Chan Seok Kang

Effective cancellation of residual magnetic interference induced from a shielded environment for precision magnetic measurements

Using the method of inverse problem, we designed a cancellation coil that prevents a strong pulsed magnetic field inside a magnetically shielded room (MSR) from magnetizing and inducing large-scale eddy currents around the shielding materials. We implemented this coil with discrete current loops and evaluated it numerically and experimentally. Without the cancellation coil, the transient residual magnetic field in the middle of the MSR was greater than 0.1 μT for 63.5 ms, while the cancellation coil reduced it to less than 0.1 μT after 10.8 ms, shortening the decay time by 83.0%.

Fabrication and magnetocardiography application of the second-order superconducting quantum interference device gradiometer made from a single-layer of YBa2Cu3O7 film

We have fabricated single-layer, second-order high Tc superconducting quantum interference device (SQUID) gradiometers on SrTiO3 substrates and investigated their noise properties and performance in magnetocardiography. The gradiometer consists of three parallel pickup loops that are directly coupled to a step-edge junction SQUID, in such a way that the coupling polarity of two side loops is opposite to that of the center loop. The overall size of the device is 17.6 mm×6 mm with a baseline of 5.8 mm. The measured gradient noises are 0.45 and 0.84 pT/cm2/√Hz at 1 Hz for the shielded and the unshielded cases, respectively, which correspond to equivalent field noises of 150 and 280 fT/√Hz, respectively. In spite of the short baseline of 5.8 mm, the high common-mode rejection ratio of the gradiometer, 103, allowed us to record a magnetocardiogram of a human subject, which demonstrates the feasibility of the design in biomagnetic studies.

Evaluation of cancellation coil for precision magnetic measurements with strong prepolarization field inside shielded environment

Many precision magnetic measurements can benefit significantly from or even require strong prepolarization fields (Bp) and magnetically shielded environments. We describe here in detail a cancellation coil (CC) which can neutralize the Bp on the electrically conductive shield walls that may otherwise induce currents on the walls to produce a lingering transient residual field (Btr) inside the shielded environment and disrupt the measurement operations. The CC was designed using the inverse problem method to effectively neutralize magnetic fields generated on the shield walls by the Bp coil. The implemented CC was evaluated by measuring Btr using a fluxgate magnetometer at different magnetometer positions and cancellation coil currents (ICC). Multi-mode component analysis on the Btr measurements revealed two dominant components, where the component with shorter time constant comes from the current induced around the shield side walls and the other with longer time constant from the current induced on the c...

Square Loop Coil System for Balancing and Calibration of Second-Order SQUID Gradiometers

We have designed and fabricated a uniaxial square- loop coil system to calibrate second-order SQUID gradiometers. The coil system consists of 2 pairs of identical square loops, the first pair which is Helmholtz coils for generation of uniform fields and the second pair for the 2nd-order gradient only in combination with the first pair. We obtained analytical expressions up to the 4th-order of the fields in the midplane and along the coil axis. The coefficient of the normalized second-order field term is calculated to be c(5 - 11gamma2 - 18gamma4 - 6gamma6)/(2 + 3gamma2 + gamma4)2 near the coil center, where c is -1 for the fields along the coil axis and +1/2 for those in the midplane, where gamma is the ratio of the inter-coil distance to the side length of the coil. The Helmholtz condition of a coil pair is gamma = 0.5445. The pure second-order gradient field is generated by subtracting the Helmholtz field from the field of a coil pair off the Helmholtz condition with equal magnitudes of the center fields of the two pairs. The coil system is useful for evaluating second- order SQUID gradiometers.

Magnetocardiographic performance of single-layer second-order YBCO SQUID gradiometers

We have studied noise properties and magnetocardiographic performance of single-layer second-order YBCO SQUID gradiometers made on STO substrates. A three-loop pickup coil is directly coupled to a step-edge junction SQUID with the coupling polarity of the center loop opposite to that of two side loops. Optimized device parameters for balancing were obtained from 8.8 mm /spl times/ 3.0 mm devices on 10 mm /spl times/ 10 mm substrates and scaled up to the larger devices for use in cardiomagnetic measurements. A 17.6 mm /spl times/ 6 mm gradiometer with a 5.8 mm baseline showed an unshielded gradient noise of 0.84 pT/cm/sup 2//Hz/sup 1/2/ at 1 Hz, which corresponds to an equivalent field noise of 280 fT/Hz/sup 1/2/. A large balancing factor of 10/sup 3/ allowed us to record magnetocardiogram of a human subject.

Development of a bio-magnetic measurement system and sensor configuration analysis for rats

Magnetoencephalography (MEG) based on superconducting quantum interference devices enables the measurement of very weak magnetic fields (10-1000 fT) generated from the human or animal brain. In this article, we introduce a small MEG system that we developed specifically for use with rats. Our system has the following characteristics: (1) variable distance between the pick-up coil and outer Dewar bottom (∼5 mm), (2) small pick-up coil (4 mm) for high spatial resolution, (3) good field sensitivity (45∼ 80fT/cm/Hz), (4) the sensor interval satisfies the Nyquist spatial sampling theorem, and (5) small source localization error for the region to be investigated. To reduce source localization error, it is necessary to establish an optimal sensor layout. To this end, we simulated confidence volumes at each point on a grid on the surface of a virtual rat head. In this simulation, we used locally fitted spheres as model rat heads. This enabled us to consider more realistic volume currents. We constrained the model such that the dipoles could have only four possible orientations: the x- and y-axes from the original coordinates, and two tangentially layered dipoles (local x- and y-axes) in the locally fitted spheres. We considered the confidence volumes according to the sensor layout and dipole orientation and positions. We then conducted a preliminary test with a 4-channel MEG system prior to manufacturing the multi-channel system. Using the 4-channel MEG system, we measured rat magnetocardiograms. We obtained well defined P-, QRS-, and T-waves in rats with a maximum value of 15 pT/cm. Finally, we measured auditory evoked fields and steady state auditory evoked fields with maximum values 400 fT/cm and 250 fT/cm, respectively.

Single-layer 2nd-order SQUID gradiometer and fabrication of YBa2Cu3O7 nanobridges as the Josephson elements

We have studied fabrication of second-order SQUID gradiometers from single-layer of high-Tc. film. The gradiometer contains three parallel-connected pickup coils which are directly coupled to a step-edge junction SQUID. In the study of short-baseline devices, we achieved an unshielded noise of 0.84 pT/cm2/Hz½ at 1 Hz for a well-balanced gradiometer. As Josephson elements of lone-baseline devices we made submicron YBCO bridges by using a focused ion beam method. In temperature-dependent critical currents, Ic(T), and normal state resistances, RN(T), the bridge showed an SIS-type behavior, which is believed to be due to naturally formed grain boundaries crossing the bridges.