  Spherical tokamak plasmas are typically overdense and thus inaccessible to
externally-injected microwaves in the electron cyclotron range. The
electrostatic electron Bernstein wave (EBW), however, provides a method to
access the plasma core for heating and diagnostic purposes. Understanding the
details of the coupling process to electromagnetic waves is thus important both
for the interpretation of microwave diagnostic data and for assessing the
feasibility of EBW heating and current drive. While the coupling is reasonably
well-understood in the linear regime, nonlinear physics arising from high input
power has not been previously quantified. To tackle this problem, we have
performed one- and two-dimensional fully kinetic particle-in-cell simulations
of the two possible coupling mechanisms, namely X-B and O-X-B mode conversion.
We find that the ion dynamics has a profound effect on the field structure in
the nonlinear regime, as high amplitude short-scale oscillations of the
longitudinal electric field are excited in the region below the high-density
cut-off prior to the arrival of the EBW. We identify this effect as the
instability of the X wave with respect to resonant scattering into an EBW and a
lower-hybrid wave. We calculate the instability rate analytically and find this
basic theory to be in reasonable agreement with our simulation results.
