Impurity Production and Transport in the Prototype Material Plasma Exposure eXperiment

The Prototype Material Plasma Exposure eXperiment (Proto-MPEX) is a linear pulse plasma device at Oak Ridge National Laboratory with the purpose of doing the research and development for the heating concepts on the planned full MPEX device. The goal of MPEX is to perform material studies at fusion relevant conditions. To understand the conditions at the material target for performing plasma-material interaction studies the ion temperature and density, the electron temperature and density, and the particle flux and fluence must be known. Impurities within Proto-MPEX can alter the desired conditions at the material target and need to be understood for PMI studies to come in MPEX. This dissertation aims to quantify the target conditions, the impurity generation location and quantities, and the impurity transport within Proto-MPEX.

The radio-frequency (RF) helicon antenna used as the ionization source on Proto-MPEX creates a rectified RF sheath on the aluminum nitride ceramic RF window that is the vacuum boundary for the helicon. Experiments on Proto-MPEX revealed Al and O impurities on target which were found to come from the helicon window. Experiments with capacitive probe measurements showed the start-up low density phase of the plasma pulse contributing to a high RF sheath potential leading to enhanced sputtering. Fluence scans also showed that more impurities made their way to the target from the beginning of the pulse but the higher density steady state time period of the pulse was not negligible. The impurities were found to be centrally peaked in the plasma core following classical radial convection. Electron cyclotron heating was found to reduce the impurities on the target due to temperature screening and introducing Kr gas to the plasma further reduced the impurities on the target by sputtering deposited material.

A dielectric layer within a COMSOL simulation was used to model the RF sheath and get the 3D mapping of the sheath potentials for the start-up and steady state time periods of the plasma pulse. The potentials were found to be high enough to give ions impacting the helicon window more energy than the displacement threshold needed for physical sputtering of the helicon window. To track the window material that becomes impurities and is transported through Proto-MPEX a Global Impurity TRansport (GITR) simulation was performed. The impurity deposition profiles on the helicon window and at the target were obtained and were found to have good agreement with experiments.