Topic/Type: 1. Plasma Simulation, Invited

Challenges in self-consistent full wave simulations of lower hybrid waves

J. C. Wright

MIT-PSFC

Analysis of wave propagation in the lower hybrid range of frequencies
(LHRF) in the past has been done using ray tracing and the
Wentzel-Kramers-Brillouin approximation taking advantage of the very
small scale of those waves. ?To include the effects of wave
diffraction and focusing in this regime full wave simulation is
necessary, but requires significantly more computational power. In
both ray tracing and full wave simulations in the LHRF it is also
essential to include the self-consistent evolution of the electron
distribution in response to the waves. This adds a considerable
computational burden in constructing the stiffness matrix for the
system [Valeo et al., 'Full-wave Simulations of LH wave propagation in
toroidal plasma with non-Maxwellian electron distributions', 18th
Topical Conference on Radio Frequency Power in Plasmas, AIP Conference
Proceedings (2007)].

Advances in algorithms and the availability of massively parallel
computer architectures have permitted the solving of the
Maxwell--Vlasov system for wave propagation directly [Wright et al.,
Phys. Plasmas (2009), 16, July]. We will discuss the various modeling
advances that have led to the capability including various memory
management approaches, physics motivated algorithm adaptions
appropriate to the LHRF, and improvements in the matrix solver to
minimize communication overhead when using 1000s of cores on
leadership class computer platforms. Of particular importance have
been the reformulation of the wave equations to avoid numerical
pollution from the strongly evanescent ion plasma mode, analytic
approximations to the real (principal value) contribution to the
plasma dielectric response, and ensuring positive definite behavior of
the plasma response using the FLR approximation for electrons in the
LHRF.