Low-frequency MHD mode activity was studied at the TJ-II stellarator. A spatiotemporal Fourier technique was used to resolve frequency-degenerate modes. By means of this technique, several MHD modes could be identified in discharges with a spontaneous confinement transition in different but similar magnetic configurations. The configurations differed mainly with respect to the radial position of the rational surfaces, thus allowing the reconstruction of a poloidal mode rotation profile based on the mode activity, which was found to be consistent with earlier work. The detected mode spectrum also provided an explanation for the bicoherence observed in one of the configurations after the confinement transition. Both the mode spectrum and the velocity profile were closely reproduced by nonlinear resistive MHD calculations in simplified geometry. As a consequence, the magnetic Reynolds stress is hypothesized to play an important role in the establishment of the velocity shear layer in TJ-II and the concomitant confinement transition.
低頻率磁流体动力学（MHD）模態活動在TJ-II仿星器中被研究。採用時空傅里葉技術來分辨頻率簡併的模態。通過該技術，在具有自發約束轉變的不同但相似的磁組態中，識別出了多個MHD模態。這些組態的主要差異在於有理面的徑向位置，從而允許基於模態活動重建極向旋轉輪廓，該輪廓與先前的工作一致。檢測到的模態譜也為約束轉變後其中一種組態中觀測到的雙相干性提供了解釋。模態譜和速度輪廓兩者均與非線性電阻MHD簡化幾何計算緊密吻合。因此，推測磁雷諾應力在TJ-II中速度剪切層的建立及伴隨的約束轉變中起著重要作用。