Abstract
Linear plasma devices are cost-effective alternatives for testing materials under reactor-relevant divertor plasma conditions. An intense radio-frequency (RF) plasma source concept for the Material Plasma Exposure eXperiment (MPEX) is under development at Oak Ridge National Laboratory. The source concept, Proto-MPEX, aims to produce high-density background deuterium helicon plasmas (ne>3×1019 m−3) that are subsequently heated with additional RF and microwave systems to deliver reactor-relevant conditions for studies on plasma-material interaction. In this work, we focus on the plasma-producing stage and its effectiveness in converting input neutral gas into plasma, namely, the neutral gas ionization efficiency. We provide a direct quantitative measurement of the ionization efficiency by measuring the total ion flux arriving at the target region relative to the neutral gas injected at the source. Using 80 kW at 13.56 MHz and a source magnetic field of 0.05 T, the helicon plasma source delivers ion fluxes up to 1×1024 m−2 s−1 and heat fluxes greater than 1 MW m−2 to a target plate located 2 m away from the source. Under these conditions, we observe that the plasma source converts ~89% of the input neutral gas into plasma that arrives at the target as ion flux at a rate of 5×1020 s−1. We demonstrate that because of the large pumping capacity of the plasma, neutral gas pumping systems are required only in the target region to maintain optimal plasma operation.
