Micro-instabilities are often invoked to account for the anomalous transport of energy and particles in fusion plasmas, whose origin and contributions to anomalous transport remain uncertain. The dynamic features of the microturbulence in the plasma core and the gradient region are investigated simultaneously by a poloidal CO2 laser coherent scattering diagnostic system on the experimental advanced superconducting tokamak (EAST). In 2018, an improved confinement mode characterized by a peaked density profile is observed in the current ramp-down phase with no auxiliary heating, which has rarely been studied. The experimental result suggests that the gradual shut off of the LHW heating might trigger the mode transition. The transition is accompanied with a great suppression of microturbulence and significant change in the cross-correlation spectrum. A possible mechanism for the suppression could be ascribed to the increasing shear flow. These results could shed light on the understanding of the nonlinear mechanism of confinement mode transition on EAST.
微不稳定性常被用来解释聚变等离子体中能量和粒子的反常输运，但其起源及对反常输运的贡献仍不确定。通过实验先进超导托卡马克（EAST）上的多普勒相干散射诊断系统，同时研究了等离子体芯部和梯度区的微观湍流动态特征。在2018年，观察到一种以密度分布峰化为特征的改进约束模式，该模式出现在无辅助加热的电流下降阶段，此类情况此前很少被研究。实验结果表明，低杂波加热的逐渐关闭可能触发了该模式转变。该转变伴随着微观湍流的显著抑制以及交叉相关谱的明显变化。这种抑制的可能机制可归因于剪切流的增强。这些结果有助于理解EAST上约束模式转变的非线性机制。