Description
Plasma disruptions are large catastrophic events in tokamak nuclear fusion burning plasmas. Disruptions result in a sudden confinement loss, so that heat and particles are rapidly expelled to the device wall. Disruptions can abruptly destroy the plasma facing components, and terminate the fusion reaction. Disruption issues are of central importance to future fusion rectors such as ITER. Meanwhile, high plasma density ($ne$) is essential for the access to high fusion gain since the fusion power density scales as $ne^2$. However, there is a limit (known as Greenwald density limit) for tokamak high density discharges. The Greenwald density limit is an empirical limit for the achievable line-averaged plasma density on experiments, namely $ne_G=I_p/\pi a^2$, where $ne_G$ is the line-averaged plasma density in units of $10^{20}m^{-3}$, $I_p$ the plasma current in $MA$ and $a$ the minor radius in $m$. Generally, when the Greenwald density is reached, the bulk plasma frequently disrupts as well as the discharge halts, namely so called density limit disruptions. Density limit disruptions have been an active area of research for decades. Many previous experimental results indicate that the density limit disruption occurrence is correlated to the plasma edge cooling, MARFE, current channel shrinkage, MHD activities (mainly tearing modes), edge turbulence, and so forth. These results indicate that density limit disruptions originate from the plasma edge region. Some experimental results also suggest the density limit can be exceeded by the plasma core fuelling, edge pumping, or modification of particle transport lead to peaked density profiles. In the light of them, many theories are proposed to unravel the mechanism of density limit disruptions, however, the underlying physics is not yet fully covered and understood. In this paper, we present a new experimental evidence of density limit disruptions triggered by core-localized Alfv{\'e}nic ion temperature gradient (AITG) instabilities. It is found for the first time that there are multiple-branch MHD instabilities in the core plasmas while $ne/ne_G>0.85$. The simulation analysis suggests that the core-localized MHD activities belong to AITG modes, and on experiment firstly, it is discovered that they trigger the minor or major disruption of bulk plasmas while the density is peaked. These new findings are of great importance to figure out and understand the origin of density limit disruptions, as well as to forecast and avoid them for future fusion rectors