Beam modulation effects on Alfvn eigenmode stability have been investigated in a recent DIII-D experiment and show that variations in neutral beam modulation period can have an impact on the beam driven Alfvn eigenmode spectrum and resultant fast ion transport despite similar time-averaged input power. The experiment was carried out during the current ramp phase of L-mode discharges heated with sub-Alfvnic 50–80 kV deuterium neutral beams that drive a variety of Alfvn eigenmodes unstable. The modulation period of two interleaved beams with different tangency radii was varied from shot to shot in order to modify the relative time dependent mix of the beam pitch angle distribution as well as the persistence of a bump-on-tail feature near the injection energy (a feature confirmed by imaging neutral particle analyzer measurements). As the beam modulation period is varied from 7ms to 30ms on/off (typical full energy slowing down time of τ _slow ≈ 50ms at mid-radius), toroidicity-induced Alfvn eigenmodes (TAEs) located in the outer periphery of the plasma become intermittent and coincident with the more tangential beam. Core mode activity changes from reversed shear Alfvn eigenmodes (RSAEs) to a mix of RSAE and beta-induced Alfvn eigenmodes. Discharges with 30ms on/off period do not have a persistent bump-on-tail feature, have the lowest average mode amplitude and least fast ion transport. Detailed analysis of an individual TAE using TRANSP kick modeling (Monte Carlo evolution of the distribution function with probabilistic ‘kicks’ by the AEs) and the resistive MHD code with kinetic fast ions, MEGA, find no strong role of energy gradient drive due to bump-on-tail features. Instead, the observed TAE modulation with interleaved beams is likely a pitch angle dependent result combined with slowing down of the tangential beam between pulses. For the conditions investigated, bump-on-tail contributions to TAE drive were found to be 5% or less of the total drive at any given time.

在最近的 DIII-D 实验中研究了射束调制对 Alfvn 特征模稳定性的影响，并表明中性束调制周期的变化会对束驱动的 Alfvn 特征模谱和由此产生的快速离子传输产生影响，尽管时间平均输入功率相似。该实验是在 L 模式放电的电流斜坡阶段进行的，该电流由亚 Alfvnic 50-80 kV 氘中性束加热，这些束驱动各种不稳定的 Alfvn 本征模式。具有不同切线半径的两个交错光束的调制周期在每次发射时都不同，以修改光束俯仰角分布的相对时间相关混合以及注入能量附近的尾部撞击特征的持久性（通过成像中性粒子分析仪测量证实的特征）τ _慢≈ 50 毫秒（在中半径处）），位于等离子体外围的环面诱导的 Alfvn 特征模态 (TAE) 变得断断续续，并与更切向的光束重合。核心模式活动从反向剪切 Alfvn 特征模式 (RSAE) 变为 RSAE 和β诱导的 Alfvn 特征模式的混合。具有 30 毫秒开/关周期的放电不具有持续的尾部碰撞特征，具有最低的平均模式振幅和最慢的离子传输速度。使用 TRANSP 踢模型（AE 概率“踢”的分布函数的蒙特卡洛演化）和具有动力学快离子 MEGA 的电阻 MHD 代码对单个 TAE 进行详细分析，发现由于碰撞没有发现能量梯度驱动的强作用尾特征。反而，观察到的交错波束的 TAE 调制可能是俯仰角相关的结果，结合脉冲之间切向波束的减慢。对于所研究的条件，发现在任何给定时间，尾部碰撞对 TAE 驱动的贡献为总驱动的 5% 或更少。