One of the main results of the Wendelstein 7-AS stellarator (major radius2 m, average plasma radius 0.18 m, magnetic field 2.5 T, low shear) is theachievement of the H-mode confinement in ECR- and NBI-heated plasmas. Thereis a strong dependence on the external rotational transform; the H-modeconfinement can be found only in a narrow window close to ι = 1/2.Viscous damping and the interaction with a neutral background are the onlydamping mechanisms proposed so far, which inhibit the poloidal shear flow.Viscous damping is computed using collisional and weakly collisionlessapproximations (plateau regime). Large values of the poloidal viscosity arefound on rational magnetic surfaces, while in the neighbourhood oflow-order rational surfaces the viscosity is very small. Islands provide amechanism for enhanced momentum transport in the radial direction, whichleads to an effective shear viscosity. In Wendelstein 7-AS islands exist onthe `natural' rational surfaces with ι = 5/9,5/10,5/11,....Furthermore, a next generation of islands exist on ι = 10/19,10/21.Experimental results in Wendelstein 7-AS confirm the hypothesis that theH-mode only can arise if none of these islands exist in the plasma. Theregions of rotational transform predicted by this hypothesis roughly agreewith those of the experiment.The extrapolation towards Wendelstein 7-Xshows that a similar case is expected there. Numerical calculations ofislands are made in vacuum fields; the evolution of the islands with risingplasma pressure is unknown. Finally a qualitative model of H-modedevelopment is discussed.
Wendelstein 7-AS仿星器（大半径2 m，平均等离子体半径0.18 m，磁场2.5 T，低剪切）的主要结果之一是在ECR和NBI加热等离子体中实现了H模约束。对外部旋转变换存在强依赖性；H模约束仅能在接近ι = 1/2的狭窄窗口内出现。粘性阻尼和与中性背景的相互作用是迄今提出的抑制极向剪切流的唯一阻尼机制。粘性阻尼采用碰撞和弱无碰撞近似（坪区）计算。在有理磁面上发现大的极向粘性，而在低阶有理面附近粘性非常小。岛为径向方向的增强动量输运提供了一种机制，从而导致有效的剪切粘性。在Wendelstein 7-AS中，岛存在于ι = 5/9、5/10、5/11、...的“自然”有理面上。此外，下一代岛存在于ι = 10/19、10/21处。Wendelstein 7-AS的实验结果证实了以下假设：只有当这些岛在等离子体中不存在时，H模才能出现。该假设预测的旋转变换区域与实验大致相符。外推至Wendelstein 7-X表明，那里预计会出现类似情况。在真空场中进行了岛的数值计算；随着等离子体压力升高，岛的演化尚不清楚。最后，讨论了一个H模发展的定性模型。