Study on divertor plasma behavior through sweeping strike point in new lower divertor on EAST

A series of L-mode discharges have been conducted in the new ‘corner slot’ divertor on the Experimental Advanced Superconducting Tokamak (EAST) to study the divertor plasma behavior through sweeping strike point. The plasma control system controls the strike point sweeping from the horizontal target to the vertical target through poloidal field coils, with keeping the main plasma stability. The surface temperature of the divertor target cools down as the strike point moves away, indicating that sweeping strike point mitigates the heat load. To avoid the negative effect of probe tip damage, a method based on sweeping strike point is used to get the normalized profile and study the decay length of particle and heat flux on the divertor target λ_js, λ_q. In the discharges with high radio-frequency (RF) heating power, electron temperature T_e is lower and λ_js is larger when the strike point locates on the horizontal target compared to the vertical target, probably due to the corner effect. In the Ohmic discharges, λ_js, λ_q are much larger compared to the discharges with high RF heating power, which may be attributed to lower edge T_e.     

Evolution of the high-field-side radiation belts during the neon seeding plasma discharge in EAST tokamak

Divertor detachment achieved by injecting impurities or increasing density is always accompanied with various local radiation phenomena in the boundary or core plasma. This paper presents the formation and evolution of the high-field-side (HFS) radiation belts during the neon seeding plasma discharge in upper single null configuration with two directions of toroidal magnetic field in EAST tokamak. The neon mixed with deuterium seeding can induce the divertor detachment with strong radiation belts in the HFS scrape-off layer (SOL) region. With the increase of the radiation power, the plasma discharge will transit from H-mode to L-mode, and meanwhile the radiation belts move away from the near X-point to HFS SOL. When the radiation power is high enough, the radiation belts begin to move further to the other X-point along the HFS SOL, and even cause plasma disruption. The results indicate that the behavior of the radiation belts is related to the radiation power, plasma confinement performance and state of divertor detachment, which is useful for developing better feedback control methods to achieve high-performance radiative divertor operation mode.     

Experimental investigation on divertor tungsten sputtering with neon seeding in ELMy H-mode plasma in EAST tokamak

Neon (Ne) seeding is used to cool the edge plasma by radiation to protect the divertor tungsten (W) target in the Experimental Advanced Superconducting Tokamak (EAST). The W sputtering in the outer divertor target with Ne seeding is assessed by the divertor visible spectroscopy system. It is observed that the W sputtering flux initially increases with Ne concentration in the divertor despite the decreasing plasma temperature. After reaching a maximum around 25 eV, the W sputtering rate starts to decrease, presenting a suppression effect. The effect on the divertor W sputtering flux and yield due to the competition between the increase of the Ne concentration and the decrease of the plasma temperature is discussed. The results show that enough Ne seeding is essential to effectively reduce the electron temperature and thus to suppress W sputtering. Moreover, ELM suppression is observed when Ne and W impurities enter the core plasma, which could be correlated to the enhanced turbulence transport in the pedestal.     

EAST边界和芯部杂质谱线的时间延迟研究

边界杂质注入是未来聚变装置ITER用于增强边界辐射,减少第一壁热负荷的一种重要方法。但部分注入的杂质会被输运到芯部,造成主等离子体辐射损失以及约束下降。光谱观测可以获取杂质种类、含量和分布等信息,在理解等离子体中杂质输运方面起着重要作用。在EAST(experimental advanced superconducting tokamak)偏滤器氩气(Ar)注入实验中,利用偏滤器可见光谱和芯部极紫外光谱监测边界的Ar¹⁺离子谱线ArⅡ(401.36 nm)和芯部的Ar¹⁵⁺离子谱线ArⅩⅥ(35.39 nm),并获得两者强度随时间的变化。其中,ArⅡ和ArⅩⅥ的电离能分别为27和918 eV,因此,ArⅡ和ArⅩⅥ分别对应分布于等离子体边界和芯部Ar离子。为了分析二者谱线强度随时间变化的特征,发展了一种基于正则Pearson积矩相关系数的相关分析方法,计算得到两者谱线强度变化的相对延迟时间,以此表征杂质从边界向芯部输运的时间。结果显示,偏滤器注入Ar杂质后,芯部ArⅩⅥ辐射增长滞后于边界ArⅡ辐射的增长,并且在具有较高的低杂波加热功率的放电中...