64 research outputs found
The monoenergetic approximation in stellarator neoclassical calculations
In the standard "monoenergetic" approach to numerical calculation of
stellarator neoclassical transport, to expedite computation, ad-hoc changes are
made to the kinetic equation so speed enters only as a parameter. Here we
examine the validity of this approach by considering the effective particle
trajectories in a model magnetic field. We find monoenergetic codes
systematically under-predict the true trapped particle fraction, with the error
in the trapped ion fraction being of order unity when the electric field is
large, suggesting some results of these codes may be unreliable in this regime.
This inaccuracy is independent of any errors introduced by approximation of the
collision operator.Comment: 8 pages, 2 figures, submitted to Plasma Phys. Controlled Fusio
Local and global Fokker-Planck neoclassical calculations showing flow and bootstrap current modification in a pedestal
In transport barriers, particularly H-mode edge pedestals, radial scale
lengths can become comparable to the ion orbit width, causing neoclassical
physics to become radially nonlocal. In this work, the resulting changes to
neoclassical flow and current are examined both analytically and numerically.
Steep density gradients are considered, with scale lengths comparable to the
poloidal ion gyroradius, together with strong radial electric fields sufficient
to electrostatically confine the ions. Attention is restricted to relatively
weak ion temperature gradients (but permitting arbitrary electron temperature
gradients), since in this limit a delta-f (small departures from a Maxwellian
distribution) rather than full-f approach is justified. This assumption is in
fact consistent with measured inter-ELM H-Mode edge pedestal density and ion
temperature profiles in many present experiments, and is expected to be
increasingly valid in future lower collisionality experiments. In the numerical
analysis, the distribution function and Rosenbluth potentials are solved for
simultaneously, allowing use of the exact field term in the linearized
Fokker-Planck collision operator. In the pedestal, the parallel and poloidal
flows are found to deviate strongly from the best available conventional
neoclassical prediction, with large poloidal variation of a different form than
in the local theory. These predicted effects may be observable experimentally.
In the local limit, the Sauter bootstrap current formulae appear accurate at
low collisionality, but they can overestimate the bootstrap current near the
plateau regime. In the pedestal ordering, ion contributions to the bootstrap
and Pfirsch-Schluter currents are also modified
Calculation of the bootstrap current profile for the TJ-II stellarator
Calculations of the bootstrap current for the TJ-II stellarator are
presented. DKES and NEO-MC codes are employed; the latter has allowed, for the
first time, the precise computation of the bootstrap transport coefficient in
the long mean free path regime of this device. The low error bars allow a
precise convolution of the monoenergetic coefficients, which is confirmed by
error analysis. The radial profile of the bootstrap current is presented for
the first time for the 100_44_64 configuration of TJ-II for three different
collisionality regimes. The bootstrap coefficient is then compared to that of
other configurations of TJ-II regularly operated. The results show qualitative
agreement with toroidal current measurements; precise comparison with real
discharges is ongoing
Study of the neoclassical radial electric field of the TJ-II flexible heliac
Calculations of the monoenergetic radial diffusion coefficients are presented
for several configurations of the TJ-II stellarator usually explored in
operation. The neoclassical radial fluxes and the ambipolar electric field for
the standard configuration are then studied for three different collisionality
regimes, obtaining precise results in all cases
Transverse Nernst-Ettingshausen effect, resonant scattering, and superconductivity in SnTe: In
Improvement of collisionless particle confinement in a non-quasi-symmetric stellarator vacuum magnetic field
Study of neoclassical transport and bootstrap current for W7-X in the 1/ν regime, using results from the PIES code
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