Speaker
Description
There is evidence that axisymmetric modes can be driven by alpha particles in tokamaks [H. J. C. Oliver, Phys. Rev. Lett. \textbf{136}, 055101 (2026)]. It is therefore of interest to investigate the main mechanism behind the drive of the modes. When the distribution function of the non-thermal ions, $f_h$, is given as a function of the invariants of the unperturbed motion energy $\mathcal{E}$, pitch angle variable $\Lambda=\mu B_0/\mathcal{E}$, ($\mu$ is the magnetic momentum) and the toroidal angular momentum $P_\phi$, i.e. $f_h(\mathcal{E},\Lambda,P_\phi)$, the drive of axisymmetric modes requires either a bump on tail in the energy direction (with $\Lambda$ and $P_\phi$ held constant) or an anisotropy characterised by a variation of $f_h$ with $\Lambda$. In the simplest model with only collisional slowing down of alpha particles, $f_h(\mathcal{E},\Lambda,P_\phi)$ always decreases monotonically with $\mathcal{E}$, i.e. there is no drive caused by $\partial f_h / \partial \mathcal{E} |_{\Lambda, P_\phi=const}$. Instead, it is shown that finite orbit width effects lead to an anisotropy with a peak of $\partial f_h/\partial\Lambda$ in the region of trapped orbits. The ability of this anisotropy to drive axisymmetric modes is analysed with a “toy model” where only the slowing down of alpha particles on background electrons is taken into account. The variation of the drive with plasma current and plasma size is investigated.