460 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
Control of the Radial Electric Field Shear by Modification of the Magnetic Field Configuration in LHD
Model for Incomplete Reconnection in Sawtooth Crashes
A model for incomplete reconnection in sawtooth crashes is presented. The
reconnection inflow during the crash phase of sawteeth self-consistently
convects the high pressure core toward the reconnection site, raising the
pressure gradient there. Reconnection shuts off if the diamagnetic drift speed
at the reconnection site exceeds a threshold, which may explain incomplete
reconnection. The relaxation of magnetic shear after reconnection stops may
explain the destabilization of ideal interchange instabilities reported
previously. Proof-of-principle two-fluid simulations confirm this basic
picture. Predictions of the model compare favorably to data from the Mega
Ampere Spherical Tokamak. Applications to transport modeling of sawteeth are
discussed. The results should apply across tokamaks, including ITER.Comment: 5 pages, 3 figures, accepted for publication in PR
Bootstrap current calculations with the SPBSC and the VENUS+δf codes for the Large Helical Device
Total bootstrap current calculations with the updated VENUS+δf code that incorporates energy convolution and the momentum correction technique have been performed for the reference tokamak JT-60U cases and for the experimental Large Helical Device (LHD, NIFS, Japan) configurations with different magnetic axis positions. The VENUS+δf results have been compared with the corresponding tokamak results of the neoclassical bootstrap current models for the general axisymmetric equilibria and arbitrary collisionality regime, as well as with the corresponding 3D SPBSC code numerical predictions and with the LHD experimental tendency
Effect of Neoclassical Transport Optimization on Electron Heat Transport in the Low-collisionality LHD Plasmas
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