Beam modulation and bump-on-tail effects on Alfvén eigenmode stability in DIII-D
Beam modulation and bump-on-tail effects on Alfvén eigenmode stability in DIII-D
复制标题
光束调制和尾部碰撞对 DIII-D 中 Alfvén 本征模稳定性的影响
DOI：
10.1088/1741-4326/abf174
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发表时间：
2021
期刊：
影响因子：
3.3
通讯作者：
X. Wang
中科院分区：
文献类型：
--
作者：
M. V. Van Zeeland;L. Bardoczi;J. Gonzalez;W. Heidbrink;M. Podestà;M. Austin;C. Collins;X. Du;V. Duarte;M. Garcia;S. Munaretto;K. Thome;Y. Todo;X. Wang
在最近的DIII-D实验中，研究了束调制对Alfvén本征模稳定性的影响，结果表明，尽管时间平均输入功率相似，但中性束调制周期的变化可以影响束驱动的Alfvén本征模频谱和由此产生的快速离子输运。实验是在50-80kV次AlfvéNic50-80KV中性束加热的L模放电的电流斜坡阶段进行的，这种中性束驱动各种Alfvén本征模不稳定。具有不同切线半径的两个交错光束的调制周期随发射而变化，以修正光束俯仰角分布的相对随时间变化的混合以及注入能量附近尾部凸起特征的持久性(成像中性粒子分析仪测量证实了这一特征)。当束调制周期从7ms变化到30ms开/关时(典型的τSlow≈中径50ms的全能量慢化时间)，位于等离子体外周的环面诱导的Alfvén本征模(TAE)变得间歇，并与更切向的束流重合。芯模活动从反向剪切Alfvén本征模(RSAE)转变为RSAE和β诱导Alfvén本征模的混合。具有30ms开/关周期的放电不具有持久的尾部凹凸特征，具有最低的平均模式幅度和最不快的离子传输。使用TRANSP Kick模型(用AEs的概率‘Kick’对分布函数的蒙特卡罗演化)和使用动能快离子MHD代码的详细分析发现，由于尾部碰撞的特征，能量梯度驱动没有很强的作用。相反，观察到的交错光束的TAE调制很可能是与俯仰角相关的结果，以及脉冲之间切向光束的减速。在所调查的条件下，在任何给定时间，对TAE驱动器的尾部撞击贡献都不超过总驱动器的5%。
Beam modulation effects on Alfvén 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 Alfvén 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-Alfvénic 50–80 kV deuterium neutral beams that drive a variety of Alfvén 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 7 ms to 30 ms on/off (typical full energy slowing down time of τ slow ≈ 50 ms at mid-radius), toroidicity-induced Alfvén 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 Alfvén eigenmodes (RSAEs) to a mix of RSAE and beta-induced Alfvén eigenmodes. Discharges with 30 ms 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.