Helical pulse-forming transmission line stack for compact pulsed power applications — Design and simulation

Abstract

Design considerations and initial static charge and transient discharge simulations using COMSOL Multiphysics™ are presented for pulse-forming transmission (T-) line modules designed to be stacked, charged in parallel, and discharged in series. Each module is designed to use a rigid injection-molded dielectric cast in halves to accommodate the center conductor, and with a helical discharge path of constant real impedance Z. High peak energy density U0 for high initial charge voltage V0 is possible with such materials made of ceramic or ceramic powder-polymer composite. The helical path permits a high volume utilization efficiency η (effective system mean energy density/U0) for compact applications. Given the system's cylindrical return conductor housing of outer radius R and height H, TV02 = 4πR2HηU0Z for an impedance-matched load. Here, T is the time interval for which the load current and voltage are within the ranges for which the load is effectively driven (neglecting rise and fall times). The model is fully parameterized so, for example, each module's rectangular cross-section T-line aspect ratio AT (width/height) and helical aspect ratio AH (T-line center to helical axis distance/T-line half-width) are free to be varied. This allows for a wide range of system configurations to be studied with minimal effort. Given an optimized T-line inner conductor shape, the contribution to η from the T-line itself is about 1/3 for the AT = 1 — 4 range studied. The minimum AH considered is 2, giving an T-line volume fraction upper bound of 8/9 relative to the minimum cylindrical volume containing it. Their product implies an upper bound on η of about 0.3 Other system requirements, such as extra length and possibly higher AH needed to accommodate a low-inductance multi-channel spark-gap switch between modules and a spark-gap trigger circuit interior to the helix, respectively, and insulation for the erected voltage breakout and between the stages and return conductor, lower η further.