Magnetic field penetration and magnetohydrodynamic acceleration in opening switch plasmas

Magnetic field penetration in current-carrying plasmas is being studied in a plasma opening switch geometry. Several Marshall guns 1 are used to inject single or multi-species plasmas between coaxial conductors connected to the output of NRL's Hawk pulsed-power generator. Following injection of the plasma, the generator is used to apply an electrical pulse with a peak current of 700 kA, a peak voltage of 640 kV, and a rise time of 1.2 µs. Initially the plasma acts as a short, conducting all of the current. Over time, the resulting magnetic field interacts with the plasma through a combination of magnetohydrodynamic (MHD) plasma translation and field penetration that is not well understood 2–4 . Eventually a quasi-neutral gap forms in the plasma 5,6 , allowing electrical power to flow downstream. The quality of this switching is affected by the manner in which the gap is formed. This process is monitored using magnetic probes and a ribbon-beam interferometer running parallel to the axis of the accelerator and spanning the inter-electrode plasma region. Particle-in-cell (PIC) modeling shows that the relative importance of MHD translation and field penetration in the gap formation process is dependent upon the radial density gradient and composition of the plasma 7 . These parameters of the initial injected plasma are adjusted experimentally using the Marshall guns for light (hydrogen), heavy (argon), and mixed light and heavy components. The experimentally-observed behavior of the resulting opening switch plasmas in the presence of the interacting magnetic field is compared with results from PIC modeling.