Adventures in the experimental development of an ultrahigh speed plasma flow

In 1991, Turchi et al. [1] reported evidence for a 2,000 km/s aluminum plasma that originated from the upstream boundary of a wire array armature in a plasma flow switch (PFS) [2]. The 2008 article by Turchi et al. [3] posits that if such high Z plasma could instead be composed of deuterium or a deuterium-tritium mixture then the resultant multi-keV plasma would make an effective target for magnetized plasma compression to fusion conditions. This report documents several exploratory tests executed in an effort to achieve significant energy transfer from a plasma flow switch to a deuterium plasma. The first phase of this research concentrated on extension of the earlier work [1, 2] to a lower current system that would emulate the PFS used in series with an imploding liner load. The apparatus was also modified to permit pulsed injection of deuterium gas along the insulated coaxial electrodes between the PFS armature and the vacuum power feed. In analyzing the armature behavior, the initial conditions used in 2-D axisymmetric MHD simulations to approximate the wire-array/polymer film composite armature resulted in significant uncertainty in the validity of the calculations. This uncertainty confounded efforts to improve the opening switch behavior of the armature. Low density foams, commonly used in other high energy density plasma experiments, were seen as a candidate material for the armature that would facilitate greater fidelity between simulations and the experiment. Two subsequent tests were conducted using foam armatures. In both cases, current prematurely shunted upstream in the vacuum feed. Several possible causes were explored for the shunting of the current. Among the modifications implemented, the gas injection system was altered to increase both the quantity of gas adjacent to the armature while facilitating an increased pressure gradient between the armature and the current feed. A series of low energy shots were conducted to examine the impact of severa- proposed design modifications on current delivery to the armature. One conclusion of these experiments was that it has been very difficult to forestall breakdown in the injected gas as required by Turchi et al. [3]. Nevertheless, two experiments were conducted to evaluate performance with foam armatures. Both experiments exhibited good current delivery to the armature, behaving initially like the low energy experiments. The magnetic flux convected downstream was greater than in any of the prior experiments, though significant work remains to demonstrate the ultra-high-speed plasma flow concept.