Fast rise-time magnetic field coil study

Abstract

The preliminary design and testing of a fast magnetic field coil is reported. The present goal is a 3-cm-diameter coil that will produce a magnetic field that changes by roughly 3 kG within 100 ns. Its purpose is to impart a varying canonical angular momentum to an electron beam produced by a field-emission cathode, to stabilize the resistive hose instability. The present design consists of an underdamped LC circuit with a 100-ns quarter-period. The coil is a single turn to minimize inductance, which is about 70 nH. This requires an extremely low capacitance, with a charging voltage of about 4 kV to obtain a large enough peak current. Several problems may have to be overcome. Stray inductances and capacitances may significantly reduce the resonant frequency of the circuit; the inductance of the circuit may have to be reduced. The magnetic field may not penetrate sufficiently far or quickly into the diode region, because of either the geometry or plasma near the anode; the coil may have to be put into the diode region itself. Timing of the coil with respect to beam formation may be difficult; the fast coil circuit may have to be triggered by a signal directly from the pulse-forming line of the accelerator. >