[论文解读] Mechanisms of plasma disruption and runaway electron losses in tokamaks

Sherzod Abdullaev, K.H. Finken+6 more|arXiv (Cornell University)|Jan 7, 2015

Magnetic confinement fusion research参考文献 37被引用 3

一句话总结

本文提出了一种由低模数数MHD不稳定性引起的随机磁场驱动的托卡马克等离子体破裂期间逃逸电子（RE）束形成与损失的机制。该机制解释了RE在完整磁面内由环向电场加速，而损失则源于向外漂移及由m/n=1/1 MHD模激发的束边缘随机层，成功再现了内束约束和RE电流缓慢衰减等关键实验特征。

ABSTRACT

Based on the analysis of data from the numerous dedicated experiments on plasma disruptions in the TEXTOR tokamak the mechanisms of the formation of runaway electron beams and their losses are proposed. The plasma disruption is caused by strong stochastic magnetic field formed due to nonlinearly excited low-mode number magnetohydrodynamic (MHD) modes. It is hypothesized that the runaway electron beam is formed in the central plasma region confined by an intact magnetic surface due to the acceleration of electrons by the inductive toroidal electric field. In the case of plasmas with the safety factor $q(0)<1$ the most stable runaway electron beams are formed by the intact magnetic surface located between the magnetic surface $q=1$ and the closest low--order rational surface $q=m/n>1$ ($q=5/4$, $q=4/3$, ...). The thermal quench time the current quench time are estimated. The runaway electron beam current is modeled as a sum of toroidally symmetric part and a small amplitude helical current with a predominant $m/n=1/1$ component. The runaway electrons are lost due to two effects: ($i$) by outward drift of electrons in a toroidal electric field until they touch wall and ($ii$) by the formation of stochastic layer of runaway electrons at the beam edge. Such a stochastic layer for high--energy runaway electrons is formed in the presence of the $m/n=1/1$ MHD mode. It has a mixed topological structure with a stochastic region open to wall. The effect of external resonant magnetic perturbations on runaway electron loss is discussed. A possible cause of the sudden MHD signals accompanied by runaway electron bursts is explained by the redistribution of runaway current during the resonant interaction of high--energetic electron orbits with the $m/n=1/1$ MHD mode.

研究动机与目标

• 理解托卡马克等离子体破裂期间逃逸电子（RE）束形成的物理机制。
• 解释不同放电中观测到的RE束变异性与稳定性的原因。
• 利用随机磁场中的碰撞输运模型模拟热弛豫和电流弛豫阶段。
• 识别高能RE在破裂后等离子体中的主要损失机制。
• 评估外部共振磁扰动（RMPs）对RE动力学的影响。

提出的方法

• 通过低模数数MHD模的叠加，模拟破裂后磁场所产生的大尺度随机性。
• 应用碰撞扩散输运模型，计算随机场中的径向热与粒子输运。
• 在感应环向电场和MHD扰动下，模拟逃逸电子的导心轨道。
• 将逃逸电流分解为轴对称与螺旋分量，其中主导模数为m/n=1/1。
• 分析由m/n=1/1 MHD模在RE束边缘形成的随机层，从而实现壁面损失。
• 利用TEXTOR的实验数据，验证模型对观测到的RE约束、能量弛豫尖峰和电流衰减剖面的拟合效果。

实验结果

研究问题

• RQ1 是什么导致了托卡马克等离子体破裂期间稳定逃逸电子束的形成？
• RQ2 MHD模引起的随机磁场如何影响电子输运与RE束约束？
• RQ3 为何逃逸电子主要在等离子体内区被观测到，而从外区损失？
• RQ4 什么机制控制了平台期RE电流的缓慢衰减？
• RQ5 外部共振磁扰动如何影响RE的损失与动力学行为？

主要发现

• 热弛豫主要由随机磁场中的碰撞电子输运驱动，其时间尺度与实验观测一致。
• 电流弛豫由同一随机场中的无旋粒子输运主导，离子动力学显著影响电流衰减时间。
• 逃逸电子束在中心等离子体区域形成，受完整磁面约束，尤其在q=1与最近的有理面（如q=5/4、4/3）之间，尤其当q(0)<1时。
• RE电流的缓慢衰减由两种损失机制解释：(i) 持续加速引起的向外漂移；(ii) 由m/n=1/1 MHD模在束边缘形成的随机层。
• 该模型成功再现了实验观测中能量弛豫阶段的短暂尖峰，归因于放电起始时从遍历区损失RE。
• 外部RMPs对低能RE（至5–10 MeV）影响甚微，表明除非直接扰动RE束核心区域，否则其抑制效果有限。