Assessment of Transport in NCSX

Abstract

We explore whether the energy confinement and planned heating in NCSX are sufficient to test MHD stability limits, and whether the configuration is sufficiently quasi-axisymmetric to reduce the neoclassical ripple transport to low levels, thereby allowing tokamak-like transport. A 0-D model with fixed profile shapes is related to global energy confinement scalings for stellarators and tokamaks; neoclassical transport properties are assessed with the DKES, NEO, and NCLASS codes; and a power balance code is used to predict temperature profiles. Reaching the NCSX goal of <beta>=4% at low collisionality will require H_ISS-95=3, which is higher than the best achieved in present stellarators. However, this level of confinement is actuallỹ 10% lower than that predicted by the ITER-97P tokamak L-mode scaling. By operating near the stellarator density limit, the required H_ISS-95 is reduced by 35%. The high degree of quasiaxisymmetry of the configuration and the self-consistent 'ambipolar' electric field reduce the neoclassical ripple transport to a small fraction of the neoclassical axisymmetric transport. A combination of neoclassical and anomalous transport models produces pressure profile shapes that are within the range of those used to study the MHD stability of NCSX. We find that <beta>=4% plasmas are 'neoclassically accessible', and are compatible with large levels of anomalous transport in the plasma periphery. 7-2

Neoclassical Viscosities in NCSX and QPS with Few Toroidal Periods and Low Aspect Ratios

“…Even for N * in the 1/ν regime, the numerical examples for more general cases had not been shown. For investigating these effects in configurations including B mn of n 0, 1 and with extremely low toroidal period numbers (required for low aspect ratios) resulting in larger ι/N, we show recent calculation examples in the National Compact Stellarator Experiment (NCSX) [10,13] and the Quasi-Poloidal Stellarator (QPS) [9,10,14] in this paper.…”

“…Faster and easier estimation methods for the neoclassical quantities are desirable in integrated simulation systems using iterative calculations of the equilibrium and transport [12], configuration optimizations [9], and experimental studies investigating dependences on configurations. Since the viscosities (or resulting neoclassical transport coefficients) are direct consequences of guiding center drift motions, evaluating them is important in understanding the characteristics of the designed magnetic configurations [9,10,13,14]. Even in tokamak experiments, neoclassical toroidal viscosity effects due to breaking the axisymmetry have recently been c 2008 The Japan Society of Plasma Science and Nuclear Fusion Research S1059-1 studied [15], and thus the framework of the moment approach for non-symmetric configurations and the methods for calculating the viscosity coefficients are now required for all studies of toroidal plasma confinements.…”

Effect of Ambipolar Plasma Flow on the Penetration of Resonant Magnetic Perturbations in a Quasi-axisymmetric Stellarator

“…The transport model is described in more detail in Ref. [7]. The calculated electron and ion temperature profiles for our reference NCSX equilibrium are plotted in Fig.…”

“…It also modifies the unperturbed rotation velocity of the plasma in the absence of a resonant perturbation. Our modeling of these effects employs a 1D transport code [7], as well as the DEGAS code for estimating the momentum transfer rate to neutrals, and the PIES code [8] for calculating the magnitude of the resonant perturbation.…”

Ion temperature gradient turbulence modification in quasi-axisymmetry

“…The numerical optimization of stellarator configurations for improved transport properties [1] has yielded devices with tokamak levels of neoclassical transport [2][3][4] and suggests that devices with reduced turbulent transport may be developed as well [5]. Stellarators have been shown to benefit from three-dimensional effects on a flux surface, causing a reduction of turbulence levels [6].…”