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Thermal ions dilution and ITG suppression in ASDEX Upgrade ion ITBs

机译：ASDEX升级离子ITB中的热离子稀释和ITG抑制

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摘要

Internal transport barriers (ITBs) in the ion channel in the tokamak ASDEX Upgrade allow for high energy confinement but collapse after only several energy confinement times. In this paper we show that in most cases the ITB phase is terminated clearly before the first ELM burst, thereby ruling out the ELMs as the main trigger of the ITB collapse. For the first time, the ITB formation and sustainment are found to be associated with a mechanism of transport suppression based on thermal ions dilution by the injected fast ions. Interestingly, such ITBs do not require reversed magnetic shear. The linear growth rate of the ion temperature gradient driven mode is computed as a function of the fast ion fraction with gyrokinetic stability analysis. Monte Carlo simulations predict the fast ion population to be above the gyrokinetic critical fraction in a region consistent with the experimental ITB width. The density threshold documented for the onset of ASDEX Upgrade ion ITBs is explained. The role of T_i/T_e and of the plasma sheared rotation for ITB sustainment are analysed. The stabilization mechanism presented here is consistent with the observed ITB lifetime of the order of the beam slowing down time. A possible runaway mechanism leading to ITB collapse is described. Finally, the relevance of this particular ITB scheme for ITER is discussed.
机译：托卡马克ASDEX升级版中离子通道中的内部传输壁垒（ITB）允许高能量限制，但仅经过几次能量限制时间便崩溃。在本文中，我们表明，在大多数情况下，ITB阶段明显在第一个ELM突发之前终止，从而排除了ELM作为ITB崩溃的主要触发因素。首次发现，ITB的形成和维持与基于热离子被注入的快速离子稀释而抑制迁移的机制有关。有趣的是，此类ITB不需要反向磁剪。离子温度梯度驱动模式的线性增长率是通过动态动力学稳定性分析得出的快速离子分数的函数。蒙特卡洛模拟预测，在与实验性ITB宽度一致的区域中，快速离子的数量将高于陀螺运动的关键分数。解释了ASDEX升级版离子ITB出现时记录的密度阈值。分析了T_i / T_e和等离子剪切旋转对ITB维持的作用。这里介绍的稳定机制与观察到的ITB寿命（光束减慢时间的数量级）一致。描述了导致ITB崩溃的一种可能的失控机制。最后，讨论了该特定ITB方案与ITER的相关性。

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