Characteristics of electron temperature profile stiffness in electron-heated plasmas on EAST

A very high core electron temperature (T (e0) similar to 10 keV) plasma has been established and stably sustained by applying both lower hybrid wave (LHW) and on-axis electron cyclotron resonance heating (ECRH) in the Experimental Advanced Superconducting Tokamak (EAST). In this work, power balance analysis shows that the increase of ECRH power can increase the normalized T (e) gradient significantly at the plasma core region (rho < 0.6), but does not change the T (e) profile stiffness in the low-density L-mode plasmas. This has been considered to be due to a strong synergistic effect between ECRH and LHW. Furthermore, three distinguishable stages characterized by different T (e) profile stiffnesses can be identified from the density ramp-up in the electron-heated plasma on EAST. A stronger T (e) profile stiffness at rho = 0.3 has been observed in the Stage-II, where the LHW power deposition gradually moves away from the plasma core region, following the electron density increases. Furthermore, the formation of an internal plasma density transport barrier inside rho similar to 0.6, accompanied by a sudden drop in core T (e) and a rise in both core plasma density and ion temperature, has been observed for the first time during the transition from the Stage-II to the Stage-III when the central line-averaged plasma density reaches a threshold of 2.2 x 10(19) m(-3). This finding strongly affects further development of high-performance gas-fueled electron-heated plasma scenarios in EAST and suggests an advanced operational regime with a wide internal plasma density transport barrier.