Speaker
Description
The presence of the radio frequency (RF) sheath can cause enhanced sputtering of plasma facing materials. Compared to a DC sheath with the same average potential, an RF sheath causes a broadening of the ion energy distribution function (IEDF) and results in enhanced light ion sputtering, especially for average RF sheath potentials close to the material sputtering threshold. The effect of RF sheaths on the erosion of fusion research relevant materials is being studied on the Radio Frequency Plasma Interaction Experiment (RF PIE). The RF PIE consists of an electron cyclotron resonance plasma source (2.45 GHz, 5 kW) with a biased and heated RF electrode that is used to simulate antenna surfaces in contact with the edge plasma for RF biases up to 500 V. The erosion of the material surface is being studied spectroscopically as a function of ion energy using a mirror-linked UV imaging spectrometer for measuring plasma line emission in and near the sheath. Materials of interest include plasma-sprayed tantalum pentoxide on silicon nitride, which is relevant for understanding the erosion of the plasma-exposed window material in the helicon plasma source being developed for the Material Plasma Exposure eXperiment (MPEX). Other plasma facing antenna materials being explored include tungsten, carbide dispersoid-strengthened tungsten , and titanium diboride. Calculations of the IEDF using hPIC2 predict a high energy tail of the distribution that can be close to 2X higher than the average sheath energy, which results in finite sputtering when the average sheath energy is below the sputter threshold energy. Predictions of the expected sputtering yield for DC and RF sheath conditions using semi-empirical sputtering formulas for helium and deuterium sputtering of these materials are consistent with experimental observations of changes in the line emission intensity as a function of ion energy. The SDTrimSP code is being used to predict sputtering of compounds (e.g., tantalum pentoxide) by calculating partial sputter yields in steady-state conditions, and results compare well with experimental measurements.
*This work sponsored by US DOE under DE-AC05-00OR22725