High-Energy Ion Generation During Difference-Frequency ICRF Wave Heating in GAMMA 10/PDX

One of the greatest challenges of simulating the high heat flux on the divertor in linear plasma devices is the achievement of ion temperatures of > 100 eV under high densities (≥1019 m−3). The slow wave in the ion cyclotron range of frequency has been employed for ion heating in GAMMA 10/PDX and demonstrated its high efficiency at middle densities of 1018 m−3. However, at high densities (≥1019 m−3), exciting the slow wave in the center of the plasma by external antennas becomes challenging. Alternatively, we have demonstrated the excitation of the difference-frequency (DF) slow wave using two high-density-applicable fast waves. In this study, we first applied the charge-exchange neutral particle analyzer (CX-NPA) to a DF wave heating experiment. Even when the bulk temperature did not increase significantly, the high-energy ions in the > 0.5 keV range were measured during the experiment. Such high-energy ions were only measured in the central line of sight of CX-NPA, indicating that the power deposition from the DF wave was largely centered and differed greatly from the normal heating scheme using external antennas. This result will motivate further pursue of exploiting nonlinearity of plasma media to produce different heating accessibilities.