline.

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SQUID Performance in a HV Environment


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    1. SQUID Performance in a HV Environment Young Jin Kim, Chen-Yu Liu May 21, 2008

    2. SQUIDs pretest Cryoelectronics magnetometer Quantum Design Superacon Cast Pb can Inserted the probe into the He supply dewar

    3. 1. Superacon SQUID Noise Signal FFT @ 1kHz White Noise baseline microphonics Time trace 1/f noise (pink)

    4. 2. Star Cryoelectronics SQUID FFT Time trace

    5. First layer of Lead shielding Second layer of Lead shielding HV feedthrough (ceramic isolation) Lead shielding Ferromagnetic Ni Plated thread G10 Macor (gradiometer) SQUID Superacon SQUID test HV in (requires RF shield) Pb superconducting foil Superconducting Shield: Pb foil (not liquid tight)

    6. Test Sequence Outer magnetic shield 6.6  3.85 0/Hz (1) (2 layers of Pb) Brass Electrode (2mm gap): 7.75 0/Hz (0.9mm gap): 19.2 0/Hz Closed 3.85 0/Hz (SF: 2.88 0/Hz) Add Semitron 6.92 0/Hz (SF: 5 0/Hz ) Open shield 6.92 0/Hz

    7. Scope to monitor the SQUID, direct current in the ground electrode, and Induced emf in the pick-up coil Faraday Cage HV power supply Glassman e-series (Powered by ac power, Grounded to the Faraday Cage) PCI-1000 SQUID controller 12 V Car batteries to power the PCI-1000 Serial to Optical converter There is no AC power inside the faraday cage

    8. A lot of Improvements since the last year • With improved magnetic shields (2 layers of Pb foils + shield penetrations), and RF shields (around HV input, Faraday cage) • Flux noise of StarCryoelectronics SQUID is 5 times lower than previous one. • No peak at 60Hz. • Microphonics from 400Hz to 800Hz has been highly suppressed. • However, • Some vibrations are still present (from He boil-off). previous measurement current measurement

    9. Tests • AC power vs Battery powered SQUID control • 1 vs 2 layers of Pb shielding • Normal State Liquid Helium vs Superfluid Helium • Induced vibrations • RF Shielding • Faraday cage, electrical isolation from the AC power ground. • Grounding • HV test results • Positive vs Negative Polarities

    10. AC power vs. 12 V battery (floating ground)Supracon SQUID (One layer Pb shielding) 69Hz 71Hz No peak 1/f noise is smaller with battery power

    11. 1 vs. 2 layers of Pb shields Supracon SQUID In two layers of Pb foil In one layer of Pb foil

    12. Pb Superconducting shieldingSupracon SQUID 1 layer 1 layer 2 layers 2 layers Dramatically reduces microphonics; suppress 1/f noise.

    13. Normal State He vs Superfluid He In superfluid, No semitron electrode With Semitron electrode, There is no vibration detected in superfluid (w/ pump off).

    14. Normal State He vs Superfluid (with semitron) The temperature of pumped SF He is around 1.9K. In superfluid He, no microphonics in the low region of frequency (<100Hz), But 1/f noise is increased due to continuous temperature change. Beat phenomena

    15. Induce vibrations in Normal State He Peaks stay the same amplitudes: He boiling 4K liquid helium Vibration induced from knocking on the cryostat increases noise spectrum below 2kHz.

    16. Induced vibrations in SF (with semitron) Microphonics are excited when knocking on the vacuum chamber. No vibrational peaks associated with He boiling.

    17. Vibrations: With and Without the Semitron Electrode Normal State He 3.85 0/Hz (no semitron)

    18. Ground Study (with semitron + open Pb shield) When the ground electrode is connected to the power-line ground (building ground), flux noise is 3 times larger than that when connected to the chamber as the ground (chamber floats).

    19. RF Shielding Study 1: Termination of unused BNC connectors These peaks are suppressed by termination of BNC jack Termination of PZT connector on BNC box can suppress peaks around from 500Hz to 800Hz (High frequency aliasing?). RF noise leaks through the un-terminated BNC connectors.

    20. RF Shielding Study 2: HV feedthrough with Brass HV electrode

    21. HV-SQUID test 1: AC powered PCI-1000, outside the RF cage At HV=7kV, we heard discharge peak’s sound and saw SQUID’s jump. At that time the level of He is low (32%), the HV electrode was probably exposed to the He gas. The SQUID survived the spark.

    22. HV-SQUID test 2 (negative polarity) Gap size: 2mm Cathode: Brass sphere Anode: semitron planar electrode cathode anode White Noise measured @ 779Hz

    23. positive polarity HV-SQUID test 3(positive polarity) Gap size: 0.9mm Anode: sphere electrode Cathode: semitron electrode Anode (+) Cathode (ground) We heard sparks (breakdown) at 78kV/cm and the supracon SQUID is dead.

    24. HV-SQUID test 2 (1/f noise) Negative polarity (Brass Cathode, Semitron Anode) These peaks are suppressed at higher voltage. We can do Gaussian fitting to find width.

    25. HV-SQUID test 3 (positive polarity): Positive polarity (Brass Anode, Semitron Cathode) Gap size: 2mm Anode: sphere electrode Cathode: semitron electrode anode cathode

    26. Analysis of the noise spectrum 15.6Hz Peak 1/f noise (spherical cathode) disappearing 1/f noise (planar cathode) 15.6Hz Peak

    27. HV-SQUID test 4: Star Cryoelectronics SQUID Comparing noise spectrum of SQUID under High Voltage Gap size: 0.9mm Anode: sphere electrode Cathode: semitron electrode Position of peak is shifted Structural change under electrical stress? (not seen with the Supracon SQUID.)

    28. Electrostatic force Charge on the electrode: Q=CV=0.1pF x 10kV = 10-9 Columb Force between electrodes: F=kQ^2/r^2 = 8.89 x 109 * (10-9)2/(0.001)2 = 0.01N Equivalent to 1 gram weight, not very large!

    29. Micro discharge or HV noise There is no HV dependence of these non-random excitations.

    30. HV noise Periodic pulses in both the direct current monitor and the pick-up coil: 110 kHz (related to the HV supply)

    31. SQUID Noise Spectrum Squid rms noise vs HV Low frequency filtered signal: integrated signal from 2.5MHz ~ 500 MHz (HV related, but could be averaged out).

    32. Breakdown A breakdown spark occured at - 12kV. E=121kV/0.9mm = 133kV/cm Negative polarity on the Brass spherical electrode The instantaneous current of the spark is measured to be Amplitude > 80V/3 = 27 Amps. Time scale < 100 ns. This killed the Star Cryoelectronics SQUID. (Battery power is floating).

    33. FFT of break down’s spark

    34. Summary • Sufficient shielding (both magnetic & RF) can be implemented to operate SQUIDs. • Applying HV does not increase the white noise baseline (up to 20kV, 130kV/cm). • However, it does increase the 1/f noise. • HV affects some features on the microphonics. • structural deformation or something else? • Provide a mean to monitor the HV magtinude • SQUID survival under sparks • When the SQUID and the electronics is kept floating, sparks readily kill SQUID sensors. Tying the SQUID ground to the RF shield might alleviate the SQUID failure rate (need more tests).

    35. Backup slides

    36. Star Cryoelectronics Magnetometer Measured at November in 2007

    37. 3. Quantum Design SQUID FFT Time trace

    38. AC power vs Battery power (with semitron) Warming up from SF to Normal State Helium: 1/f noise is increased due to temperature change Noise spectrum using AC power was measured with no termination of PZT BNC connector, while rest one was measured with termination. So noise spectrum in the region of from 500Hz to 800Hz on rest one is strongly suppressed. Therefore we can conclude that noise peaks at this region come from pickup signals by BNC connector.

    39. Summary: Supracon SQUID Spectrum Comparing noise spectrum of SQUID under different conditions Flux noise of SQUID measured in clean state has obviously less than others

    40. RF Shielding Study 2: (Brass electrode) Comparing noise signal of SQUID under different RF shielding Measurement under upper Ni with RF cage looks worst between them. But difference is not big.

    41. SQUID Noise Spectrum(positive polarity)

    42. HV-SQUID test 2 Negative polarity Gap size: 2mm Cathode: sphere electrode Anode: semitron electrode cathode anode We didn’t see any micro-discharge.

    43. HV-SQUID test 2 Negative polarity 1/f noise starts increasing? In the region of E>75kV/cm, 1/f noise keeps increasing.

    44. Normal State He vs Superfluid He 4K liquid He, no pump Superfluid w/ pump on Superfluid, no pump

    45. RF shielding Study 2: HV feedthrough semitron open Pb shield

    46. Studies on RF shielding of the HV line (Semitron + open Pb shield) Measurement under upper Ni with RF cage looks worst between them. But difference is not big.

    47. AC power vs. 12 V batterySupracon SQUID (One layer Pb shielding) With feedback on, 120Vac power PCI-1000 12V battery power PCI-1000

    48. SQUID with and without Faraday cage The baseline of SQUID with Faraday cage is larger than that without Faraday cage. But we guess that this comes from the gap size of electrodes. With Faraday cage it’s hard to see SQUID jumped. So it is obvious that we can reduce RF interference by Faraday cage.

    49. Tune vs Lock mode In the tune mode (feedback circuit is off) Supracon SQUID (one layer Pb shielding) Amplitude of noise signal in the tune mode is 10 times larger than that in the lock mode

    50. Supracon SQUID (one layer Pb shielding) Tune vs Lock mode Noise Spectrum with feedback off Noise Spectrum with feedback on