The recently developed free-plasma-boundary version of the SIESTA MHD equilibrium code (Hirshman et al 2011 Phys. Plasmas18 062504; Peraza-Rodriguez et al 2017 Phys. Plasmas24 082516) is used for the first time to study scenarios with considerable bootstrap currents for the Wendelstein 7-X (W7-X) stellarator. Bootstrap currents in the range of tens of kAs can lead to the formation of unwanted magnetic island chains or stochastic regions within the plasma and alter the boundary rotational transform due to the small shear in W7-X. The latter issue is of relevance since the island divertor operation of W7-X relies on a proper positioning of magnetic island chains at the plasma edge to control the particle and energy exhaust towards the divertor plates. Two scenarios are examined with the new free-plasma-boundary capabilities of SIESTA: a freely evolving bootstrap current one that illustrates the difficulties arising from the dislocation of the boundary islands, and a second one in which off-axis electron cyclotron current drive (ECCD) is applied to compensate the effects of the bootstrap current and keep the island divertor configuration intact. SIESTA finds that off-axis ECCD is indeed able to keep the location and phase of the edge magnetic island chain unchanged, but it may also lead to an undesired stochastization of parts of the confined plasma if the EC deposition radial profile becomes too narrow.
最近开发的SIESTA MHD平衡代码的自由等离子体边界版本（Hirshman等人，2011年《物理等离子体》18, 062504；Peraza-Rodriguez等人，2017年《物理等离子体》24, 082516）首次被用于研究Wendelstein 7-X（W7-X）仿星器中具有显著自举电流的情景。由于W7-X中的剪切较小，数十千安培量级的自举电流可能导致等离子体内部出现不期望的磁岛链或随机化区域，并改变边界旋转变换。后者的问题尤为重要，因为W7-X的岛偏滤器运行依赖于等离子体边缘磁岛链的正确定位，以控制流向偏滤器板的粒子和能量排出。利用SIESTA的新自由等离子体边界能力，研究了两种情景：一种是自由演化的自举电流情景，展示了边界岛位移所带来的困难；另一种是施加离轴电子回旋电流驱动（ECCD）以补偿自举电流效应并保持岛偏滤器位形完整的情景。SIESTA发现，离轴ECCD确实能够保持边缘磁岛链的位置和相位不变，但如果EC沉积径向分布过窄，则可能导致部分约束等离子体出现不期望的随机化。