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arXiv 2101.05976physics.plasm-ph
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球形托卡马克亚回旋压缩和整体Alfvén本征模稳定性的混合模拟

Hybrid simulations of sub-cyclotron compressional and global Alfvén Eigenmode stability in spherical tokamaks

J. B. Lestz, E. V. Belova, N. N. Gorelenkov

摘要:

为了研究束流驱动的亚回旋频率压缩(CAE)和全局(GAE)Alfvén本征模在宽束流参数范围内的稳定性,进行了一项全面的数值研究。在NSTX实验中,CAE和GAE的存在与高光束功率下的异常电子温度分布平坦化有关,这促使进一步研究其激发条件。线性模拟进行混合MHD动力学的初始值代码HYM,以捕捉一般多普勒频移回旋共振驱动的模式。在模拟中发现三种不同类型的模式-co-CAE,cntr-GAE和co-GAE-具有不同的光谱和稳定性。模拟结果表明,不稳定的GAE比不稳定的CAE更普遍存在,与实验观察一致,因为它们在较低的光束能量下被激发,并且通常具有较大的增长率。局部解析理论被用来解释模拟结果的关键特征,包括基于束注入几何形状的不同模式的优先激发和对束注入速度、临界速度和速度空间各向异性程度的增长率依赖。背景阻尼率是从模拟中推断出来的,并对模拟模型中不存在的相关源进行分析估计,这表明协同CAE比回旋共振驱动的模式更接近边缘稳定性。

英文摘要:

A comprehensive numerical study has been conducted in order to investigate the stability of beam-driven, sub-cyclotron frequency compressional (CAE) and global (GAE) Alfvén Eigenmodes in low aspect ratio plasmas for a wide range of beam parameters. The presence of CAEs and GAEs has previously been linked to anomalous electron temperature profile flattening at high beam power in NSTX experiments, prompting further examination of the conditions for their excitation. Linear simulations are performed with the hybrid MHD-kinetic initial value code HYM in order to capture the general Doppler-shifted cyclotron resonance that drives the modes. Three distinct types of modes are found in simulations -- co-CAEs, cntr-GAEs, and co-GAEs -- with differing spectral and stability properties. The simulations reveal that unstable GAEs are more ubiquitous than unstable CAEs, consistent with experimental observations, as they are excited at lower beam energies and generally have larger growth rates. Local analytic theory is used to explain key features of the simulation results, including the preferential excitation of different modes based on beam injection geometry and the growth rate dependence on the beam injection velocity, critical velocity, and degree of velocity space anisotropy. The background damping rate is inferred from simulations and estimated analytically for relevant sources not present in the simulation model, indicating that co-CAEs are closer to marginal stability than modes driven by the cyclotron resonances.

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