Small Helical Magnetic Flux-Compression Generators: Experiments and Analysis

In order to gain experience in explosive pulsed power and to provide experimental data for modeling, a small high-explosive-driven helical magnetic flux-compression generator (FCG) was designed at the Swedish Defence Research Agency (FOI). The generator, of which three have been built, has an overall length of 300 mm and a diameter of 70 mm. It could serve as the energy source in a pulse-forming network to generate high power pulses for various loads. This paper presents the design of, and tests with, this helical FCG. The generator had an initial inductance of 23 muH and was operated into a load of 0.2 muH. The generator is charged with 0.27 kg of high explosives (PBXN-5). Various types of diagnostics were used to monitor the operation of the generator, including current probes, optical fibers, and piezo gauges. With seed currents of 5.7 and 11.2 kA, final currents of 269 and 436 kA were obtained, corresponding to current amplification factors of 47 and 39. The peak of the current was reached about 30 mus after the time of crowbar. The two generators showed only small losses in terms of 2pi-clocking. Using signals from optical fibers, the deflection angle of the armature could be determined to be 10 ^deg in good agreement with hydrodynamic simulations of the detonation process and the detonation velocity to be 8.7 km/s in agreement with tabulated value.

I. Introduction

Small helical magnetic flux-compression generators (FCGs) are attractive energy sources for compact pulsed-power systems [1], [2]. A helical FCG usually consists of a conducting cylindrical coil (stator) and a conducting cylindrical tube (armature) filled with high explosives. The stator and armature are connected via, in most cases, an inductive load into which the magnetic flux is to be concentrated. The stator is magnetized by a seed current having its return path via the load and armature. When initiated at one end, the explosives in the armature will rapidly expand the armature in a conical fashion. When the armature contacts the crowbar (a time which will be referred to as the crowbar time here in), it will short out the seed-current source and trap the magnetic flux in the volume between the stator and the armature. As the detonation moves forward, the stator coil is shorted out turn by turn by the armature, reducing the inductance of the circuit. If the flux is conserved, the current and energy in the circuit will increase and can be used for various applications. Of special interest is to use an FCG together with a pulse-forming network to provide a high-voltage pulse to drive microwave generators to generate a high-power-microwave pulse [3].