Experimental Studies of and Theoretical Models for Detachment in Helical Fusion Devices

Abstract

Good plasma performance in magnetic fusion devices of different types, both tokamaks and helical devices, is achieved normally if the plasma density does not exceed a certain limit. In devices with a divertor, such as tokamaks JET, JT-60U, and heliotron large helical device (LHD), by approaching the density limit, the plasma detaches from the divertor target plates so that the particle and heat fluxes onto the targets reduce dramatically. This is an attractive scenario for fusion reactors, offering a solution to the plasma-wall interaction problem. However, the main concerns by realizing such a scenario are the stability of the detached zone. The activity on the heliotron LHD aimed on detachment stabilization, by applying a resonant magnetic perturbation (RMP) and generating a wide magnetic island at the plasma edge, will be reviewed. Also, theoretical models, explaining the detachment conditions, low-frequency oscillations at the detachment onset, and mechanisms of the detached plasma stabilization by RMP, will be discussed.

Time‐dependent plasma transport simulation for the study of edge impurity radiation dynamics with magnetic island in large helical device

“…Thus impurity radiation is usually larger in the stochastic layer than in the divertor legs as analyzed in ref. [5,8] . The effects of the stochastic layer are not included in the present simplified geometry, but rather we focus on the effects of island structure as mentioned above.…”

“…The recent experiments of W7-X with island divertor configuration have also realized a stable operation of complete detachment [4] . The mechanisms of the detachment stabilization have been discussed, such as core-edge coupling through neutral penetration [3] , or competition between impurity confinement time and radiation penetration time [5] , etc. However, the analyses have been limited to rather simplified models so far, while in order to understand the stability of the detachment, time-dependent analysis with appropriate geometrical effects is essential.…”

Conditions and benefits of X-point radiation for the island divertor

“…Although the underlying physics is not fully clear, EMC3-Eirene modeling for all three devices has shown that the distribution of impurity radiation closely correlates with the island geometry, which is in line with experimental observations. In particular, experiments and modeling have shown that stable detachment conditions achieved at LHD and W7-X are associated with significant X-point radiation (XPR) [31,32,[34][35][36][37][38]. XPR is also a topic of renewed intense interest to the tokamak community as a potential power dissipation regime for future tokamaks [39][40][41].…”

“…One of the design metrics for a divertor is the capability of stable operation of a highly dissipative plasma, known as 'detachment' [25][26][27][28]. Divertor experiments from W7-AS, W7-X, and LHD consistently showed that stable detachment requires large magnetic islands with strong perturbation fields [29][30][31][32][33]. This cross-machine result demonstrates the importance of magnetic islands for the performance of 3D divertors.…”

Confinement improvement during detached phase with RMP application in deuterium plasmas of LHD

“…The divertor detachment is also a bifurcation process in terms of an energy balance between power input and loss channels, the latter of which are the energy transport loss to divertor plates as well as impurity radiation. Both of the processes have strong nonlinearity with respect to temperature, resulting in bifurcation between high and low temperature branches [9]. Therefore, the realization of compatibility between the good core confinement and divertor heat load mitigation with detachment is to search for an operating regime, where the branches in both of the bifurcations are satisfied with the ETB formation and the low edge temperature simultaneously.…”