Dynamics of Internal Reconnection Events in Versatile Experiment Spherical Torus

Internal Reconnection Events (IREs) [1] in spherical tokamaks have been widely studied through experiments [2–4] and simulations [5], with core magnetohydrodynamic (MHD) activity often identified as the main mechanism. However, recent observations suggest that edge phenomena such as blobs and filaments can also play a significant role in IRE dynamics [6]. In this work, we present a detailed analysis of core and edge fluctuations in the Versatile Experiment Spherical Torus (VEST), using soft X-ray arrays and fast camera imaging. Synthetic soft X-ray analysis indicates that coherent edge blob structures are influenced by magnetic perturbations generated by multiple core-rotating magnetic islands. As plasma approaches an IRE onset, both the core mode and edge blob structure were intensified, with a simultaneous rise in their correlation. Notably, the phase relationship between core and edge fluctuations gradually synchronized prior to the IRE onset. With this phase synchronization, portions of the edge blobs were observed to propagate inward along the trajectory of the rotating magnetic islands, appearing as inward-moving filaments in fast camera images. These filaments ultimately terminated in a burst near the center stack at the onset of the IRE. These findings suggest that the interaction between core and edge fluctuations can significantly influence the IRE process. A comprehensive set of analysis results, including spatiotemporal characteristics of the fluctuations and their correlation with the IRE onset, will be discussed in the presentation.



This work was supported by National Grant no. 2021M3F7A1084418 and RS-2024-00409564 through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT, Korea.