Accessing the density-free regime with ECRH-assisted ohmic start-up on EAST.

Abstract

High plasma density operation is crucial for a tokamak to achieve energy breakeven and burning plasma. However, there is often an empirical upper limit of electron density in tokamak operation, namely, the Greenwald density limit [Formula: see text], above which tokamaks generally disrupt. Achieving high-density operation above the density limit has been a long-standing challenge in magnetic confinement fusion research. Here, we report experimental results on the Experimental Advanced Superconducting Tokamak (EAST) achieving line-averaged electron density in the range of (1.3 to 1.65) [Formula: see text], significantly above the typical EAST operational range of (0.8 to 1.0) [Formula: see text]. This is performed with electron cyclotron resonance heating (ECRH)-assisted ohmic start-up and sufficiently high initial neutral density. These experiments are shown to operate in the density-free regime first predicted by a recent plasma-wall self-organization theory. These results suggest a promising scheme for substantially increasing the density limit in tokamaks, a critical advancement toward achieving burning plasma.