High-frequency operation of the coils for standard magnetic-field generation using the bulk-current technique

The sum of the currents in the individual wires of a multiturn coil (here referred to as the bulk current) possesses properties that can extend the useful operating range of the device to frequencies substantially higher that those obtainable through the traditional control of the terminal current. This paper theoretically analyzes and experimentally confirms some basic results about the high-frequency behavior of coils. The analysis is initially based on the multiconductor transmission line theory applied to an equivalent line model obtained through a simple geometrical transformation of the coil; then closed-form formulas predicting the essential deviations from ideal at high frequencies (well beyond the first resonance) are presented. The response to different types of feed (balanced or single ended) is also described. The "bunch" or "bulk" current concept is a new finding, not existing in the previous literature. The analysis of the bulk current reveals that the magnetic field remains constant and predictable up to frequencies many times higher than the resonant frequency of the coil. Experimental results confirming the theoretical predictions are presented and discussed.