Observation of the “Self-Healing” of an Error Field Island in the Large Helical Device

Abstract

It was observed that the vacuum magnetic island produced by an external error magnetic field in the large helical device shrank in the presence of plasma. This was evidenced by the disappearance of flat regions in the electron temperature profile obtained by Thomson scattering. This island behavior depended on the magnetic configuration in which the plasmas were produced. DOI: 10.1103/PhysRevLett.87.135002 PACS numbers: 52.55.HcExistence of a set of nested magnetic surfaces (magnetic configuration) is a prerequisite for almost all toroidal magnetic confinement devices [1]. In the tokamak, a magnetic configuration is generated by plasma current plus external coil currents, and hence the concept of a vacuum magnetic configuration does not exist. On the other hand, the stellarator, by its definition, possesses a vacuum magnetic configuration that is able to confine plasma just as Nature possesses a vacuum space-time that allows the existence of matter. This fact makes it easier in the stellarator than in the tokamak to study the interactions between the magnetic configuration and plasma such as the formation of topological defects and their reactions on the plasma behavior. The vacuum magnetic configuration of a stellarator is inevitably accompanied by topological defects such as magnetic islands and stochastic regions as a result of a weak violation of a helical symmetry. If these defects are large in size or numerous, they will deteriorate the plasma confinement considerably. This was a central issue in stellarator development. However, nowadays, we can make these topological defects small enough not to degrade the plasma confinement over a wide region by optimizing the coil-winding law [2]. Although the vacuum magnetic configuration problem was thus solved, the question whether a fairly good magnetic configuration is preserved in the presence of plasma has not yet been settled. Cary and Kotschenreuther [3] showed that sharply peaked currents near the island enhance or limit the island size, depending on whether the average curvature is bad or good. They also showed that in the hill region the islands overlap for arbitrary small beta b (kinetic pressure/magnetic pressure). Hegna and Bhattacharjee [4] generalized the above statement for higher b plasma with the result that the well/hill criterion should be replaced by the resistive interchange stability criterion. Hayashi et al. [5] computationally found that the vacuum magnetic surfaces of an L 2͞M 10 heliotron/torsatron configuration are, indeed, broken at high b, giving a more stringent limit on b than that imposed by equilibrium. Hayashi et al. [6] also discovered that as b increases the islands present in the vacuum heliac configuration diminish ("self-heal") and then reappear with the flipped phase at a higher b. Bhattacharjee et al. [7] formulated a theory to explain the above "self-healing" phenomenon. Hegna [8] further generalized the above theories to incorporate the effect of a bootstrap current. Since these theoretical and numerical predictions hav...

Assessment of Transport in NCSX

“…Energy confinement in LHD has ranged beyond twice the ISS-95 expectation [8,9] even at high <b> [7,10], and has no apparent dependence on heating method [11]. The enhanced confinement is attributed to an edge temperature pedestal [12,13,14], which is not generally observed in other stellarators but is not attributed to an H-mode in LHD. The pedestal may be associated with a chain of magnetic islands at or near the plasma edge [12], while avoidance of island chains at low order rational i values is associated with H-modes in W7-AS [15,16].…”

Transport Modeling of Edge Plasma in an m/n=1/1 Magnetic Island

“…Recent Large Helical Device experiments show that the transport modeling based only on the fluid description is not sufficient for expressing transport phenomena in/around magnetic islands formed at the edge. For example, it is observed that an m/n = 1/1 magnetic island, which is formed at the edge by using external coils, is healed [1][2][3][4], where m and n are the poloidal and toroidal mode numbers respectively, and in the experiments the island temperature is 500 eV and the island density ∼ 10 19 m −3 . The island width depends on plasma pressure in the edge region, i.e., the width reduces as the pressure increases.…”

The effect of anisotropic heat transport on magnetic islands in 3-D configurations

“…[9][10][11] Additionally, analytical island theories have been proven useful to interpreting experimental data of magnetic island dynamics in stellarators. [12][13][14] In the majority of these studies, however, an assumption of equilibration along magnetic field lines B Á $p ¼ 0 is employed. In the current work, the analytic theory of pressure-induced magnetic islands in 3-D equilibria is revisited, where the rigid requirement of parallel equilibration along field lines is weakened by allowing for the presence of finite, but large, parallel heat conduction.…”