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Overview of the inverse problem. Vertical cross-section of the B p coil (A: pair of crossed rectangles, just below the center), field cancellation space (B: oblique-lined area), and the CC space (C: thick black dashed line). Interior of the MSR was 2 m × 2 m × 2.6 m, with CC placed just inside the MSR walls (gray shade) with 30 mm clearance from the top and the bottom and 60 mm from the sides. The field cancellation space was 4 m × 4 m × 4 m outside and 1.96 m × 1.96 m × 2.56 m inside. It was set up to contain the MSR walls but not overlap with the CC space. The current density distribution in the CC space was restricted to be the same as the B p coil current. The middle of the B p coil bore was 1 m above the bottom MSR wall with its axis along the z-axis. The B p coil was designed to generate 0.1 T in the middle of its bore with a 20 A current and measured 18 cm in its inner diameter, 5 cm in its thickness, and 13 cm in its height. The B p coil was enclosed in a cryogenic dewar and immersed in liquid nitrogen to reduce its electrical resistance and allow greater amount of current flow. Arrow in the middle (F) indicates the fluxgate magnetometer position for B tr measurements.
(Color online) Inverse J(r′) solution and a discretized CC implementation corresponding to the solution inside the MSR. Current loop sizes and locations were chosen to mimic the current density distribution solution (outset graphs) as closely as possible. Magenta loops on gray plates indicate the top and bottom loops, and blue loops indicate the side loops.
(Color online) Numerical evaluation of the implemented CC. (a) Magnetic field component normal to the MSR walls (positive towards the z-axis for the top and bottom plates and towards the outside for the side plates) inside the field cancellation space in Fig. 1, with 20 A current driven to the B p coil only. (b) Same as (a) with the same current driven to the B p coil and the CC, as described in Fig. 2, in series. Hollow cylinders show the locations and relative sizes of the B p coil, and arrows indicate the direction of the B p coil current and B p inside its bore.
(Color online) Experimental evaluation of the CC implementation. (a) CC evaluation protocol: 20 A current was driven to the B p coil for 5 s, and then the current was ramped down non-adiabatically to 0 A in 8 ms. The B p coil current reached 0 A at t = 0. The solid line is for the B p towards the top and the dashed line is for the opposite B p . (b) and (c) B tr measured with the FGM inside the B p coil bore with B p towards the MSR top (b; as indicated in Fig. 1) and with the B p polarity reversed (c). Insets in (b) and (c) show close-ups of the long-term traces. Black dashed lines indicate baseline shift artifacts from the FGM exposed to strong B p , fitted to straight lines between 3 s and 30 s in the corresponding graphs. (d) and (e) B tr measurements with the artifacts (black dashed lines in (b) and (c)) removed. Horizontal axes represent the time as defined in (a) in logarithmic scale and vertical axes represent the FGM readout. Red solid lines represent measurements with current driven to the B p coil alone and blue dotted lines with current driven to the B p coil and the CC in serial.
B tr decay times with and without the CC, with 3 field thresholds.
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