Regarding the optimization of O1-mode ECRH and the feasibility of EBW startup on NSTX-U

Recently published scenarios for fully non-inductive startup and operation on the National Spherical
Torus eXperiment Upgrade (NSTX-U) (Menard et al 2012 Nucl. Fusion 52 083015) show Electron
Cyclotron Resonance Heating (ECRH) as an important component in preparing a target plasma for
efficient High Harmonic Fast Wave and Neutral Beam heating. The modeling of the propagation and
absorption of EC waves in the evolving plasma is required to define the most effective window of
operation, and to optimize the launcher geometry for maximal heating and current drive during this
window. Here, we extend a previous optimization of O1-mode ECRH on NSTX-U to account for the
full time-dependent performance of the ECRH using simulations performed with TRANSP. We find
that the evolution of the density profile has a prominent role in the optimization by defining the time
window of operation, which in certain cases may be a more important metric to compare launcher
performance than the average power absorption. This feature cannot be captured by analysis on static
profiles, and should be accounted for when optimizing ECRH on any device that operates near the
cutoff density. Additionally, the utility of the electron Bernstein wave (EBW) in driving current and
generating closed flux surfaces in the early startup phase has been demonstrated on a number of
devices. Using standalone GENRAY simulations, we find that efficient EBW current drive is
possible on NSTX-U if the injection angle is shifted below the midplane and aimed towards the top
half of the vacuum vessel. However, collisional damping of the EBW is projected to be significant, in
some cases accounting for up to 97% of the absorbed EBW power

近日发表于国家球形环面实验装置升级款（National Spherical Torus eXperiment Upgrade，NSTX-U）的完全无感应启动与运行场景研究（Menard等人2012年《Nucl. Fusion》52卷083015期）表明，电子回旋共振加热（Electron Cyclotron Resonance Heating，ECRH）是为高效高谐波快波与中性束加热制备目标等离子体的关键手段。为确定该运行窗口的最优参数，并优化发射天线几何结构以实现该窗口内最大的加热效果与电流驱动效率，需对演化等离子体中EC波的传播与吸收过程进行建模。本文基于TRANSP模拟代码开展的仿真，将此前针对NSTX-U的O1模ECRH优化工作拓展至覆盖ECRH全时变性能的场景。研究发现，密度剖面的演化通过定义有效运行窗口对优化过程起到显著作用，在部分场景中，该窗口相较于平均吸收功率，是衡量天线性能更关键的指标。这一特性无法通过静态剖面分析捕捉，因此在任何运行于截止密度附近的装置中开展ECRH优化时，均需纳入考量。此外，电子伯恩斯坦波（electron Bernstein wave，EBW）在早期启动阶段驱动电流与生成闭合磁面的效用，已在多款实验装置中得到验证。通过独立的GENRAY仿真，我们发现，若将注入角度调整至中平面以下并对准真空室上半区域，NSTX-U上可实现高效的EBW电流驱动。但研究同时指出，EBW的碰撞阻尼效应预计将十分显著，在部分场景中其吸收功率占比可达97%。