Sawtooth crash in tokamak as a sequence of multi-region relaxed MHD equilibria

This study examines the sawtooth crash phenomenon in tokamak plasmas by modeling it as a sequence of multi-region relaxed magnetohydrodynamic (MRxMHD) equilibria. Using the stepped-pressure equilibrium code, we constructed a series of equilibria representing intermediate states during the sawtooth crash, with progressively increasing reconnection regions. Numerical results demonstrated that the system prefers the lower energy non-axisymmetric equilibria with islands and is eventually back to an axisymmetric state, capturing key features of the reconnection process. Comparisons with the nonlinear MHD code M3D-C1 showed remarkable agreement on the field-line topology, the safety factor, and the current profile. However, the simplified MRxMHD model does not resolve the detailed structure of the current sheet. Despite this limitation, MRxMHD offers an insightful approach and a complementary perspective to initial-value MHD simulations.

本研究将托卡马克等离子体中的锯齿崩落（sawtooth crash）现象建模为多区域弛豫磁流体动力学（MRxMHD）平衡序列，以此开展相关研究。本研究借助阶梯压强平衡代码（stepped-pressure equilibrium code），构建了一系列表征锯齿崩落过程中间态的平衡态，其中重联区域逐步扩大。数值结果表明，该系统更倾向于处于带有磁岛的低能量非轴对称平衡态，并最终回归至轴对称状态，该过程捕捉到了磁重联过程的关键特征。与非线性磁流体动力学代码M3D-C1的对比结果显示，二者在磁力线拓扑结构、安全因子以及电流分布上均表现出极佳的一致性。不过，简化后的MRxMHD模型无法解析电流片的精细结构。尽管存在这一局限，MRxMHD仍为初值磁流体动力学模拟提供了极具启发性的研究路径与互补视角。