Characterization of density scanning experiments with NBI and LHW on EAST

Fubin Zhong,Gongshun Li,Fei Wen,Yao Yang,Xiang Gao,Tao Zhang,Jia Huang,Kangning Geng,Xiang Han,Haoming Xiang,Xuexi Zhang,Yukai Liu,Xiaohe Wu,Kaixuan Ye,Mingfu Wu

doi:10.1063/5.0062690

Abstract

This paper presents the results of the density scanning experiment on the 2018 EAST campaign to study the effect of gas fueling on energy confinement of the ELMy H-mode. The experiment is carried out in a USN configuration, with neutral beam and lower hybrid wave heating and gas fueling, with the upper triangularity δup ∼ 0.47. The total stored energy, H98, and βN decrease with normalized density. Compared to the variations in temperature at the pedestal, the core temperature decreases more significantly for both Te and Ti, leading to a large reduction in core pressure and an increase in the pedestal electron collisionality ν*e,ped. The increase in ν*e,ped could reduce the pedestal current and result in a decrease in the value of q in the core region. It was observed that the frequency of type I ELMs increases with density and the edge localized mode size becomes smaller at high density plasma. An m/n = 2/2 tearing mode was observed at the core of the plasma and can coexist with a sawtooth at low density plasma while this tearing mode disappeared at high gas fueling plasma. The reversal radius of the sawtooth (where q = 1) moves toward the magnetic axis as density increases. The degradation in performance with density may be due to two reasons: the more monotonic shear q profile and the weakening of the stabilizing effect of fast ions on ion temperature gradient modes at high density by D2 gas fueling. It seems that there is a strong link between core transport and pedestal parameters which are influenced by gas fueling, resulting in a significant degradation of energy confinement.

Highlights

IntroductionResults in JET, ASDEX Upgrade, DIII-D,18 and JT-60U19 have shown that pedestal pressure pped increases with increasing plasma triangularity, leading to higher stored thermal energy Wth and the energy confinement enhancement factor H98 due to the access to the second stable regime for ideal ballooning modes
The explanation for the transition from type I to type III edge localized mode (ELM) is that a transition from second to first ballooning stability controlled by the resistive rather than ideal ballooning modes is triggered
In the experiment described in this paper, the density increases from pulse to pulse by increasing the D2 gas fueling rate

Summary

Introduction

Results in JET, ASDEX Upgrade, DIII-D,18 and JT-60U19 have shown that pedestal pressure pped increases with increasing plasma triangularity, leading to higher stored thermal energy Wth and the energy confinement enhancement factor H98 due to the access to the second stable regime for ideal ballooning modes.. A transition from type I ELMs to small ELMs (type II or Grassy ELM) was observed at high density high confinement plasma with strong shaping on JET, ASDEX Upgrade, and JT-60U25 due to the change in stability in the pedestal region. For the high ratio of Pin/PL–H (where PL–H is the threshold power of L–H transition) discharge in JET, a slight degradation of pedestal pressure rather than the transition to the type III ELM regime and large consequent loss of confinement at high density was observed.. MHD stability analysis of highly powered plasma demonstrated that a higher ballooning stability threshold was achieved with a peeling/kink current limit that increased as the pressure gradient increased

Methods

Results

Conclusion