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 been achieved. 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 electron temperature are chosen for modeling. In phase 1, the electron heating is by LHW alone. The modeling confirms that the LHW can only sustain the core electron temperature T (e) approximate to 5.5 keV, which is consistent with the experiment. In phase 2, the electron temperature increases rapidly after the first 0.4 MW ECW is injected. The result shows that the rapid increase of the electron temperature 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) and the 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 slow increase is mainly due to the flattening of the density profile. The flattening of the density profile can decrease the thermal diffusivity of the electrons mainly induced by the TEMs leading 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 physical mechanism 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 is sufficiently high. The slow rise of the electron temperature is caused by the improvement of the electron energy confinement as thermal diffusivity of the electrons is decreased due to the flattening of the electron density profile, which is similar to the main reason in phase 3.