Experimental study on up-down asymmetry of tungsten impurities in EAST tokamak

Abstract

By using the high-performance extreme ultraviolet spatial resolution impurity spectrometer, the up-down asymmetric distribution of tungsten impurity radiation in EAST tokamak is studied experimentally for the first time. The results show that during the co-directional neutral beam injection, the central toroidal rotation velocity is large, the up-down asymmetry is strong, and the side with strong radiation deviates from ion <inline-formula><tex-math id="Z-20240121170250">\begin{document}$B\times \nabla B$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20231448_Z-20240121170250.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20231448_Z-20240121170250.png"/></alternatives></inline-formula> drift direction. However, after injecting deuterated methane CD₄ into plasma through the valve of the upper divertor outer plate, the central toroidal rotation velocity decreases rapidly, and the asymmetry of the original tungsten impurity radiation decreases soon and finally reverses. In this work, a further statistical study of the W³²⁺ impurity radiation asymmetry factor <i>I</i>_u/<i>I</i>_d depending on the central toroidal rotation velocity <i>V</i>_t0 is performed. The results show that when <i>V</i>_t0 is larger than 30 km/s, the side with strong radiation deviates from ion <inline-formula><tex-math id="Z-20240121170255">\begin{document}$B\times \nabla B$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20231448_Z-20240121170255.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20231448_Z-20240121170255.png"/></alternatives></inline-formula> drift direction, however, when <i>V</i>_t0 decreases to below 20 km/s, the asymmetry can be reversed. The relation of toroidal rotation velocity with impurity radiation asymmetry validates the prediction from drift-kinetic theory, and demonstrates that the centrifugal force induced by the toroidal rotation directly causes the asymmetric distribution of tungsten impurities through affecting the momentum conservation parallel to the magnetic field. The experimental observation of the asymmetric distribution of tungsten impurities in this work lays a solid foundation for further studying the poloidal transport of high-<i>Z</i> impurities and provides some important references for controlling the high-<i>Z</i> impurities in future fusion reactors.