Speaker
Dr
Jeff Candy
(General Atomics)
Description
Sonic toroidal plasma flow, on the order of the ion sound speed, arises in
tokamaks due to external torque driven by neutral beam injection. This flow
has a profound effect on drift-wave turbulence and corresponding radial
transport fluxes. Historically, gyrokinetic theory and simulation
operate (almost) exclusively in the weak rotation limit, retaining
only the E$\times$B flow, Coriolis drift and toroidal rotation shear.
However, correct treatment of the sonic rotation regime requires the
inclusion of centrifugal effects, which are quadratic in the Mach number.
In 1998, Sugama formulated a comprehensive and rigorous gyrokinetic
system that describes sonic rotation and associated centrifugal terms,
and is valid for general electromagnetic perturbations [1]. This
formulation, importantly, includes the corresponding particle, energy, momentum
and exchange transport coefficients which are required to obtain the correct
equations for profile evolution. We show that the most general implementation
is critically important for the study of heavy impurity transport. In particular,
using the more accurate theory, nonlinear turbulent fluxes for tungsten are
radically different than in the weak-rotation regime.
In this presentation we give particular emphasis to a discussion of a
new approach for the implementation of shear in the E$\times$B flow. This shear is
different than the previous rotation terms in that it cannot be treated simply
or directly in a flux-tube. In the past, E$\times$B shear has been treated using
either *non-periodic boundary conditions*, or in the case of flux-tube
codes, using a discontinuous *wavenumber shift* method [2]. We report
on the development of a new discrete *wavenumber advection* algorithm
that treats the shear with spectral accuracy without spurious boundary
effects or a discontinuous time-history. Because the new algorithm may also be
used to treat profile shear, it is well-suited to treat multiscale gyrokinetic
simulations in the steep-gradient pedestal region.
Primary author
Dr
Jeff Candy
(General Atomics)
Co-author
Dr
Emily Belli
(General Atomics)
