A resonant biaxial Helmholtz coil employing a fractal capacitor bank

The design and construction of a series resonant biaxial Helmholtz coil for the production of magnetic fields as large as 500 G in the range of 100-2500 Hz is described. Important aspects of ac coil design are discussed, including: minimizing power losses due to the expected Joule heating, self-induced eddy currents, and skin resistance; controlling the stray capacitance; maximizing field homogeneity; and keeping peak voltages at acceptable levels. The design and construction of a computer-controlled, optically isolated fractal capacitor bank is then treated, and various aspects of capacitor selection and characterization were discussed. The system performance is demonstrated, including stability and the possibility of field component dephasing with typical magnetic samples.