Revisiting ballooning modes in levitated dipole geometries

Inspired by the relatively recent revival of the levitated dipole concept as a commercial fusion reactor, we report the results of an investigation into the relationship between interchange and ballooning stability in levitated dipole fusion plasmas. One major previous investigation into the subject came to the conclusion that dipole plasmas are rendered ballooning stable by satisfying interchange stability, but relied on a potentially incomplete stability methodology. We seek to reexamine the topic from a more complete approach. This study uses a technique based on the Rayleigh quotient and the ideal MHD energy principle, applied to equilibria computed with our custom Grad–Shafranov solver for static levitated-dipole plasmas. Contrary to previous results, we find that stability to interchange modes does not necessarily imply stability to ballooning modes. In the outer flux surfaces within the bad curvature regions where ballooning modes can develop, it is possible for surfaces to be interchange stable while ballooning unstable. We suggest that this occurs due to the plasma exploiting strong curvature variation along the purely poloidal field lines, overcoming the stabilizing influence of field-line bending.