  The motion of an ion in a coherent lower hybrid wave (characterized by
|k_parallel| << |k_perp| and omega >> Omega_i) in a tokamak plasma is studied.
For ions satisfying v_perp > omega/k_perp, the Lorentz force law for the ions
is reduced to a set of difference equations which give the Larmor radius and
phase of an ion on one cyclotron orbit in terms of these quantities a cyclotron
period earlier. From these difference equations an earlier result [Phys. Fluids
21, 1584 (1978)] that above a certain wave amplitude the ion motion is
stochastic, is readily obtained. The stochasticity threshold is given a simple
physical interpretation. In addition, the difference equations are used to
derive a diffusion equation governing the heating of the ions above the
stochasticity threshold. By including the effects of collisions, the heating
rate for the bulk ions is obtained.
