Nonlinear excitation of subcritical fast ion-driven modes
Nonlinear excitation of subcritical fast ion-driven modes
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DOI：
10.1088/0029-5515/56/5/056009
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发表时间：
2016-04
期刊：
影响因子：
3.3
通讯作者：
M. Lesur;K. Itoh;T. Ido;S. Itoh;Y. Kosuga;M. Sasaki;S. Inagaki;M. Osakabe;K. Ogawa;A. Shimizu;K. Ida
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文献类型：
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作者：
M. Lesur;K. Itoh;T. Ido;S. Itoh;Y. Kosuga;M. Sasaki;S. Inagaki;M. Osakabe;K. Ogawa;A. Shimizu;K. Ida
在无碰撞等离子体中，众所周知，线性稳定模式可能因相空间中结构的存在而不稳定（亚临界）。这种结构的生长是一种非线性的动力学机制，与传统的逆朗道阻尼不同，它提供了自由能提取的通道。然而，这种非线性生长需要存在具有相对大的振幅阈值的种子结构。我们证明，在存在另一种线性不稳定（超临界）模式的情况下，波-波耦合可以提供种子，这可以通过两种机制中的任何一种导致亚临界不稳定。这两种机制都取决于流体非线性和动力学非线性之间的协作。如果碰撞扩散速度足够低，超临界模式提供的种子就克服了相空间结构非线性生长的阈值。然后，超临界模式触发传统的亚临界不稳定性。如果碰撞速度扩散太大，则种子明显低于阈值，但仍然可以通过流体和动力学非线性之间的持续协作来生长。即使超临界模式的频率快速扫描，这两种亚临界不稳定性也可能被触发。这些结果是通过对亚临界模式动力学建模以及通过简单的波-波耦合方程对超临界模式的影响进行建模而获得的。该模型应用于 LHD 实验中测地线声学模式的突发爆发。该模型恢复了几个关键特征，例如相对幅度、时间尺度和相位关系。它表明最强的爆发是亚临界不稳定性，流体和动力学非线性之间存在持续的协作。
In collisionless plasma, it is known that linearly stable modes can be destabilized (subcritically) by the presence of structures in phase-space. The growth of such structures is a nonlinear, kinetic mechanism, which provides a channel for free-energy extraction, different from conventional inverse Landau damping. However, such nonlinear growth requires the presence of a seed structure with a relatively large threshold in amplitude. We demonstrate that, in the presence of another, linearly unstable (supercritical) mode, wave–wave coupling can provide a seed, which can lead to subcritical instability by either one of two mechanisms. Both mechanisms hinge on a collaboration between fluid nonlinearity and kinetic nonlinearity. If collisional velocity diffusion is low enough, the seed provided by the supercritical mode overcomes the threshold for nonlinear growth of phase-space structure. Then, the supercritical mode triggers the conventional subcritical instability. If collisional velocity diffusion is too large, the seed is significantly below the threshold, but can still grow by a sustained collaboration between fluid and kinetic nonlinearities. Both of these subcritical instabilities can be triggered, even when the frequency of the supercritical mode is rapidly sweeping. These results were obtained by modeling the subcritical mode kinetically, and the impact of the supercritical mode by simple wave–wave coupling equations. This model is applied to bursty onset of geodesic acoustic modes in an LHD experiment. The model recovers several key features such as relative amplitude, timescales, and phase relations. It suggests that the strongest bursts are subcritical instabilities, with sustained collaboration between fluid and kinetic nonlinearities.