A study of pulsed activation of trapped field magnets: effects of multiple pulsing

Experimental studies are reported for pulsed activation of trapped field magnets (TFMs), using a sequence of N equal pulses. These experiments are done on low field TFMs for which temperature increase is not yet a dominant limitation. For low and moderate amplitudes of activation pulses heights, BA, the trapped field at any distance r from the TFM centre increases as BT(r,N) ∝ klogN, where k is not a function of N. Thus the increment in trapped field is ∝1/N. Since logN increases without limit as N → ∞, while BT is limited by finite JC, limits are expected on the logN increase. The limiting behaviour is observed at higher values of BT(r,N), and is very well fitted by the multiplicative factor [1 − BT(r,N − 1)/B*(r)]. The resulting phenomenological equation fits the data at essentially all values of N, r, and BA. In this equation, B*(r) plays the role of the limiting value of BT(r) as N → ∞. A study of B*(r) leads to its identification as the trapped field reached in a zero-field-cool activation using a constant field of the same magnitude as the pulsed field. (paper)