Experiments and Modeling of Plasma Ionization and Acceleration in a Coaxial Plasma Accelerator

Abstract

We report an experimental investigation and theoretical modeling of ionizing current sheet dynamics in a 1-m coaxial plasma accelerator on the FuZE experiment at the University of Washington. The accelerator contains an embedded longitudinal magnetic probe array that measures the time evolution of the azimuthal magnetic field profile $B(z,t)$ ) with 5-cm spatial resolution, allowing reconstruction of the detailed structure of the current sheet propagating into neutral gas. The radial current density is $J_{R^{\sim}}$ -dB/dz and the spatial distribution of the electric potential V (z, t) is found from dV/dz = -L’ dI/dt. Thus, the local power available for plasma ionization and acceleration (J R V) and the local $jxB$ force profiles are known for the plasma in the experiment, while the plasma density and velocity profiles are unknown. We reconstruct the plasma density and velocity profiles by solving a 1-D MHD fluid model with a density source term to represent the ionization of the neutral gas. The experimental magnetic field $B(z,t)$ and potential $V(z,t)$ are used as inputs to generate the solution. The local ionization rate is found by assuming that resistive power dissipation $(RJ_{R}^{2})$ couples to ionization only; resistivity $R$ is obtained from generalized Ohm's law. The predicted plasma velocity and density are compared to direct measurements at the exit of the accelerator, where velocity is measured by ion Doppler spectroscopy and density by laser interferometry. The reconstruction approach is applied to characterize how the initial neutral density profile (controlled via timing of fast gas injection valves) affects the propagation of the ionizing current sheet and the performance of the accelerator. 1 D modeling of the plasma flow also allows for the local resistivity of the current sheet to be inferred, which is compared to existing resistivity models of partially-ionized plasma.