Detachment stabilization with n/m=1/1 resonant magnetic perturbation field applied to the stochastic magnetic boundary of the Large Helical Device

Abstract

It is found that the remnant island structure created by n/m=1/1 resonant magnetic perturbation field in the stochastic magnetic boundary of the Large Helical Device (LHD) [A. Komori et al., Nucl. Fusion 49, 104015 (2009).] has a stabilizing effect on formation of radiating plasma, realizing stably sustained divertor detachment operation with the core plasma being unaffected. The data from the several diagnostics, (profiles of electron temperature & density, radiation and temporal evolution of divertor particle flux) indicate selective cooling around X-point of the island and thus peaked radiation there, which is stabilized outside of the last closed flux surface throughout the detachment phase. The VUV spectroscopy measurements of high Z impurity (iron) emission shows significant decrease during the detachment, indicating core plasma decontamination. The results from the 3D edge transport code EMC3 (Edge Monte-Carlo 3D) [Y. Feng et al., Contributions to Plasma Physics, 44, 57 (2004).]-EIRENE [D. Reiter et al., Fusion Sci. Technol., 47 172 (2005).] show similar tendency in the radiation pattern. The island size and its radial location are varied to investigate the magnetic topology effects on the detachment control. The divertor particle flux and neutral pressure exhibit intermittent oscillation as well as modification of recycling pattern during the detachment, which are found to reflect the island structure.

Comparison between stellarator and tokamak divertor transport

“…The island chains in helical SOLs provide a multi-null divertor configuration where the number, location and geometry of the X-points can be adjusted externally, thus having the potential of more flexible control of the radiation location for optimum power removal. Indeed, both W7-AS and LHD have demonstrated success in controlling and stabilizing the radiation layer outside the confinement region by externally manipulating the divertor geometry [54,55]. Numerically it is shown that the plasma-neutral interaction is not the main reason for the rollover of the recycling flux and downstream density in helical devices.…”

Imaging Bolometers for Visualization of Plasma Radiation and Cross-Validation of Three Dimensional Impurity Transport Models for the Large Helical Device

“…As an example of a comparison between the EMC3/Eirene-based synthetic images and the IRVB images, data is taken from discharges in which an m/n = 1/1 magnetic island (where m and n are the indicies of the poloidal and toroidal harmonics, respectively) is used to break the m = 2, n = 10 symmetry of the LHD magnetic field and induce and stabilize a detached plasma [7][8][9]. In these discharges it has been observed that during the detachment with the magnetic island the radiation is localized in the x-point of the magnetic island which is toroidally localized and centered on the inboard side at the same field period as the o-point which is centered on the outboard side.…”

Behavior of Plasma Response Field in Detached Plasma

“…While the detached plasma is established in the LHD experiment, the radiation region exists at the stochastic region which is upstream from the divertor leg. In particular, in the case that the magnetic island is imposed, AXUVD and bolometer diagnostics have shown that the radiation region is localized around the X-point of the magnetic island [3,7]. This detachment in LHD can be thought to be a phenomenon similar to an X-MARFE in a tokamak [8], which can be sustained in the stochastic magnetic field at the X-point of the magnetic island.…”

“…In the previous study in LHD, the 1/3∼1/10 reduction of particle flux and heat load on the divertor plate has been observed by means of the divertor-probe measurement [3], which can be thought of as a detachment. While the detached plasma is established in the LHD experiment, the radiation region exists at the stochastic region which is upstream from the divertor leg.…”

“…Although a lot of studies about externally controlled methods have been done to maintain detached plasmas in tokamaks [1,2], a reliable method to obtain a detached plasma has not been found. In recent LHD experiments, a resonant magnetic perturbation (RMP) has been utilized to establish a detached plasma [3], which is one of the candidates to reduce the divertor heat load. When the RMP is imposed on the plasma, a significant plasma response field is observed to modify the magnetic island [4].…”