The bulk current concept: a new technique for high-frequency operation of the coils in the standard magnetic field generation

In this paper are theoretically analyzed, and experimentally confirmed, some preliminary results about the high-frequency behaviour of coils. The analysis is based on the well known multiconductor transmission line theory, applied to an equivalent line model obtained through a simple geometrical transformation of the coil. 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 feeding (balanced or single ended) is also described. The bunch or bulk current concept is introduced. The analysis of the bulk current reveals that the magnetic field generated by the coil remains constant and predictable at frequencies up to eight times higher than the resonant frequency of the coil. Experimental results confirming the theoretical predictions are presented and discussed.