Modeling study of synergistic effects between lower hybrid and electron cyclotron current drive on EAST

Recent radio frequency (RF) current drive experiments in EAST provide an opportunity to investigate RF synergy effects between Electron Cyclotron (EC) and Lower Hybrid (LH) current drive. Synergy current is the excess current that can arise from overlapping two wave-particle resonance bands in the velocity phase space. In this work, a ray-tracing and Fokker-Planck code package, GENRAY/CQL3D, is used to model and investigate the phase space interaction of two waves for EAST. The target plasma features a high central electron temperature of 6.5 keV, promoting Landau damping of the LH power on-axis that overlaps with the EC power resonance region. The GENRAY/CQL3D code evaluates a self-consistent electron distribution function in the presence of two RF quasilinear diffusion coefficients. A systematic scan in collisional dissipative LH power loss and fast-electron radial transport is performed. Including the divertor SOL in GENRAY and fast-electron radial diffusion in CQL3D can quantitatively provide model total RF currents in line with the experimental range. Radial transport operator broadens and smooths the radial region of synergistic current generation. The impact of LH wave scattering on the synergistic interaction is examined by introducing the LH wave scattering angle in a heuristic manner. The sensitivity analysis is presented. Furthermore, a numerical scan of the ECCD injection angles is conducted to evaluate the synergistic interactions. The roles of LHCD and ECCD in synergistic current generation are further examined through a power scan of both power sources.

近期在EAST装置上开展的射频（Radio Frequency, RF）电流驱动实验，为探究电子回旋（Electron Cyclotron, EC）与低混杂（Lower Hybrid, LH）电流驱动间的射频协同效应提供了契机。协同电流指在速度相空间中，两种波-粒子共振带发生重叠时产生的额外电流。本研究采用射线追踪（ray-tracing）与福克-普朗克（Fokker-Planck）代码套件GENRAY/CQL3D，对EAST装置上双波的相空间相互作用进行建模与分析。目标等离子体具有6.5 keV的高中心电子温度，这使得轴向上的低混杂功率朗道阻尼（Landau damping）与电子回旋功率的共振区域发生重叠。GENRAY/CQL3D代码可在双射频准线性扩散系数（quasilinear diffusion coefficients）存在的条件下，求解自洽的电子分布函数。本研究针对碰撞耗散型低混杂功率损耗与快电子径向输运开展了系统性扫描。在GENRAY中纳入偏滤器刮削层（Divertor Scrape-Off Layer, SOL）、并在CQL3D中引入快电子径向扩散后，模型计算得到的总射频电流可与实验观测范围定量吻合。径向输运算子会拓宽并平滑协同电流的产生径向区域。本研究通过启发式引入低混杂波散射角，探究了低混杂波散射对协同相互作用的影响，并给出了相应的敏感性分析结果。此外，本研究还通过对电子回旋电流驱动（Electron Cyclotron Current Drive, ECCD）的注入角度开展数值扫描，评估了其与低混杂电流驱动（Lower Hybrid Current Drive, LHCD）间的协同相互作用。通过对两类射频源的功率进行扫描，本研究进一步阐明了LHCD与ECCD在协同电流产生过程中的具体作用。