Jin Mok Kim

Double Relaxation Oscillation SQUID Systems for Biomagnetic Multichannel Measurements

Multichannel superconducting quantum interference device (SQUID) systems based on double relaxation oscillation SQUIDs (DROS) were developed for measuring magnetocardiography (MCG) and magnetoencephalography (MEG) signals. Since DROS provides large flux-to-voltage transfer coefficients, about 10 times larger than the DC SQUIDs, direct readout of the SQUID output was possible using compact roomtemperature electronics. Using DROSs, we fabricated two types of multichannel systems; a 37-channel magnetometer system with circular sensor distribution for measuring radial components of MEG signals, and two planar gradiometer systems of 40-channel and 62-channel measuring tangential components of MCG or MEG signals. The magnetometer system has external feedback to eliminate magnetic coupling with adjacent channels, and reference vector magnetometers were installed to form software gradiometers. The field noise of the magnetometers is around 3 fT/ Hz at 100 Hz inside a magnetically shielded room. The planar gradiometer systems have integrated first-order gradiometer in thin-film form with a baseline of 40 mm. The magnetic field gradient noise of the planar gradiometers is about 1 fT/cm/Hz at 100 Hz. The planar gradiometers were arranged to measure field components tangential to the body surface, providing efficient measurement of especially MCG signals with smaller sensor coverage than the conventional normal component measurements.

Compact readout electronics for 62-channel DROS magnetocardiogram system

Compact and low-cost SQUID electronics to operate double relaxation oscillation SQUIDs (DROSs) has been constructed for detecting magnetocardiogram (MCG) fields. SQUID electronics consists of low-noise preamplifier with a white noise of 0.6 nV//spl radic/Hz, flux-locked loop (FLL) circuit with auto-reset, and interface circuit to control FLL using 3 digital lines from a computer. The automatic control software adjusts SQUID parameters to the optimum operating condition within 15 seconds for 62 channels. Outputs of FLL circuits are passed through the hardware filter box to the 64-channel data acquisition board. Each SQUID electronics for FLL and digital control is built together on a printed circuit board of 45 mm /spl times/ 95 mm. When SQUID electronics is connected to DROS gradiometer with typical flux-to-voltage transfers of 1 mV//spl Phi//sub o/, the noise contribution of FLL circuit is about 0.6 /spl mu//spl Phi//sub o///spl radic/Hz or 0.6 fT//spl radic/Hz at 100 Hz, low enough to measure MCG fields.

Closed-cycle cryocooled SQUID system with superconductive shield for biomagnetism

We developed a cryocooled SQUID system with which human magnetocardiogram (MCG) and possibly magnetoenceparogram (MEG) can be measured. To reduce cyclic magnetic noises originating from the regenerator of the cold heads of the cryocooler, a superconductive shield (99.5% Pb) was used to protect the SQUID sensors, and a ferromagnetic shield (78% Ni alloy) was used to screen the cold head. In addition, the SQUID sensors chamber was placed at a distance of 1.8 m from the cold head chamber to install the cold-head chamber outside the magnetically shielded room (MSR) for future development. The loss in cooling power due to the increased distance was compensated by increasing the number of thermal rods, and thus the SQUID sensor and superconductive shield could be refrigerated to 4.8 K and 5 K, respectively. The superconductive shield successfully rejected thermal noise emitted from metallic blocks used to improve thermal conduction. The noise of the SQUID system was 3 fT/Hz1/2, and the cyclic magnetic noise could be reduced to 1.7 pT. We could obtain a clear MCG signal while the entire cryogenics was in operation without any special digital processing.

Conducted EMI Reduction on the Pulse Tube Cryocooler

Pulse tube (PT) cryocoolers can be used to cool SQUIDs, however their electromagnetic interference (EMI) through the conducted high frequency noise currents are normally too large to operate SQUIDs. In this paper, high frequency noise currents of a pulse tube cryocooler system are simultaneously observed at several critical points to analyze conducting paths of the noise currents and to find a solution for this problem. High frequency noise currents are detected by current sensors consisting of a current transformer and a terminating resistor at the secondary side. By covering three-phase power lines and a neutral line connecting the valve motor to the inverter with a copper mesh, and by using it as an additional neutral line, conducted EMI (noise currents) to the cryostat has bee reduced by one fifth. With this treatment, SQUIDs have been operated normally.

Characteristics of Directly-Coupled High-Tc Superconducting Quantum Interference Device Magnetometers for Multichannel Applications

We have characterized directly-coupled dc superconducting quantum interference device (SQUID) magnetometers for multichannel applications. YBa2Cu3O7-δ single layers were deposited on 1 cm×1 cm SrTiO3 bicrystal substrates by laser ablation and patterned by Ar ion milling. From the comparison of the experimental data with the theoretical predictions, we have shown that the degradation of the modulation voltage is caused by the thermal noise flux and thermal noise rounding in the I–V curve. In addition, the SQUID noise at low frequencies could be explained well by critical current fluctuations of the Josephson junctions with a value of SI0 /I02≈(1–4)×10-8/Hz. We have achieved a magnetic field noise of 170 fT/√ Hz at 1 Hz. We demonstrated the use of our magnetometers to obtain realtime traces of a magnetocardiogram inside a magnetically shielded room.

AIN ceramic thermal guide for cooling a SQUID and its effect on thermal magnetic noise

†We evaluated the thermal conductivity of AlN ceramic plates as a potential material of a thermal guide for cooling SQUID in cryocooler cooled SQUID systems. We found that the AlN ceramic plate does not show a good thermal conductivity at very low temperature. We tried to enhance its thermal conductivity by attaching a lot of thin insulated copper wires in parallel with each other to cover a back plane of the plate. With this enhancement we succeeded to operate a double relaxation oscillation SQUID (DROS) first-order gradiometer. Comparison of measurement results on white noise level between two cases where DROS was attached to a copper plate thermal guide and where it was attached to a modified AlN thermal guide shows almost 5 time reduction in noise floor with the latter plate.