  FOXTAIL is a new hybrid magnetohydrodynamic-kinetic code used to describe
interactions between energetic particles and Alfv\'en eigenmodes in tokamaks
with realistic geometries. The code simulates the nonlinear dynamics of the
amplitudes of individual eigenmodes and of a set of discrete markers in
five-dimensional phase space representing the energetic particle distribution.
Action-angle coordinates of the equilibrium system are used for efficient
tracing of energetic particles, and the particle acceleration by the wave
fields of the eigenmodes is Fourier decomposed in the same angles. The
eigenmodes are described using temporally constant eigenfunctions with dynamic
complex amplitudes. Possible applications of the code are presented, e.g.,
making a quantitative validity evaluation of the one-dimensional bump-on-tail
approximation of the system. Expected effects of the fulfillment of the
Chirikov criterion in two-mode scenarios have also been verified.
