Design of Pickup Coil Made of Litz Wire and Cooled at 77 K for High Sensitive Measurement of AC Magnetic Fields

概要

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We studied the performance of a copper pickup coil for high sensitive measurements of ac magnetic fields. The coil was made of Litz wire to prevent the skin effect, and was cooled at 77 K to reduce thermal noise. We first obtained an analytical expression for the coil resistance by analyzing the eddy current loss of the coil. The analytical result explains well the measured dependences of the coil resistance on the frequency, number of turns of the coil ($N$), and filament diameter of the Litz wire ($d_{\text{f}}$). Next, we show the design of the pickup coil. When appropriate values are selected for $N$ and $d_{\text{f}}$, we can expect the magnetic field noise $S_{\text{B}}{}^{1/2} < 10$ fT/Hz1/2 for $f > 1$ kHz and $S_{\text{B}}{}^{1/2} < 2$ fT/Hz1/2 for $f > 10$ kHz for the pickup coil with diameter of 50 mm. The experimental results agreed with the designed values.
2011-07-25