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 wellunderstood 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 shortscale 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.
In this work, we propose a positivity-preserving scheme for solving two-dimensional advection-diffusion equations including mixed derivative terms, in order to improve the accuracy of lower-order methods. The solution to these equations, in the absence of mixed derivatives, has been studied in detail, while positivity-preserving solutions to mixed derivative terms have received much less attention. A two-dimensional diffusion equation, for which the analytical solution is known, is solved numerically to show the applicability of the scheme. It is further applied to the Fokker-Planck collision operator in two-dimensional cylindrical coordinates under the assumption of local thermal equilibrium. For a thermal equilibration problem, it is shown that the scheme conserves particle number and energy, while the preservation of positivity is ensured and the steady-state solution is the Maxwellian distribution.
We have performed fully-kinetic simulations of X-B and O-X-B mode conversion
in one and two dimensional setups using the PIC code EPOCH. We have recovered
the linear dispersion relation for electron Bernstein waves by employing
relatively low amplitude incoming waves. The setups presented here can be used
to study non-linear regimes of X-B and O-X-B mode conversion.Comment: 4 pages, 3 figure
We investigate the cluster emission of Ne isotopes from the nuclei
and
. Experiments are unable to distinguish between the isotopes
and
, but by establishing a method of determining the cluster, with associated preformation probability, and using a binary cluster formalism, we deduce that the most likely emitted cluster is
.
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