Fast Particle driven Alfven Quasimodes in Tokamaks

Upward sweeping Alfv\'en cascade eigenmodes have long been observed in tokamak reverse shear discharges. These upward sweeping modes have been explained as eigenmodes, localized around $q_{\mbox{min}}$ driven by fast ion inhomogeneities in the plasma~[Physics of Plasmas 10 3649]. However in flat $q$-profile discharges an accompanying downward sweeping mode, not provided for by the previous work, are occaisionally observed[Physics of Plasmas 12 112506]. We explain these modes as quasimodes supported by energetic particle drive against their weak radiative decay. The computation of the fast particle drive is simplified using an action/angle variable drift kinetic formalism to treat the trapped fast particle population.