Modelling of the EAST lower-hybrid current drive experiment using GENRAY/CQL3D and TORLH/CQL3D

The coupled GENRAY‐CQL3D code has been used to do systematic ray‐tracing and Fokker‐Planck analysis for EAST Lower Hybrid wave Current Drive (LHCD) experiments. Despite being in the weak absorption regime, the experimental level of LH current drive is successfully simulated, by taking into account the variations in the parallel wavenumber due to the toroidal effect. The effect of radial transport of the fast LH electrons in EAST has also been studied, which shows that a modest amount of radial transport diffusion can redistribute the fast LH current significantly. Taking advantage of the new capability in GENRAY, the actual Scrape Off Layer (SOL) model with magnetic field, density, temperature, and geometry is included in the simulation for both the lower and the higher density cases, so that the collisional losses of Lower Hybrid Wave (LHW) power in the SOL has been accounted for, which together with fast electron losses can reproduce the LHCD experimental observations in different discharges of EAST. We have also analyzed EAST discharges where there is a significant ohmic contribution to the total current, and good agreement with experiment in terms of total current has been obtained. Also, the full‐wave code TORLH has been used for the simulation of the LH physics in the EAST, including full‐wave effects such as diffraction and focusing which may also play an important role in bridging the spectral gap. The comparisons between the GENRAY and the TORLH codes are done for both the Maxwellian and the quasi‐linear electron Landau damping cases. These simulations represent an important addition to the validation studies of the GENRAY‐CQL3D and TORLH models being used in weak absorption scenarios of tokamaks with large aspect ratio.本研究采用耦合GENRAY-CQL3D代码，针对EAST装置的低杂波电流驱动（Lower Hybrid wave Current Drive, LHCD）实验开展系统性射线追踪（ray-tracing）与福克-普朗克分析（Fokker-Planck analysis）。尽管处于弱吸收工况（weak absorption regime），研究通过考虑环向效应（toroidal effect）引发的平行波数（parallel wavenumber）变化，成功复现了实验中的低杂波电流驱动水平。本研究还探究了EAST装置中快速低杂波电子的径向输运（radial transport）效应，结果表明适度的径向输运扩散可显著重塑快速低杂波电流的分布。借助GENRAY新增的功能，仿真中纳入了包含磁场、密度、温度与几何构型的真实刮削层（Scrape Off Layer, SOL）模型，覆盖高低密度两类工况，从而考虑了低杂波（Lower Hybrid Wave, LHW）功率在刮削层中的碰撞损耗；结合快速电子损耗效应，该模型可复现EAST不同放电实验中的低杂波电流驱动观测结果。本研究还分析了总电流存在显著欧姆分量（ohmic contribution）的EAST放电实验，最终在总电流层面实现了与实验的良好吻合。此外，本研究还采用全波代码（full-wave code）TORLH对EAST中的低杂波物理过程开展仿真，涵盖衍射（diffraction）、聚焦（focusing）等全波效应——这些效应在弥合频谱间隙（spectral gap）方面同样发挥关键作用。针对麦克斯韦分布（Maxwellian）与准线性电子朗道阻尼（quasi-linear electron Landau damping）两种场景，本研究完成了GENRAY与TORLH两类代码的对比分析。上述仿真工作为采用大纵横比（aspect ratio）托卡马克（tokamak）弱吸收工况下的GENRAY-CQL3D与TORLH模型的验证研究提供了重要补充。