Studies of edge poloidal rotation and turbulence momentum transport during divertor detachment on HL-2A tokamak
Authors: Long Ting , Ke Rui , Wu Ting , Gao Jin-Ming , Cai Lai-Zhong , Wang Zhan-Hui , Xu Min ,   Southwestern Institute of Physics, Chengdu 610225, China
Published: February 2024

In a magnetic confinement fusion device, the plasma undergoing nuclear fusion reaction must be maintained in a high-temperature and high-density confinement state for a long enough time to release high energy, while the heat loads on the divertor target plates need to be reduced to avoid damage to wall at the same time. The latter is one of the key challenges of ITER and commercial fusion reactors in future. Divertor detachment provides an effective solution to reduce the heat load on the target plate of tokamak. However, this may result in the change of plasma states at the boundary, thus affecting the plasma confinement. In this paper, edge plasma poloidal rotation and turbulence momentum transport are studied experimentally during the divertor detachment in the L-mode discharge of HL-2A tokamak. The detachment is achieved by injecting a mixture of gas (60% nitrogen+40% deuterium) into the divertor. The gas mixture is injected by pulsed injection, with pulse length being in a range of 5–20 ms. During the divertor detached phase, both the ion saturation current density and the heat flux to the outer target plate decrease considerably. The enhanced radiation is also observed in the divertor and <i>X</i>-point region. It is found that in the process of attachment-to-pre-detachement, the <inline-formula><tex-math id="M3">\begin{document}$ \boldsymbol{E}\times \boldsymbol{B} $\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="8-20231749_M3.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="8-20231749_M3.png"/></alternatives></inline-formula> poloidal flow velocity in the near scrape-off layer (SOL) changes from ion magnetic drift direction to electron magnetic drift direction. The turbulent driving force of poloidal flow, which is characterized by the negative radial gradient of momentum transfer flux (Reynolds stress), shows the same trend. In the detached phase, both the <inline-formula><tex-math id="M4">\begin{document}$ \boldsymbol{E}\times \boldsymbol{B} $\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="8-20231749_M4.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="8-20231749_M4.png"/></alternatives></inline-formula>flow and the Reynolds force become very small. Therefore, the dynamics of <inline-formula><tex-math id="M5">\begin{document}$ \boldsymbol{E}\times \boldsymbol{B} $\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="8-20231749_M5.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="8-20231749_M5.png"/></alternatives></inline-formula> poloidal flow velocity in the SOL is consistent with the evolution of rotation driving effect induced by the turbulent momentum transport. Combined with the <inline-formula><tex-math id="M6">\begin{document}$ \boldsymbol{E}\times \boldsymbol{B} $\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="8-20231749_M6.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="8-20231749_M6.png"/></alternatives></inline-formula> poloidal flow measured by the probe in the SOL and the beam emission spectrum inside the LCFS, the <inline-formula><tex-math id="M7">\begin{document}$ \boldsymbol{E}\times \boldsymbol{B} $\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="8-20231749_M7.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="8-20231749_M7.png"/></alternatives></inline-formula> poloidal velocity shearing rate near the LCFS can be inferred. Compared with the attached state, when the divertor is detached, the edge poloidal flow shearing rate decreases significantly, leading to the obviously enhanced turbulence level. Under the influence of both enhanced turbulent transport and radiation, the global confinement degrades moderately. The energy confinement time decreases about 15% and the confinement factor <inline-formula><tex-math id="M8">\begin{document}$ {H}_{89-P} $\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="8-20231749_M8.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="8-20231749_M8.png"/></alternatives></inline-formula> decreases about 10%. These results indicate that edge turbulent transport and plasma rotation dynamics play a role in the core-edge coupling process in which the divertor detachment affects the global confinement.