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arXiv 2404.14520physics.plasm-ph
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低磁剪切托卡马克中湍流涡旋的自相互作用

Self-interaction of turbulent eddies in tokamaks with low magnetic shear

Arnas Volčokas, Justin Ball, Stephan Brunner

AI总结:

通过局部非线性回旋动理学模拟,发现低磁剪切托卡马克中沿磁力线延伸的超长湍流涡旋会引发平行自相互作用,影响输运并导致“涡旋挤压”效应,为改善约束和触发ITBs提供了新机制。

摘要:

使用局部非线性回旋动理学模拟,我们证明了在具有弱磁剪切或零磁剪切$\hat{s}$的托卡马克中,湍流涡旋可以沿磁力线延伸数百个极向圈。我们观察到这种平行涡旋长度与磁剪切呈反比缩放关系,并且在$\hat{s}=0$时受电子热速度$v_{th,e}$限制。我们考察了这些“超长”涡旋对湍流输运的影响,特别是磁力线拓扑如何介导强烈的平行自相互作用。我们的研究表明,通过这一过程,磁力线拓扑能够强烈影响输运。它可以导致不同湍流不稳定性之间的转换,在某些情况下,会使驱动给定热通量所需的对数梯度增加两倍。我们还发现了一种新的“涡旋挤压”效应,该效应减小了涡旋的垂直尺寸及其输运能量的能力,从而代表了一种改善约束的新方法。最后,我们使用低磁剪切模拟研究了Internal Transport Barriers (ITBs)的触发机制,阐明了为什么ITBs通常在安全因子具有低阶有理值时更容易被触发。

英文摘要:

Using local nonlinear gyrokinetic simulations, we demonstrate that turbulent eddies can extend along magnetic field lines for hundreds of poloidal turns in tokamaks with weak or zero magnetic shear $\hat{s}$. We observe that this parallel eddy length scales inversely with magnetic shear and at $\hat{s}=0$ is limited by the thermal speed of electrons $v_{th,e}$. We examine the consequences of these "ultra long" eddies on turbulent transport, in particular, how field line topology mediates strong parallel self-interaction. Our investigation reveals that, through this process, field line topology can strongly affect transport. It can cause transitions between different turbulent instabilities and in some cases triple the logarithmic gradient needed to drive a given amount of heat flux. We also identify a novel "eddy squeezing" effect, which reduces the perpendicular size of eddies and their ability to transport energy, thus representing a novel approach to improve confinement. Finally, we investigate the triggering mechanism of Internal Transport Barriers (ITBs) using low magnetic shear simulations, shedding light on why ITBs are often easier to trigger where the safety factor has a low-order rational value.

机构、总结、中文摘要均由AI分析而来,可能存在错误,请以论文正文为准。

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