Effects of electron cyclotron resonance heating modulation on edge turbulence driving and spreading in HL-2A tokamak

The plasma flow generated by turbulent nonlinear interaction can improve plasma confinement by suppressing turbulence and its driven transport. Turbulence can be driven by local gradients and propagate radially from far beyond its relevant length. Effects of electron cyclotron resonance heating (ECRH) modulation on edge turbulence driving and spreading are observed for the first time in the edge plasma of the HL-2A tokamak. These experiments are performed by a fast reciprocating Langmuir probe array. When ECRH modulation is applied, both the edge temperature and the edge plasma density are higher, and the radial electric field is stronger. The edge radial electric field, turbulence, and Reynolds stresses are all enhanced when the ECRH is applied, while the ion-ion collision rate is reduced. Figures (a)－(g) show the conditional averages of the ECRH power, turbulence intensity, turbulent Reynolds stress gradient, <inline-formula><tex-math id="M1">\begin{document}$ {\boldsymbol{E}}_{r}\times \boldsymbol{B} $\end{document}</tex-math></inline-formula> poloidal velocity, density gradient, turbulence drive rate and turbulence spreading rate, respectively. With ECRH applied, both the turbulence intensity and the Reynolds stress gradients increase. The maximum turbulence intensity appears at the beginning of the ECRH switch-off while the maximum stress gradient occurs at the end of the ECRH. The evolution of the <inline-formula><tex-math id="M2">\begin{document}$ {\boldsymbol{E}}_{r}\times \boldsymbol{B} $\end{document}</tex-math></inline-formula> poloidal velocity is very similar to that of the Reynolds stress gradient. This observation suggests that the poloidal flow is the result of the combined effect of turbulence nonlinear driving and damping. The enhancement of Reynolds stress during ECRH modulation mainly depends on the increase of the turbulence intensity, with the increase in radial velocity fluctuation intensity being more significant. The turbulence drive and spreading rates also increase with ECRH. The maximum drive rate appears at the beginning of the ECRH switch-off, while the maximum spreading rate occurs at the end of the ECRH. This analysis indicates that turbulence driving and spreading are enhanced, with the former being dominant. This result suggests that the enhancements of turbulence driving and spreading lead the turbulence and Reynolds stress to increase, and thus producing the stronger edge flows.