Alpha particle physics studies in JET D-$^{\mathrm{3}}$He plasmas in preparation for D-T

We report results of studies in D-$^{\mathrm{3}}$He plasmas on JET ($n(^{\mathrm{3}}$He)/$n_{\mathrm{e}}\approx $20-25{\%}) using the 3-ion radio frequency (RF) scheme [1,~2] for the core acceleration of deuterons from neutral beam injection (NBI) to higher energies to generate alpha particles from D$+^{\mathrm{3}}$He fusion reactions. The fast-ion distribution of RF-accelerated D-NBI ions was controlled by varying the RF/NBI power ratio ($P_{\mathrm{RF}}\approx $4-6MW, $P_{\mathrm{NBI}}\approx $3-20MW). With as little as $P_{\mathrm{RF}}\approx $6~MW and $P_{\mathrm{NBI}}\approx $7-10~MW, rather high D-D neutron ($\approx $1x10$^{\mathrm{16}}$ 1/s) and D-$^{\mathrm{3}}$He alpha rates ($\approx $2\texttimes 10$^{\mathrm{16}}$ 1/s) were achieved. For the first time in JET-ILW, the upgraded gamma-ray diagnostic [3] provided spatial images of the alpha birth profile, validating this essential diagnostic for its use in future D-T experiments. Dominant fast-ion heating and a rich variety of fast-ion driven Alfv\'{e}n eigenmodes (AEs) were observed in these discharges, which allowed mimicking several aspects of future burning plasmas, where alphas will largely affect heating and global confinement. The sawtooth period in D-$^{\mathrm{3}}$He pulses on JET varied between $\approx $200-300~ms and 3.9~s, depending on the RF/NBI power ratio, $n_{\mathrm{e}}$, etc. During long period sawteeth (\textgreater $\approx $1s), reversed shear AEs were additionally detected by magnetic coils. This indicates that a non-monotonic $q$-profile was developed and sustained by large fast-ion populations. AE dynamics were different in pulses with short sawteeth periods. In these conditions, $n$~$=$~0 global AEs were regularly observed, hinting at the presence of fast ions with an inverted distribution. The possibility that some of the observed AEs were driven by fusion-born alphas is under investigation. Surprisingly, this complex AE activity is not detrimental for the thermal confinement, which is actually enhanced in the inner core, especially for the ions. [1] Kazakov, Y \textit{et al.} \textit{Nature Phys} \textbf{13, }973--978 (2017) [2] Ongena, J \textit{et al.} \textit{EPJ Web of Conferences} \textbf{157}, 02006 (2017) [3] Rigamonti, D \textit{et al.} \textit{Rev. Sci. Instrum.} \textbf{89}, 10I116 (2018)