  A novel channel for fuel ions heating in tokamak core plasma is proposed and
analyzed using nonlinear gyrokinetic theory. The channel is achieved via
spontaneous decay of reversed shear Alfv\'en eigenmode (RSAE) into low
frequency Alfv\'en modes (LFAM), which then heat fuel ions via collisionless
ion Landau damping. The conditions for RSAE spontaneous decay are investigated,
and the saturation level and the consequent fuel ion heating rate are also
derived. The channel is expected to be crucial for future reactors operating
under reversed shear configurations, where fusion alpha particles are generated
in the tokamak core where the magnetic shear is typically reversed, and there
is a dense RSAE spectrum due to the small alpha particle characteristic
dimensionless orbits.
