Dynamics of filaments during the edge-localized mode crash on NSTX

Edge localized modes (ELMs) are routinely observed in H-mode plasma regimes of the National Spherical Torus Experiment (NSTX). Due to the explosive nature of the instability, only diagnostics with high temporal and spatial resolution could provide a detailed insight into the dynamics associated with the ELMs. Gas-puff imaging (GPI) at NSTX provides 2D measurements of the magnetic field aligned fluctuations (e.g. ELM filaments) in the scrape-off layer and the at the plasma edge with 2.5 us temporal and 10 mm optical resolution.A novel analysis technique was developed to estimate the frame-by-frame velocities and the spatial parameters of the dominant structures associated with the ELMs. The analysis was applied to single ELM events to characterize the ELM crash dynamics, and then extended to a database of 169 ELM events.Statistical analysis was performed in order to find the characterizing dynamics of the ELM crash. The results show that on average an ELM crash consists of a filament with a circular cross-section which is propelled outwards with a characterizing peak radial velocity of ~3.3 km/s. The radial velocity was found to be linearly dependent on the distance of the filament from the separatrix, which has never been seen before. The ELM filament is characterized by propagation in the ion-diamagnetic direction poloidally with a peak velocity of 11.4 km/s. The ELM crash lasts for approximately 100us until the radial propulsion settles back to the pre-ELM level. The experimental findings were compared with analytical theory. Two possible mechanisms were identified for explaining the observations: the curvature interchange model and the current-filament interaction model.

边缘局域模（Edge localized modes, ELMs）常在国家球形环面实验装置（National Spherical Torus Experiment, NSTX）的H模等离子体运行模式中被常规观测到。由于该不稳定性具有爆发性特征，唯有具备高时间与空间分辨率的诊断系统，才能为与ELMs相关的动力学过程提供细致的观测视角。国家球形环面实验装置的喷气成像（Gas-puff imaging, GPI）系统可对刮削层及等离子体边缘处与磁场对齐的涨落（如ELM丝结构）进行二维测量，其时间分辨率达2.5微秒，光学空间分辨率为10毫米。

研究人员开发了一种新型分析技术，用以估算与ELMs相关的主导结构的逐帧速度及空间参数。该分析方法首先应用于单个ELM事件以表征ELM爆发动力学过程，随后扩展至包含169个ELM事件的数据集。

为揭示ELM爆发的特征动力学行为，研究人员开展了统计分析。结果显示，平均而言，一次ELM爆发由一个截面呈圆形的丝结构构成，该结构以约3.3 km/s的特征峰值径向速度向外推进。研究还发现，径向速度与丝结构到磁分界面的距离呈线性相关，这一现象此前从未被观测到。ELM丝结构沿离子抗磁方向进行极向传播，其峰值速度达11.4 km/s。ELM爆发过程持续约100微秒，直至径向推进速度回落至ELM爆发前的水平。研究将实验观测结果与解析理论进行了对比，最终确定了两种可解释该观测现象的潜在机制：曲率交换模型与电流丝相互作用模型。