Non-thermal (``Cool'') Fusion Burning Plasma Regimes

Radially localized (``captive'') ballooning modes [1], capable of sustaining the transfer of energy from fusion reaction products to the reacting nuclei, have been identified. These modes, which involve high power transfers with acceptable particle density fluctuation levels, can lead to so-called ``cool fusion'' scenarios with considerably lower temperatures of the fusing nuclei than those associated with simple Maxwellian distributions. For DT plasmas the appropriate frequency of these modes is close to the deuterium cyclotron frequency for $k_{\perp}d_{i}{\sim}1$ and $d_{i}=c/{\omega_{pi}}$. Thus, the needed electron temperatures have to be adequate (e.g., around the ideal ignition temperature or higher) in order to avoid significant electron damping by the relevant ballooning mode-particle resonances [1]. These findings, which are consistent with recent experimental observations [2], suggest that evidence for non-thermal fusion should continue to be looked for and a serious effort should be devoted to exploit the advantages of this kind of fusion burning processes.

[1] B. Coppi, A. Cardinali, and B. Basu, Paper TH/P 3-5, IAEA, FEC 2021, to be submitted Nucl. Fus. (2021).

[2] R. M. Magee, A. Necas, R. Clary et al., Nature 15, 281 (2019).