Pulsed power supply for magnetic field to generate HPM from slow wave structures

Abstract

A strong axial magnetic field is required to guide a relativistic pulsed electron beam. The magnetic field can be supplied either by a permanent magnet or an electromagnet. In a backward wave oscillator (BWO) experiment, as the required field is 0–1.2 T in a cylindrical volume having radius of a couple of centimeters and length of about 10–12 cm, an electromagnet is preferred. A large pulsed current is passed into a solenoid to generate a pulsed magnetic field. This current is provided by a capacitor bank. In the paper, development of a pulsed magnetic field system is described. The generated magnetic field is of strength 1.25T with the pulse duration of 2 ms. Inner diameter of solenoid is 5 cm and length is 12 cm. The magnetic field guides a relativistic electron beam that excites a slow wave structure to generate high power microwaves. To generate a unidirectional magnetic field, we must have a unidirectional current pulse. This is obtained by discharging a charged capacitor into an inductor with a resistor in between them. For critical damping, we get a unidirectional pulse. Assuming inductance L and resistance R, a maximum occurs att max = 2L/R where the maximum current is t max = 0.74V 0 /R. The pulse duration is The experimental set up is as follows: Secondary of a step-up transformer gives a peak voltage of 2.5 kV. After half wave rectification, a capacitor bank having capacity of 450µF is charged through a 800 ohm hand woven resistor. Charging time of the capacitor bank is 3 sec. Charged capacitor bank is discharged into a solenoid, inductance L, through pulse forming non-inductive resistance R=1.3 Ω. A small resistance R m =0.125Ω is in series for measurements. A spherical spark gap having 2 cm sphere diameter acts as a self breaking switch. As soon as the instantaneous value of current overshoots a predefined strength, the corresponding voltage that drops across the resistor R m exceeds the comparing voltage on a comparator. This causes a timer circuit to give a turn on a relay that controls switching of a Marx capacitor bank for relativistic electron beam generation1. The voltage across R m , therefore give us the information of current pulse as well as controls the triggering of electron beam system. Peak current is controlled by the pulse forming resistor R. Table 1 given below shows various values of R and corresponding value of voltage on R m . Measured current, pulse duration and magnetic field are given in last three columns.