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The role of the radial electric field for the transition to high confinement regimes

机译：径向电场在过渡到高约束状态中的作用

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The radial electric field E_r(x,t), and particularly its gradient, has been invoked by various theories and empirical models as a crucial parameter 'per se' for determining the transition to high confinement regimes, such as the onset of an internal transport barrier (ITB) in the plasma core and of the H-mode pedestal at the plasma edge. This idea, however, does not consider the basic fact that in most experiments the transition to a steady-state higher confinement regimes is produced by applying sufficient additional heating onto a given target density and current profile. In order to test this ansatz on a more routine basis, we have developed here an analytical approximation to the neoclassical calculation of the radial electric field, adapted for the 2D toroidal geometry of JET to describe all collisionality regimes (banana, banana-plateau, Pfirsch-Schlueter) and to include averaging over the potato orbits. An analytic calculation of the error bars on E_r(x, t) has also been developed, which has allowed us to compare and successfully benchmark our calculations with the results of neoclassical codes such as JETTO and NCLASS. We are then able to demonstrate a striking similarity in the shape of E_r(x, t) in steady-state L-mode, H-mode and ITB plasmas when normalizing E_r(x, t) with respect to the total heating power flux. This clearly indicates that, experimentally, there is no direct causality relation between changes in E_r(x, t) and steady-state improved confinement, as these are brought about together by changes in the power deposition profile. Only two cases do not satisfy this general rule. First, localized and rapid transients (i.e. occurring on time scales much shorter than the momentum and energy confinement time) could be linked to non-neoclassical changes in E_r(x, t), possibly due to turbulence suppression mechanisms. Second, when comparing H-mode plasmas with forward and reversed ion ▽B-drift direction, we demonstrate the role of prompt fast ion losses in affecting E_r(x, t), most likely due to the different edge flows and their cascading effect onto the core plasma.
机译：各种理论和经验模型已将径向电场E_r（x，t）尤其是其梯度用作确定过渡到高约束状态（例如内部运输开始）的关键参数“本身”等离子体核心中的势垒（ITB）和等离子体边缘的H型基座。但是，该想法并未考虑以下基本事实：在大多数实验中，通过在给定的目标密度和电流分布图上施加足够的额外热量，可以实现向稳态更高限制状态的过渡。为了更常规地测试该ansatz，我们在此开发了径向电场新古典计算的解析近似方法，适用于JET的2D环形几何形状，以描述所有碰撞方式（香蕉，香蕉高原，普菲尔希-Schlueter），并包括对马铃薯轨道进行平均。还开发了对E_r（x，t）上的误差线的解析计算，这使我们能够与JETTO和NCLASS等新古典代码的结果进行比较，并成功地对我们的计算进行基准测试。然后，当相对于总加热功率通量标准化E_r（x，t）时，我们能够证明在稳态L模式，H模式和ITB等离子体中E_r（x，t）的形状具有惊人的相似性。这清楚地表明，从实验上看，E_r（x，t）的变化与稳态改善的约束之间没有直接因果关系，因为这些都是通过功率沉积曲线的变化而聚在一起的。只有两种情况不满足该一般规则。首先，局部和快速的瞬变（即发生在比动量和能量约束时间短得多的时间尺度上）可能与E_r（x，t）的非新古典变化有关，这可能是由于湍流抑制机制引起的。其次，当将正向和反向离子▽B漂移方向的H型等离子体进行比较时，我们证明了快速的快速离子损失在影响E_r（x，t）中的作用，这很可能是由于不同的边缘流及其级联效应核心等离子体。

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