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arXiv 2005.11013physics.plasm-ph
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球形托克马克压缩和整体阿尔芬本征模稳定性的理论与模拟

Theory and Simulations of Compressional and Global Alfven Eigenmode Stability in Spherical Tokamaks

J. B. Lestz

摘要:

中子束驱动的亚回旋压缩(CAE)和全局(GAE)\Alfven本征模在NSTX(-U)和MAST等球形托卡马克中经常被激发,在常规纵横比托卡马克DIII-D上也观察到,在ITER燃烧等离子体中可能不稳定。他们的存在已被实验联系到异常平坦的电子温度分布在高束流功率在NSTX,潜在地限制聚变性能。因此,详细了解CAE/GAE激发对于预测(并最终控制)它们对等离子体约束的影响至关重要。为此,混合动力学MHD模拟的补充与分析研究的线性稳定性的CAE和GAE。微扰,局部解析理论已被用来获得新的不稳定性条件的CAEs/GAE驱动的现实中性束分布。一套全面的模拟NSTX类等离子体已经进行了广泛的束流参数,提供了丰富的信息,在球形托卡马克的CAE和GAE稳定性。线性模拟表明,激发的CAEs与GAE有一个复杂的依赖于快速离子注入速度和几何形状,定性描述的解析理论在本论文中发展。在模拟中发现了GAE的强高能粒子修饰,表明存在一种新型的高频高能粒子模式。理论的稳定性界限,模拟结果和实验测量之间的交叉验证显示出有利的协议不稳定的CAE和GAE光谱的依赖于快离子参数。分析结果准确地解释了最近的实验发现GAE稳定与少量的离轴光束注入NSTX-U,并建议在未来的实验中控制这些不稳定性的新技术。

英文摘要:

Neutral-beam-driven, sub-cyclotron compressional (CAE) and global (GAE) \Alfven eigenmodes are routinely excited in spherical tokamaks such as NSTX(-U) and MAST, have been observed on the conventional aspect ratio tokamak DIII-D, and may be unstable in ITER burning plasmas. Their presence has been experimentally linked to the anomalous flattening of electron temperature profiles at high beam power in NSTX, potentially limiting fusion performance. A detailed understanding of CAE/GAE excitation, therefore, is vital to predicting (and ultimately controlling) their effects on plasma confinement. To this end, hybrid kinetic-MHD simulations are complemented with an analytic study of the linear stability properties of CAEs and GAEs. Perturbative, local analytic theory has been used to derive new instability conditions for CAEs/GAEs driven by realistic neutral beam distributions. A comprehensive set of simulations of NSTX-like plasmas has been performed for a wide range of beam parameters, providing a wealth of information on CAE and GAE stability in spherical tokamaks. Linear simulations show that the excitation of CAEs vs GAEs has a complex dependence on the fast ion injection velocity and geometry, qualitatively described by the analytic theory developed in this thesis. Strong energetic particle modifications of GAEs are found in simulations, indicating the existence of a new type of high frequency energetic particle mode. A cross validation between the theoretical stability bounds, simulation results, and experimental measurements shows favorable agreement for both the unstable CAE and GAE spectra's dependence on fast ion parameters. The analytic results accurately explain the recent experimental discovery of GAE stabilization with small amounts of off-axis beam injection on NSTX-U and suggest new techniques for control of these instabilities in future experiments.

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