To investigate the driving mechanism of the locked-mode-like instability observed in the large helical device, we reconstruct the magnetohydrodynamic (MHD) equilibria consistent with the measurement and identify a dominant MHD instability in the precursor phase based on linear MHD analyses. From the dependence of the linear growth rate on the magnetic Reynolds number, the radial mode structure of the electrostatic potential fluctuation and other indices, the ideal interchange mode is found to be dominant. Moreover, it is found that the Mercier parameter, DI, becomes much larger than 0.3 during the phase, while the precursor has constant frequency and fluctuation amplitude. Therefore, DI ≫ 0.3 is a good index of the on-set condition of the minor collapse itself. It is also found that the achievement of DI ≫ 0.3 is due to the movement of the resonant surface to the inner plasma region, which corresponds to the larger pressure gradient region.
为了研究大型螺旋装置中观察到的类锁定模不稳定性的驱动机制，我们重建了与测量一致的磁流体动力学（MHD）平衡态，并通过线性MHD分析识别出前驱阶段的主导不稳定性。根据线性增长率对磁雷诺数的依赖关系、静电势涨落的径向模结构以及其他指标，发现理想交换模占主导地位。此外，发现Mercier参数DI在前驱阶段远大于0.3，而前驱具有恒定频率和涨落幅度。因此，DI ≫ 0.3是次生崩塌本身 onset 的良好指标。还发现DI ≫ 0.3的实现是由于共振面向等离子体内部区域移动，该区域对应更大的压力梯度区域。