A comprehensive numerical study has been conducted in order to investigate the stability of beam-driven, sub-cyclotron-frequency compressional Alfvn eigenmodes (CAEs) and global Alfvn eigenmodes (GAEs) 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 powers in NSTX experiments, prompting a further examination of the conditions necessary for their excitation. Linear simulations have been 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 were found in the simulations—co-CAEs, cntr-GAEs, and co-GAEs—with differing spectral and stability properties. The simulations revealed that unstable GAEs are more ubiquitous than unstable CAEs, which is 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 the simulations and analytically estimated for relevant sources absent from the simulation model, indicating that co-CAEs are closer to marginal stability than modes driven by the cyclotron resonances.

进行了全面的数值研究，以研究低纵横比等离子体中光束驱动、亚回旋频率压缩 Alfvn 本征模 (CAE) 和全局 Alfvn 本征模 (GAE) 的稳定性，适用于各种光束参数. CAE 和 GAE 的存在先前已与 NSTX 实验中高光束功率下异常电子温度分布变平有关，促使进一步检查其激发所需的条件。使用混合MHD-动力学初始值代码HYM进行了线性模拟为了捕获驱动模式的一般多普勒频移回旋共振。在模拟中发现了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特性。模拟表明，不稳定的 GAE 比不稳定的 CAE 更普遍，这与实验观察一致，因为它们在较低的光束能量下被激发并且通常具有更大的增长率。局部解析理论用于解释模拟结果的关键特征，包括基于射束注入几何形状的不同模式的优先激发以及对射束注入速度、临界速度和速度空间各向异性程度的增长率依赖性。