离子温度梯度湍流从有限到弱磁剪切区域

中文摘要 AI 辅助

托卡马克是一种约束热等离子体的环形磁装置，是实现聚变能最先进的方法之一。其核心障碍是由离子温度梯度（ITG）模驱动的湍流输运。依赖弱或零磁剪切芯部的运行方案，如ITER混合方案，已实现显著改善的约束性能，但其潜在机制尚不清楚。在此，我们表明ITG极向谐波的径向宽度不仅受熟悉的平行朗道阻尼约束，还受垂直于磁场平面内微湍流各向同性的约束。在弱剪切极限下，微各向同性占主导，并将径向宽度限制在一个极向波长内，从而产生临界磁剪切$s_{\text{crit}} \approx 1/(2\pi)$。高于此阈值时，扩展的II型气球模占主导；低于此阈值时，局域的I型模出现，仅包含两到三个谐波。涵盖DIII-D、JET和ITER参数的全局回旋动力学模拟证实了这一几何判据。具有持续加热的非线性模拟表明，弱剪切等离子体自发形成内部输运垒，湍流在由$|s| < s_{\text{crit}}$确定的径向区域被抑制。这一几何判据定义了弱磁剪切区域，并为聚变等离子体中的湍流输运提供了新视角。

英文摘要

The tokamak, a toroidal magnetic device confining a hot plasma, is one of the most advanced approaches to fusion energy. A central obstacle is turbulent transport driven by the ion-temperature-gradient (ITG) mode. Operation scenarios that rely on a weak or zero magnetic shear core, such as the ITER hybrid scenario, have achieved markedly improved confinement, yet the underlying mechanism has remained unclear. Here we show that the radial width of ITG poloidal harmonics is constrained not only by the familiar parallel Landau damping, but also by the isotropy of micro-turbulence in the plane perpendicular to the magnetic field. In the weak-shear limit, micro-isotropy dominates and constrains the radial width at one poloidal wavelength, yielding a critical magnetic shear $s_{\text{crit}} \approx 1/(2π)$. Above this threshold, the extended Type II ballooning modes prevail; below it, the localized Type I modes emerge, comprising only two or three harmonics. Global gyrokinetic simulations spanning DIII-D, JET~and ITER parameters confirm this geometric criterion. Nonlinear simulations with sustained heating show that weak-shear plasmas spontaneously form internal transport barriers with the turbulence suppressed in the radial region determined by $|s| < s_{\text{crit}}$. This geometric criterion defines the weak-magnetic-shear regime and offers a fresh perspective on turbulent transport in fusion plasmas.

发表机构

• University of Science and Technology of China（中国科学技术大学）

机构由 AI 辅助整理，请以论文原文为准。

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14 pages, 9 figures