Trade-off analysis of various plasma implosion configurations for soft x-ray production on ACE 4

In this work, several possible choices for plasma radiation source loads are compared with respect to their coupling efficiency for producing soft x-rays on the ACE 4 pulse driver. ACE 4 is a 4 MJ, 6 MA inductive energy storage pulse generator. ACE 4 will be capable of driving loads at variable current rise times, from about 50 ns to over 1 microsecond, by tailoring the output voltage of its Plasma Opening Switch (POS). It is this POS voltage which effectively drives the loads under consideration. The loads evaluated are plasma implosions in the Z-pinch, theta pinch, hybrid pinch, and coaxial fast pinch configurations as described by simple dynamic equations relating the driving current with their pinching motion, and by concurrently calculating their time dependent impedances. The study is conducted by coupling self-consistently these simple representations for the loads to the circuit equations describing ACE 4 operation. The circuit representation for ACE 4 includes a model for the conduction and opening phases of its POS. The cylindrical Z-pinch is an efficient soft X-ray (SXR) radiator when driven by currents rising in about 100 ns or less. When the current rise time is longer, the Rayleigh-Taylor instability growth prevents the plasma from being compressed efficiently, and SXR emission is greatly reduced. Being able to drive SXR loads with slower rising currents would provide important benefits in designing and building the next generation of pulsed power drivers. In the case of the hybrid pinch, where the current return cage posts are spiraled around the load to produce a B, component of the magnetic field, believed to contribute to stabilization of the Rayleigh-Taylor instability, preliminary experiments were performed on BLACKJACK 3 a few years ago indicating better yields than a Z-pinch at the same peak current of 0.6 MA and 70 ns rise time