891 research outputs found
On neoclassical impurity transport in stellarator geometry
The impurity dynamics in stellarators has become an issue of moderate concern
due to the inherent tendency of the impurities to accumulate in the core when
the neoclassical ambipolar radial electric field points radially inwards (ion
root regime). This accumulation can lead to collapse of the plasma due to
radiative losses, and thus limit high performance plasma discharges in
non-axisymmetric devices.\\ A quantitative description of the neoclassical
impurity transport is complicated by the breakdown of the assumption of small
drift and trapping due to the electrostatic
potential variation on a flux surface compared to those due to
the magnetic field gradient. The present work examines the impact of this
potential variation on neoclassical impurity transport in the Large Helical
Device (LHD) stellarator. It shows that the neoclassical impurity transport can
be strongly affected by . The central numerical tool used is the
particle in cell (PIC) Monte Carlo code EUTERPE. The
used in the calculations is provided by the neoclassical code GSRAKE. The
possibility of obtaining a more general self-consistently with
EUTERPE is also addressed and a preliminary calculation is presented.Comment: 11 pages, 15 figures, presented at Joint Varenna-Lausanne
International Workshop on Theory of Fusion Plasmas, 2012. Accepted for
publication to Plasma Phys. and Control. Fusio
Comparison of particle trajectories and collision operators for collisional transport in nonaxisymmetric plasmas
In this work, we examine the validity of several common simplifying
assumptions used in numerical neoclassical calculations for nonaxisymmetric
plasmas, both by using a new continuum drift-kinetic code and by considering
analytic properties of the kinetic equation. First, neoclassical phenomena are
computed for the LHD and W7-X stellarators using several versions of the
drift-kinetic equation, including the commonly used incompressible-ExB-drift
approximation and two other variants, corresponding to different effective
particle trajectories. It is found that for electric fields below roughly one
third of the resonant value, the different formulations give nearly identical
results, demonstrating the incompressible ExB-drift approximation is quite
accurate in this regime. However, near the electric field resonance, the models
yield substantially different results. We also compare results for various
collision operators, including the full linearized Fokker-Planck operator. At
low collisionality, the radial transport driven by radial gradients is nearly
identical for the different operators, while in other cases it is found to be
important that collisions conserve momentum
Properties of a new quasi-axisymmetric configuration
A novel, compact, quasi-axisymmetric configuration is presented which
exhibits low fast-particle losses and is stable to ideal MHD instabilities. The
design has fast-particle loss rates below 8\% for flux surfaces within the
half-radius, and is shown to have an MHD-stability limit of a normalised
pressure of where is volume
averaged. The flux surfaces at various plasma betas and currents as calculated
using the SPEC equilibrium code are presented. Neoclassical transport
coefficients are shown to be similar to an equivalent tokamak, with a distinct
banana regime at half-radius. An initial coil design study is presented to
assess the feasibility of this configuration as a fusion-relevant experiment
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