Calibration and Inversion for a Plasma Impedance Tomographic Diagnostic: Initial Testing*

Plasma Impedance Tomography (PIT) aims to create a 3D map of plasma density by collecting multiple mutual impedances from an array of impedance probes, and use the measured voltages and currents to find the least square solution to a linear set of electromagnetic conservation equations. By solving for the impedance, along current paths that may fall outside of the line-of-sight between probes, the solution contains 3D information about the plasma distribution when collected from a 2D array. That capability is expected to provide a method to map the plume of an electric propulsion thruster, both in flight and in ground tests. This non-invasive tomography can be scaled and isolated from the plasma through a dielectric windows, leading to additional potential applications in fusion, hypersonic flow, dielectric barrier discharge, and space plasmas. This work presents progress on developing a PIT system with eight dipole impedance probes arranged in a ring. The de-embedding process of the mutual impedance is presented, which allows for recalibration during testing to account for cable movement and temperature changes. The inversion method using basis functions is reviewed, and the derivation of electromagnetic energy conservation equation with small perturbations in the material is presented for solving both general dielectrics and plasma densities. Current results are presented for calibrating, measuring, and inverting a test sample.