Operation and modelling of a solid-state switched coaxial plasma gun

We describe the fabrication and operation of a gas puff-injected coaxial plasma gun (CPG) using high-power solid-state switching. Understanding the topological details of the plasma formation and sheath rundown phase are critical for in-space propulsion testing and applications to plasma jet magneto-inertial fusion, for which this CPG is designed. The construction of the CPG electrodes is detailed and used to define the boundary conditions (BCs) for two-dimensional axisymmetric resistive magnetohydrodynamics (MHD) modelling. To the authors’ knowledge, this study represents the first application of the open-source modelling framework of OpenMHD (e.g. Zenitani & Miyoshi 2011 Phys. Plasmas vol. 18, 022105) to an experimental system with physical BCs. Real-time diagnostics of the gas puff injection, plasma sheath ionisation and bulk plasma propagation were synchronised to and referenced against the digital gate signal provided to the custom solid-state switching array. The array is configured to enable reliable and repeatable CPG discharges at 4 kV and 7.7 kA with global shutter camera images taken at digital delay increments throughout the plasma evolution. By tuning the initial conditions (ICs) of OpenMHD modelling, the initial density, velocity and magnetic field profiles, the imaged dynamics could be replicated to a significant degree. While the operation of the CPG deviates from the classic one-dimensional snowplough model, quantifying the multi-dimensional role of blow-by during the rundown phase was the primary result of the resistive MHD modelling. Understanding the plasma profiles through the definition of the ICs will be useful for informing the future placement of diagnostics and enforcing the interaction of the bulk plasma properties with an external magnetic field.