Modeling very high electron heating by radio frequency waves on EAST

In the 2018 EAST experimental campaign, a very high central electron heating, fully-non-inductive discharge with the core electron temperature over 9 keV has beenachieved. Such high central electron heating was realized by injecting radio frequency waves, including 1.8 MW lower hybrid wave (LHW) and 0.8 MW electron–cyclotron waves (ECW).Experimental diagnosis indicates two different time scales characterizing the electron heating process, a rapid and a slow rise of the central electron temperature after the injection of ECW.In this work, integrated modeling is performed to investigate the physical mechanisms of such high electron heating. Five characteristic phases during the increase of the electrontemperature are chosen for modeling. In phase 1, the electron heating is by LHW alone. The modeling confrms that the LHW can only sustain the core electron temperature Te ≈ 5.5 keV,which is consistent with the experiment. In phase 2, the electron temperature increases rapidly after the frst 0.4 MW ECW is injected. The result shows that the rapid increase of the electrontemperature is due to the interaction between the ECW and the electrons. With the increase of the electron temperature, the electron flux induced by the trapped electron modes (TEMs) andthe electron temperature gradient driven modes (ETGs) is enhanced in the core region. In phase 3, the electron temperature increases slowly after phase 2. It is found that the slowincrease is mainly due to the flattening of the density profle. The flattening of the density profle can decrease the thermal diffusivity of the electrons mainly induced by the TEMsleading to a higher electron temperature for a given heating source. In phase 4, the electron temperature again increases rapidly after the second 0.4 MW ECW is injected. The physicalmechanism is similar to that in phase 2. In phase 5, the LHW power deposition of the LHW remains almost unchanged compared to that in phase 4 since the electron temperature issuffciently high. The slow rise of the electron temperature is caused by the improvement of the electron energy confnement as thermal diffusivity of the electrons is decreased due to theflattening of the electron density profle, which is similar to the main reason in phase 3.

在2018年EAST装置实验运行阶段，科研人员实现了中心电子加热极强的完全非感应放电，其芯区电子温度超过9千电子伏特（keV）。该高中心电子加热效果通过注入射频波实现，包括1.8兆瓦的低混杂波（lower hybrid wave, LHW）与0.8兆瓦的电子回旋波（electron–cyclotron waves, ECW）。实验诊断结果表明，电子加热过程存在两种不同的时间尺度：在注入电子回旋波后，中心电子温度会经历快速上升与缓慢上升两个阶段。本研究通过一体化建模方法，探究此类高强度电子加热的物理机制。本次建模选取了电子温度上升过程中的五个典型阶段进行分析：在阶段1，仅通过低混杂波进行电子加热。建模结果证实，低混杂波仅能维持芯区电子温度Te≈5.5千电子伏特，与实验结果相符。在阶段2，首次注入0.4兆瓦电子回旋波后，电子温度快速上升。结果表明，电子温度的快速上升源于电子回旋波与电子的相互作用。随着电子温度升高，芯区由捕获电子模（trapped electron modes, TEMs）与电子温度梯度驱动模（electron temperature gradient driven modes, ETGs）引发的电子通量得到增强。在阶段3，电子温度在阶段2结束后进入缓慢上升阶段。研究发现，该缓慢上升主要源于密度分布的展平。密度分布展平可降低主要由捕获电子模引发的电子热扩散系数，使得在加热源功率固定的条件下，电子温度得以提升。在阶段4，第二次注入0.4兆瓦电子回旋波后，电子温度再次快速上升，其物理机制与阶段2一致。在阶段5，由于电子温度已足够高，低混杂波的功率沉积与阶段4相比几乎无变化。电子温度的缓慢上升源于电子能量约束性能的提升：电子密度分布展平导致电子热扩散系数降低，这一机制与阶段3的核心原因一致。