Dynamic evolution of periodic collapse and carbon impurity distribution at the plasma boundary with magnetic islands on J-TEXT

Abstract Periodic collapses at the edge of Ohmic-mode plasma under Resonant Magnetic Perturbation (RMP) have been observed in the J-TEXT tokamak. These collapses occur following the field penetration. During the collapse, particles and heat flow outward through the X-point of the boundary m / n = 3/1 magnetic island, where m and n are the poloidal and toroidal mode number respectively. After the collapse, the magnetic island rapidly expands to its maximum width, then gradually shrinks until it disappears. The reason why the magnetic island cannot sustain itself remains unclear. Afterward, the edge plasma returns to its initial state, and the cycle will repeat with the next collapse. Additionally, the conditions leading to periodic collapses have been identified, the amplitude of RMPs at q = 3 resonant surface is essential. No field penetration exists if the amplitude is too small. If the amplitude is too large, the magnetic island remains after field penetration and collapses no longer happen. Therefore, periodic collapses only appear when applying a moderate amplitude of RMP. Carbon impurities play a significant role in triggering instabilities of edge magnetic island and collapse events. This behavior is different from the pumping-out effect previously observed on J-TEXT, which was caused by a 2/1 magnetic island in the core.