SQUID NMR Measurements in nEDM experiment -- how to get a big signal & how to operate SQUIDs in High Voltage Environments Chen-Yu Liu

He 3 Spin Measurement ● How big is the magnetic field signal? – 2.76x gauss on the edge of the cell, where x is the He3 concentration. – X= → field signal = 2.76 fT – With a pick-up coil of 10cmx20cm, F= 1.5×10 -8 gauss-cm 2 =70m F 0. ● We have the option of gradiometer & magnetometer.

50 µ F 0 (coil)→0.5µ F 0 (SQUID) with 1% coupling efficiency 13.5 m F 0 (coil)→135µ F 0 (SQUID) with 1% coupling efficiency Magnetometer/gradiometer=270

Understanding the Magnetic Field Profile in the geometry of nEDM experiment ● A detailed magnetic field modeling is required to guide us in positioning and designing the pick-up coils. ● Available resources: – A Matlab finite element code in P-21 group. (S/C. Shield, no F/m shield) – Ansys: 3D finite element analysis – Radio (mathematica): 3D (no S/C shield) – Poisson Superfish: 2D – Other Commercial Codes

Vibration B 0 =1mG Assume a position fluctuation (due to vibration) of 1µm over a 10 cm length. The flux fluctuation into a magnetometer pick-up coil of 10cmx20cm is A gradiometer can cancel the DC field to 10 4, and the vibrational coupled flux is reduced to 10m F 0. Comparable to the He3 signal! Challenging!

Options on pickup coils – Wire-wound magnetometer ● Coils can be made as big as you want. (big signal!) ● Subject to all environmental noise and vibration induced ac noise. – Wire-wound gradiometer ● 1:100 balancing between 2 opposite wound coils. ● Coupled to a SQUID outside the field region. – On-chip PhotoLithography gradiometer ● best matching 1:10,000 ● area is limited by the size of substrate (Si wafer). (Smaller signal, but better immunity to vibrational noise and RFI.) ● Convinent, commercially available.

R 1 =2cm R 2 =2cm R 3 =  (R 1 2 +R 2 2 )=3.42cm L G =700nH for 10  m dia. wire =500nH for 100  m dia. Wire (Nb superconducting wire) Electrodes and Magnetic flux pick-up coils (planar gradiometer) for eEDM experiment Common mode rejection of external uniform B field and fluctuations. Enhancement of sample flux pick-up. + _ 0 5” 2.5” CMRR = 238  0.4% area mismatch

Lead Box Star Cryoelectronics 1165 DC SQUID Closed Pb box, containing a SQUID (solder sealed) To pickup coil To PFL circuit box, Outside the cryostat SQUID in Pb box Connection box (RF shielded) HV lines Star Cryoelectronics 1165 SQUID: Fails after a few thermal cycles. Current lock mode built-in Quantum Design DC SQUID, Model 50: Very sturdy Flux lock mode

1/f corner of SQUID (QD-50)

First-Order Planar SQUID Gradiometers with Long Baseline Robin Cantor, J. Ad Hall, Andrei. N. Matlachov and Petr L. Volegov ● Developed by Star Cryoelectronics. ● “…. The white flux noise of the improved gradiometer with 3.6 cm baseline is as low as 2.4 µ Ѱ 0 / √ Hz(rms), and the magnetic field sensitivity referred to one pickup loop is 0.63 nT/ Ѱ 0. This results in a magnetic field gradient noise of 0.42 fT/cm- √ Hz.” ● “ All of the gradiometers cold easily be operated without magnetic shielding in a typical laboratory environment without losing lock using only a Mylar film with a thin Au coating for rf shielding. Operated unshielded, the rms flux noise at 10kHz tended to be roughly two times higher.” ● Could make it 4 times bigger.

SQUID Friendly Environment ● RF shielded environment – Faraday Cage: absolutely necessary – Superconductor: only necessary around the SQUID package. ● With a samll gradiometer, S/C shield is not required. – All current, voltage leads going into the field region should be low pass filtered.

Noise in SQUID measurements ● Predictable Noise – Johnson noise (white) from nearby conductors ● Electrodes (Material still yet to be decided). ● Ferromagnetic Shields ● Excessive noise – Vibrations. – Micro-discharge in dielectrics under HV (White or pink?) ● More importantly, it imposes a critical practical issue of how to keep SQUID operational with RFI produced by micro-discharges? – Leakage current. (Continuous current flow or pulsed?)

SQUID Noise Spectrum in eEDM experiment -MACOR electrode with AeroDag coating -Pb shield 2 layers of Pb superconducting foil Vibrational peaks are suppressed. Background ~ 2x Intrinsic SQUID noise Baseline: 27.5  0 /rtHz Baseline: 5.8  0 /rtHz Vibrational peaks A proper S/C shield reduces the vibrational noise in eEDM experiment. However, this will not solve the vibrational noise in nEDM experiment, because the B field is internal.

SQUID under High Voltage Insufficient RF shield? Limits the application of field to only 3kV/cm! Effects of RFI due to micro-discharges???

Leakage current I B~ fT (contribution from 1 side of the wall) at 5cm away from the cell. This is comparable to the If the gradiometer is right in the center of symmetry, there is no field inside the pickup coil. If the leakage current is making fraction of a turn from one electrode to the other, there could be finite contribution into the pickup coil. The size of the signal depends on the exact current path. The gradiometer configuration does not cancel this field produced by local source.

SQUID R&D plan ● SQUID sensors & Pickup coil test (P-21) – Reliability tests (thermal cycles) & intrinsic noise measurements. – Sensitivity characterization of various designs of pickup coil (full size model). – Mitigate the vibration issue through proper design of a gradiometer. – Help with low concentration He 3 NMR measurements (with Tito). ● Johnson noise measurement of electrode and cell material (P-21, IU) ● HV compatibility test (IU) – Same technical issues faced in the eEDM experiment.

HV-SQUID Compatibility Tests at IU ● Scaled down HV cell – Study breakdown properties of various electrode materials of interests, with different degree of surface polish. – Effect of pressurization on breakdown in liquid helium in normal state and superfluid state. – Leakage current in different cell materials of interests. ● SQUID – Study effects of (micro-) discharge on SQUID performance. – Mitigate RFI ● by, for example, placing the pickup coil inside a thin-metal foil faraday cage with a price of increased Johnson noise. ● Low pass filter – Measure leakage current with SQUID. – Attempt to detect trapped flux inside superconducting shields??? – Possibility of using SQUID as electron detector???