Performance of a dual-stage helical flux compression generator under varying background gas and pressure

Recent efforts at the Center for Pulsed Power and Power Electronics at Texas Tech University have been focused on the development of a compact and explosively driven High Power Microwave, HPM, system. The primary energy source (other than the seed energy source) driving the microwave load in this system is a mid-sized, dual-stage helical flux compression generator, HFCG. The HFCG has a constant stator inner diameter of 7.6 cm, a length of 26 cm, with a working volume of 890 cm³. Testing at the Center has revealed energy gains in the 30's and 40's with output energy levels in the kilo-joules regime into loads of several micro-Henries. Over the last few years, close to one hundred shots have been taken with these generators into various loads consisting of dummy inductive loads, power conditioning systems, and HPM sources. Throughout these tests, the working volume of the HFCG, i.e. the volume in between the wire stator and the explosive-filled aluminum armature, was filled with SF₆ at atmospheric pressure. This was primarily done do avoid electrical breakdown in the generator volume during operation, resulting in flux loss. Recent design updates enable pressurizing the generator volume to pressures up to 0.5 MPa, which is needed, for instance, to replace the SF₆ with other gases such as air or nitrogen. The performance of the dual-stage HFCG with pressurized working volume (SF₆ and N₂) is presented in this paper along with an analysis of the maximum electric field amplitude held off in the volume during operation. The design technique to seal the HFCG will also be briefly discussed.

I. Introduction

HFCG have become some of the most favored and popular devices for explosively driven pulsed power applications due to their high energy density, compact size, portability, as well as their relative simplicity to manufacture upon the somewhat complicated design development [1], [2], [3]. Flux compression generators were developed in the early 1950's and have since evolved and been used in various applications [4], [5] with significant differences in overall size parameters. Since HFCGs require a seed magnetic field it is necessary to provide a seed energy or current in the primary stator coil to establish the initial magnetic flux to be compressed. Many techniques have been vigorously tested and discussed in literature that utilize direct and indirect seed sources, as well as a variety of solid-state [6], explosively driven, and piezoelectric seed sources. A common goal of operation in the HFCGs is to maximize output energy to accommodate the varying impedance of the loads for these different applications. One of the limiting factors of the performance of HFCGs has been flux loss due to the high electric fields established inside of the working volume between the helical stator (field coil) and expanding armature (working piston) [2]. Fig. 1. Plasma formation between armature and stator coil diagram. (a.) Initial state of HFCG upon detonation. (b.) Armature expansion with development of high-electric field between armature and stator. (c.) Plasma formation ahead of armature expansion.