Non-monotonic growth rates of sawtooth precursors evidenced with a new method on ASDEX Upgrade
Non-monotonic growth rates of sawtooth precursors evidenced with a new method on ASDEX Upgrade
复制标题
DOI：
10.1088/0029-5515/56/8/086001
复制
发表时间：
2016-07
期刊：
影响因子：
3.3
通讯作者：
D. Vezinet;V. Igochine;M. Weiland;Q. Yu;A. Gude;D. Meshcheriakov;M. Sertoli
中科院分区：
文献类型：
--
作者：
D. Vezinet;V. Igochine;M. Weiland;Q. Yu;A. Gude;D. Meshcheriakov;M. Sertoli
本文描述了一种新的方法，从软x射线（SXR）层析成像中，对岩心位移、增长率和1/1锯齿状碰撞前兆的频率进行稳健估计。该方法适用于非常尖峰的SXR剖面，并且对反演算法和旋转等离子体中钨的存在都具有鲁棒性。然后分析了三种典型的ASDEX升级崩溃。在所有情况下都观察到一个后光标，这表明不完全重新连接。尽管动力学不同，但三种情况下岩心位移的增长率表现出相似的特征。首先，它不是恒定的，这支持了非线性增长的观点。其次，它可以被划分为几个清晰的阶段，这些阶段的增长率都是准恒定的，这表明增长模式的变化是突然的，而不是平稳的过渡。其三，其演化是非单调的，由加速生长阶段转入阻尼阶段。基于MHD模拟的结果，这种阻尼在两种情况下分别被解释为快离子和模式耦合的影响。在所有情况下，模式频率都与等离子体体旋转曲线密切相关，电子反磁漂移频率的影响很小或不可见。起始判据不能被清楚地识别，并且表明压力梯度的作用并不像线性稳定性理论的朴素外推所期望的那样。
This paper describes a new method to derive, from soft x-ray (SXR) tomography, robust estimates of the core displacement, growth rate and frequency of a 1/1 sawtooth crash precursor. The method is valid for very peaked SXR profiles and is robust against both the inversion algorithm and the presence of tungsten in a rotating plasma. Three typical ASDEX Upgrade crashes are then analysed. In all cases a postcursor is observed, suggesting incomplete reconnection. Despite different dynamics, in all three cases the growth rate of the core displacement shows similar features. First, it is not constant, supporting the idea of non-linear growth. Second, it can be divided into clearly identified phases with quasi-constant growth rates, suggesting sudden change of growth regime rather than smooth transitions. Third, its evolution is non-monotonic, with phases of accelerated growth followed by damped phases. This damping is interpreted for two cases respectively as an effect of fast ions and of mode coupling, based on the result of a MHD simulation. The mode frequency is observed in all cases to be closely related to the plasma bulk rotation profile, with little or no visible effect of the electron diamagnetic drift frequency. The onset criterion could not be clearly identified and it is shown that the role of the pressure gradient is not as expected from a naive extrapolation of the linear stability theory.