Radio Frequency Driven Plasma Material Interactions of Reactor Relevant Materials

The interaction of radio frequency (RF) sheaths with fusion reactor relevant materials (e.g., tungsten) 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. Helium and/or deuterium plasmas (density of ~1e18/m³, electron temperature of 4-5 eV) are being used to explore sheath formation on tungsten surfaces at temperatures up to 850 C and biases up to 500 V. An increase in the electron temperature on magnetic field lines connected to the electrode has been observed and depends on the grounding of the RF-driven electrode. The changes in helium line ratios measured with a filterscope are being compared to Langmuir probe measurements and modeling predictions to characterize this sheath interaction. The erosion of the tungsten surface as a function of bias conditions is also being studied spectroscopically. Tungsten line emission intensity is different for DC versus RF bias for similar plasma conditions and average ion energy, which is likely due to the calculated broadening of the ion energy distribution due to RF. The effect of this broadening on the expected sputtering yield is being determined. Experimental details, as well as tungsten nano-fuzz growth under certain bias conditions, will be presented.