Significance of MHD Effects in Stellarator Confinement

Abstract

Substantial progress has been achieved to raise the plasma beta in stellarators and helical systems by high power neutral beam heating, approaching reactor relevant values [1][2][3]. The achievement of high-β operation is closely linked with configuration effects on the confinement and with magnetohydrodynamic (MHD) stability. However, the observed pressure gradients indicate some mitigation of the effects on the plasma confinement, presumably because of the high collisionality of high-β plasmas and island healing effects ). As far as operational limits by pressure driven MHD instabilities are concerned, only weak confinement degradation effects are usually observed, even in linearly unstable regimes.The impact of the results concerning high-β operation in W7-AS and LHD on the future stellarator programme will be discussed, including relations to tokamak research. Some of the future key issues appear to be: -the control of the magnetic configuration (including toroidal current control), -the modification of confinement and MHD properties towards the low collisional regime, -and the compatibility of high-β regimes with power and particle exhaust requirements to achieve steady state operation.

Non-Axisymmetric Shaping of Tokamaks Preserving Quasi-Axisymmetry

“…Configurations were studied, Figure (6), that covered a wide range of aspect ratios, from 3 to 8, for a given rotational transform and β = 4%. These configurations possess good quasi-axisymmetry and are also MHD stable to the external kinks.…”

“…Figure (7) shows the shape of flux surfaces for A = 8 at toroidal angles corresponding to the beginning of a field period and at the half-period. For A = 8 and β = 4%, the Tryon (6). Coil winding surfaces have been constructed such that the inboard midplane is displaced by 0.6a whereas the outboard miplane is displaced by 1.2a, where a is the plasma minor radius, with interpolation made for locations in between.…”

“…The experiments have found stable high volume-averaged beta, β ≡ 2µ 0 p/B 2 plasmas that last hundreds of energy confinement times [6], high density (≥ 10 21 /m 3 ) regimes exceeding the Greenwald density limit of tokamaks [7], and a scaling of confinement comparable to that of H-mode tokamaks [8]. These achievements coupled with the observed robust stability [1] without disruptive plasma terminations show that threedimensional shaping of fusion plasmas addresses important issues that must be addressed before fusion power systems can be a reality.…”

Plasma Equilibrium in a Magnetic Field with Stochastic Regions

“…4,[7][8][9] For diverted tokamaks, there has been some success in suppressing edge localized modes (ELMs) by the imposition of nonaxisymmetric fields near the plasma edge, 10 and the possible role of the stochastic layer produced near the diverter separatrix is a subject of current research. Additionally, it has recently been shown that the observed electron thermal conductivity in one type of discharge in the National Spherical Torus Experiment is in quantitative agreement with a theory of transport due to magnetic-field-line stochasticity produced by magnetic perturbations from microtearing instabilities.…”

Pressure-induced breaking of equilibrium flux surfaces in the W7AS stellarator

“…[5,6] Only in recent years have stellarators achieved values of β sufficiently high to study the issue experimentally. Both the W7AS and LHD stellarators exhibit degraded confinement at high values of β that does not appear to be caused by instabilities, [7,8] There is no diagnostic in these experiments that directly indicates whether surfaces are broken. In this paper we compare experimental observations on the W7AS stellarator with PIES code calculations to investigate this issue.…”