Impact of a resonant magnetic perturbation field on impurity radiation, divertor footprint, and core plasma transport in attached and detached plasmas in the Large Helical Device

Abstract

Effects of resonant magnetic perturbation (RMP) field on impurity radiation, divertor footprint distribution, and core plasma transport are investigated in the detachment discharges of LHD. The RMP with m/n = 1/1 mode creates edge magnetic island in the stochastic layer, which enhances the impurity emission from low charge states, C 2+ and C 3+ , and then triggers detachment transition. The emission from the higher charge states, C 4+ and C 5+ , implies enhanced penetration of impurity during detachment phase with RMP. The toroidal divertor particle flux distribution exhibits n = 1 mode structure in both attached and detached phases, but with a large toroidal phase shift between the two phases. The distribution in the attached phase is well correlated with magnetic footprint of field line connection length calculated by the vacuum approximation. During the detached phase, however, the phase shift is not well explained by the vacuum approximation, where significant plasma response to the external RMP is observed. The energy confinement time becomes systematically shorter with RMP application due to the shrinkage of plasma volume caused by the edge magnetic island. On the other hand, the pressure profile during detachment with RMP is found to be more peaked than without RMP. The analysis with the core transport code TASK3D, taking into account heating profiles of NBI, shows no significant transport degradation during detachment with RMP application, whilst the enhanced radiation, the reduced divertor flux, and the possible impurity penetration.

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

“…The toroidal distributions of the particle flux onto divertor targets with RMP normalized by the values without RMP on attached (a) and detached (b) discharge phases. Red circles correspond to the left divertor and blue diamonds to the right divertor arrays, as it is indicated in Figure 2 by the toroidal section number [27]. plasma response to the RMP, which is different in the attached and detached phases, as mentioned above.…”

“…The impact of RMP on the impurity emission intensity was also investigated in the VUV range with the diagnostic equipment [22], viewing the entire plasma toroidal cross section. Density dependence of the radiated power measured by the resistive bolometer, together with emissions from different charge states, CIII (C 2+ ), CIV (C 3+ ), CV (C 4+ ), and CVI (C 5+ ), measured with the spectrometer [27] is plotted in Figure 7. Here, the plasma density is normalized to the density limit in helical devices, n sudo [28].…”

“…The toroidal variation of the particle fluxes onto divertor plates with RMP has been investigated with Langmuir probe array installed around the mid-plane of the targets at the inboard side [17,31]. It has been found that the time evolution of the divertor particle flux exhibits substantial difference between toroidal locations, that is, some plates are becoming detached earlier than others; at some plates, the flux even increases after detachment [27]. The summary of this behavior is shown in Figure 8, where the divertor particle flux normalized to its value without RMP is plotted for different toroidal sections.…”

“…In the detached phase, the n = 1 structure remains, but the toroidal phase is shifted by one section. The relation between the divertor flux and L C profiles is presented in Figure 9 for several toroidal cross sections [27]. At the section 6L (Figure 9(a) and (b)), a bundle of flux tubes of 6.5 mm width is connected to the divertor plate.…”

“…The measured particle flux increases in the absolute values due to the longer L C , as seen also in Figure 9(a). The density is normalized to the density limit in helical devices, n sudo [27].…”