We report on a detailed study of magnetic fluctuations in the JET pedestal, employing basic theoretical considerations, gyrokinetic simulations, and experimental fluctuation data, to establish the physical basis for their origin, role, and distinctive characteristics. We demonstrate quantitative agreement between gyrokinetic simulations of microtearing modes (MTMs) and two magnetic frequency bands with corresponding toroidal mode numbers n=4 and 8. Such disparate fluctuation scales, with substantial gaps between toroidal mode numbers, are commonly observed in pedestal fluctuations. Here we provide a clear explanation, namely the alignment of the relevant rational surfaces (and not others) with the peak in the omega star profile, which is localized in the steep gradient region of the pedestal. We demonstrate that a global treatment is required to capture this effect. Nonlinear simulations suggest that the MTM fluctuations produce experimentally-relevant transport levels and saturate by relaxing the background electron temperature gradient, slightly downshifting the fluctuation frequencies from the linear predictions. Scans in collisionality are compared with simple MTM dispersion relations. At the experimental points considered, MTM growth rates can either increase or decrease with collision frequency depending on the parameters thus defying any simple characterization of collisionality dependence.

中文翻译：

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微撕裂模式作为 JET 基座中磁波动的来源

我们报告了对 JET 基座中磁波动的详细研究，采用基本理论考虑、陀螺动力学模拟和实验波动数据，为它们的起源、作用和独特特征建立物理基础。我们证明了微撕裂模式 (MTM) 的陀螺动力学模拟与具有相应环形模式数 n = 4 和 8 的两个磁频带之间的定量一致性。这种不同的波动尺度，环形模式数之间存在巨大差距，通常在基座波动中观察到。在这里，我们提供了一个清晰的解释，即相关有理表面（而不是其他表面）与 omega 星轮廓中的峰值对齐，该峰值位于基座的陡峭梯度区域。我们证明需要全局处理来捕捉这种效果。非线性模拟表明，MTM 波动产生与实验相关的传输水平，并通过放宽背景电子温度梯度而饱和，从线性预测略微降低波动频率。将碰撞扫描与简单的 MTM 色散关系进行比较。在所考虑的实验点上，MTM 增长率可以根据参数随碰撞频率增加或减少，因此无法对碰撞依赖性进行任何简单的表征。稍微降低线性预测的波动频率。将碰撞扫描与简单的 MTM 色散关系进行比较。在所考虑的实验点上，MTM 增长率可以根据参数随碰撞频率增加或减少，因此无法对碰撞依赖性进行任何简单的表征。稍微降低线性预测的波动频率。将碰撞扫描与简单的 MTM 色散关系进行比较。在考虑的实验点上，MTM 增长率可以随碰撞频率增加或减少，具体取决于参数，因此无法对碰撞依赖性进行任何简单的表征。