Design and operation of a sequentially-fired pulse forming network for non-linear loads

While the construction of a linear pulse forming network (PFN) for a constant load impedance is relatively easy, the process is more difficult for a nonlinear or time-varying load. A passive PFN can certainly be synthesized for nonlinear loads, but is usually large and lacks the flexibility to be truly useful in most practical and research applications. This investigation describes the design and construction of a sequentially-fired pulse forming network (SFPFN) that maintains constant voltage and current for a nonlinear load. Operation of the SFPFN consists of charging multiple capacitor banks (or modules) to various levels and sequentially firing these banks into the load at appropriate times. The load and its characteristics determine the module charge voltage. An added benefit with the SFPFN is real-time computer monitoring and control allowing the PFN modules to be charged from a single prime power source via a group of switching relays. The nonlinear load used in this investigation is a helical coil electromagnetic launcher (HCEL). The SFPFN is also tested with a linear load consisting of a pulsed field coil. The nonlinearity of the HCEL is well-known with a factor of 2 change in winding resistance due to joule heating and a large variation in terminal voltage due to changes in the armature back-voltage. Experimental measurements show the SFPFN can deliver a relatively constant current pulse on the order of 5-15 kA into the HCEL load for a pulse length up to 8 ms. The maximum SFPFN operating voltage is 900 V with a total stored energy of 125 kJ. Scaling the SFPFN to larger or smaller pulse amplitudes or lengths is possible.