Plasma heating and generation of energetic ions with novel three-ion ICRF scenarios on Alcator C-Mod and JET tokamak facilities

This talk will report the first experimental results of novel three-ion ICRF scenarios (two or more majority ion species and one minority) for plasma heating and generating energetic ions in fusion facilities [1]. The key feature of these scenarios is strong absorption of RF power possible at lower concentrations of minority ions than in two-ion plasmas. Effective plasma heating by injecting a small amount of $^{3}{\rm He}$ ions into H-D plasma mixtures with $n_{\rm H}/n_{e} \sim 70\%$ has been successfully demonstrated in Alcator C-Mod and JET tokamaks. In C-Mod, efficient plasma heating was observed for $^{3}{\rm He}$ concentrations from 0.4-2\%. During the discharges, a strong increase in Alfv\'{e}n eigenmode activity was found to coincide with the addition of $^{3}{\rm He}$ to the H-D plasmas [2]. Even lower $^{3}{\rm He}$ concentrations $(\sim 0.2\%)$ were utilized in recent JET experiments. The potential of the D-$(^{3}{\rm He})$-H scenario for plasma heating and generating MeV-range ions in JET plasmas was confirmed by a set of independent measurements, including stabilization of sawteeth, characteristic $\gamma$-ray emission, fast-ion loss detector. Furthermore, toroidal Alfv\'{e}n eigenmodes with a range of toroidal mode numbers $n$ were detected, which is another indication for the presence of significant population of high-energy $^{3}{\rm He}$ ions in a plasma. The discussed mechanism of resonant wave-particle interaction opens up various unexplored opportunities for ICRF system, including new scenarios for plasma heating. Three-ion ICRF scenarios are also relevant for the experimental programme of ITER. The possibility of using intrinsic $^{9}{\rm Be}$ impurities as the minority (instead of $^{3}{\rm He}$) was suggested for heating bulk ions in D-T plasmas of JET and ITER [3], as well as heating trace amounts of $^{3}{\rm He}$ and $^{4}{\rm He}$ ions in H majority plasmas of ITER. The latest results and simulation comparisons will be presented. \newline [1] Y. Kazakov et al., \textit{Nucl. Fusion} \textbf{55}, 032001 (2015) \newline [2] J. Wright et al. and Y. Lin et al., \textit{this conference} \newline [3] Y. Kazakov et al., \textit{Phys. Plasmas} \textbf{22}, 082511 (2015)