Generation of long-lived, pulsed magnetic fields using capacitive energy storage

Abstract

A relatively slow decay of current in a pulsed magnet can be obtained by applying a suitable low-impedance crowbar to the magnet at first current maximum. Semiconductors, ignitrons and spark gaps are considered as crowbarring devices and their electrical characteristics measured for currents of a few kiloamperes on a time scale of a few milliseconds. The capsule semiconductor diode was vastly superior to the ignitron and spark gap, and its near linear characteristic enabled the circuit performance to be analysed in greater detail. An optimised system was constructed comprising a 2.4 kJ capacitor discharging into a tape-reel magnet formed from 7 kg of copper strip. The initial decay of the central axial field was three per-cent. per millisecond, which was some 40% less than the decay of the magnet current. This discrepancy was shown to be caused by proximity effects in the winding. One drawback of this technique is that the decay time is proportional to the volume of conductor constituting the winding, and some magnetic flux must penetrate the conductors. Consequently, an increase in winding volume must produce an increase in decay time at the expense of some decrease in the accessible magnetic field.