Plasma stability and reproducibility for characterizing plasma-material interactions (PMI) on the ZaP-HD sheared-flow-stabilized Z-pinch device.*

An array of axially and azimuthally distributed magnetic field probes measures the spatial and temporal evolution of the magnetic field in the ZaP-HD sheared-flow-stabilized (SFS) Z pinch. Fourier analysis of the probe data yields the fluctuation amplitudes of the azimuthal modes. The resulting m = 0 mode measures the mean field amplitude, and the m = 1 mode is proportional to the radial displacement of current from the geometric axis of the probe array. These values are used to assess the axial uniformity, quiescent period, and reproducibility of the pinch. The power supply discharge time and gas puff times are varied to optimize the machine settings that produce uniform, stable, and reproducible pinches. The high current density and high energy density plasma in ZaP-HD and other SFS Z-pinch devices can result in electrode erosion, which can be detrimental to fusion performance and component lifetimes. Operational campaigns on ZaP-HD to diagnose the plasma-material interactions (PMI) at the cathode will benefit from stable, reproducible pinches by providing consistent energy deposition into the plasma. Diagnostics in development such as a two-color pyrometer will provide measurements of surface temperature and the heat flux through the cathode.