Speaker
Description
The interaction between fusion-born alpha particles and resistive MHD modes is a key stability concern for burning plasmas such as ITER. The neoclassical tearing mode — one of the most performance-limiting instabilities in these regimes — is itself driven by the helical perturbation of the bootstrap current associated with pressure flattening inside magnetic islands, which makes the perturbed bootstrap current the central quantity governing its stability. An additional perturbed bootstrap current carried by alpha particles can therefore directly affect tearing-mode and neoclassical-tearing-mode (NTM) stability. In our earlier work, the influence of the perturbed alpha-particle pressure was treated through its modification of the Mercier-index term; the perturbed bootstrap current carried by alpha particles constitutes a distinct and so-far untreated channel, and is the subject of the present study.
We carry out a drift-kinetic derivation, within the framework of Graves (2013), of the non-flux-function perturbed parallel current produced by an isotropic alpha-particle population, treating trapped and passing particles on an equal footing. Starting from the microscopic perturbed distribution function $\delta g$, the two classes are found to behave qualitatively differently. For trapped particles, the leading-order contribution cancels upon summation over the sign of the parallel velocity, and the surviving contribution originates from the first-order finite-orbit-width correction; it carries the structure of a perturbed bootstrap current. For passing particles, the resonance denominator depends on the parallel-velocity sign, so a non-zero contribution already appears at leading order. The two channels enter the tearing-mode outer-region equation with different singularity structures near the rational surface.
These results provide the analytic source terms for an ongoing quantitative evaluation of the alpha-particle contribution to $\Delta'$. The same drift-kinetic framework applies equally to NBI- and ICRF-driven energetic ions, and the results bear directly on the assessment of energetic-particle effects on neoclassical-tearing-mode onset and control in future burning-plasma devices.