Fusion Science and Technology / Volume 50 / Number 2 / August 2006 / Pages 158-170
Technical Paper / Stellarators
Substantial progress has been achieved in raising the plasma beta in stellarators and helical systems by high-power neutral beam heating, approaching reactor-relevant values. The achievement of high-beta operation is closely linked with configuration effects on the confinement and with magnetohydrodynamic (MHD) stability.
The magnetic configurations of the Wendelstein 7-AS (W7-AS) stellarator and of the Large Helical Device (LHD) and their optimization for high-beta operation within the flexibility of the devices are characterized. A comparative description of the accessible operational regimes in W7-AS and LHD is given. The finite-beta effects on the flux surfaces depend on the degree of configuration optimization. In particular, a large Shafranov shift is accompanied by formation of islands and stochastic field regions as found by numerical equilibrium studies. However, the observed pressure gradients indicate some mitigation of the effects on the plasma confinement, presumably because of the high collisionality of high-beta plasmas and island healing effects (LHD). 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-beta operation in W7-AS and LHD on the future stellarator program will be discussed, including the relationship to tokamak research. Some of the future key issues appear to be the following: the control of the magnetic configuration (including toroidal current control), the modification of confinement and MHD properties toward the low-collisional regime, and the compatibility of high-beta regimes with power and particle exhaust requirements to achieve steady-state operation.