Characterization of density limit in negative triangularity plasmas on DIII-D tokamak*

The operational density limit in negative triangularity (NT) plasmas has been explored on the DIII-D tokamak, particularly its dependence on plasma current and auxiliary power. The results show that at low power input, the density limit increases as the power input increases, but the power dependence becomes weaker once the input power exceeds 5 MW. The highest achieved density in NT plasmas does not exhibit a clear dependence on the plasma current. In this experiment, it was possible to achieve a high Greenwald fraction of the line-average density (f_G=1.5-2) by using a high neutral beam injection (NBI) power input and an unfavorable ▽B drift configuration at a lower plasma current of 0.6 MA. When the line-averaged density reached the density limit, there was a noticeable collapse of the edge electron temperature and the mean shear flow profiles, indicating edge cooling. However, in high-power discharges near the density limit, disruptive avoidance could be achieved. Furthermore, as the line-averaged density approached the Greenwald limit, the intermediate-k edge density fluctuations at midplane were observed to increase, but these fluctuations became saturated once the Greenwald limit was exceeded.