The 2018 operation phase (OP 1.2b) of the stellarator Wendelstein 7-X (W7-X) included, for the first time, neutral beam injection (NBI) to heat the plasma. Since the injection geometry at W7-X is not parallel, this generates both passing and trapped fast particles. During longer phases of NBI injection, with the primary purpose to study the heating efficiency of this system, Alfven eigenmodes (AEs) were observed by a number of diagnostics, including the phase contrast imaging (PCI) system, the magnetic pick-up coils (Mirnov coils), and the soft X-ray multi-camera tomography system (XMCTS). Alfven eigenmodes are of great interest for future fusion reactors as it has been shown that the resonant interaction of fast ions with self-excited AEs can lead to enhanced transport of fast ions and potentially to energy losses. This is especially true for so-called gap-modes, Alfven eigenmodes with frequencies in gaps of the continuous spectrum, since they lack continuum damping. These modes are commonly known to be excited by fast ions, but other destabilizing mechanisms, e.g. the electron-pressure gradient are also possible. In this article we present a first analysis of the experimentally observed frequencies from the theoretical side. The calculation of shear Alfven wave continua for selected cases and the assignment of observed frequencies to the gaps of the continuous spectra are presented. Using the ideal-MHD code CKA, we find gap modes that match the experimental measurements in terms of the observed frequencies. We emphasize the crucial roles played by the coupling of sound and Alfven waves as well as of the Doppler shift arising as a consequence of the radial electric field in W7-X. We employ the perturbative gyrokinetic code CKA-EUTERPE, using a slowing-down distribution function for the fast ions as calculated by the Monte-Carlo particle following code ASCOT to assess the fast-ion drive. We find that the fast-ion drive is insufficient to overcome the background-plasma damping. The fact that unstable modes were observed experimentally may point to problems with the modelling or indicate the existence of other destabilizing mechanisms, e.g. associated with the electron-pressure gradient that sensitively depend on the profiles of the background plasma.

中文翻译：

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Wendelstein 7-X NBI 加热实验中模式活性的研究

仿星器 Wendelstein 7-X (W7-X) 的 2018 年运行阶段 (OP 1.2b) 首次包括中性束注入 (NBI) 以加热等离子体。由于 W7-X 处的喷射几何形状不平行，这会产生通过和捕获的快速粒子。在 NBI 注入的较长阶段期间，主要目的是研究该系统的加热效率，通过许多诊断观察到 Alfven 特征模态 (AE)，包括相衬成像 (PCI) 系统、磁性拾波线圈 ( Mirnov 线圈）和软 X 射线多相机断层扫描系统（XMCTS）。阿尔芬本征模式对未来的聚变反应堆非常感兴趣，因为已经表明快离子与自激 AE 的共振相互作用会导致快离子的传输增强并可能导致能量损失。这对于所谓的间隙模式，即频率在连续谱间隙中的阿尔芬本征模式尤其如此，因为它们缺乏连续阻尼。这些模式通常被快速离子激发，但其他不稳定机制，例如电子压力梯度也是可能的。在本文中，我们从理论方面对实验观察到的频率进行了首次分析。介绍了选定情况下剪切阿尔芬波连续谱的计算以及将观察到的频率分配给连续谱的间隙。使用理想 MHD 代码 CKA，我们找到了在观察频率方面与实验测量相匹配的间隙模式。我们强调声波和阿尔文波的耦合以及 W7-X 中径向电场引起的多普勒频移所起的关键作用。我们采用微扰陀螺动力学代码 CKA-EUTERPE，使用由蒙特卡洛粒子计算的快离子的减速分布函数按照代码 ASCOT 来评估快离子驱动。我们发现快离子驱动不足以克服背景等离子体阻尼。通过实验观察到不稳定模式的事实可能表明建模存在问题或表明存在其他不稳定机制，例如与敏感地依赖于背景等离子体轮廓的电子压力梯度相关。使用由蒙特卡洛粒子按照代码 ASCOT 计算的快离子的减速分布函数来评估快离子驱动。我们发现快离子驱动不足以克服背景等离子体阻尼。通过实验观察到不稳定模式的事实可能表明建模存在问题或表明存在其他不稳定机制，例如与敏感地依赖于背景等离子体轮廓的电子压力梯度相关。使用由蒙特卡洛粒子按照代码 ASCOT 计算的快离子的减速分布函数来评估快离子驱动。我们发现快离子驱动不足以克服背景等离子体阻尼。通过实验观察到不稳定模式的事实可能表明建模存在问题或表明存在其他不稳定机制，例如与敏感地依赖于背景等离子体轮廓的电子压力梯度相关。