Cooling of the plasma edge is widely considered to be a key element in the density limit (DL) of tokamaks. This paper investigates the critical edge cooling threshold of the DL by exploring various plasma configurations in the joint-Texas experimental tokamak. Notably, significant differences in the edge electron temperature in the vicinity of the last closed flux surface were observed between the limiter and divertor configurations. However, the electron temperature drops to a similar level in the vicinity of q= 3 surface close to DL, independent of the magnetic field configuration. In addition, to evaluate the reliability of the critical edge cooling threshold, experiments were conducted by increasing the carbon impurity content to enhance the edge cooling rate. These experiments involved two approaches to increase the carbon impurity content: methane injection and penetration of a graphite solid source. Results from these experiments indicate that the temperature threshold of the q = 3 surfaces remains consistent even with a stronger edge cooling rate. The consistency observed in the electron temperature threshold near the q = 3 surface, regardless of magnetic configuration or edge cooling rate, could help refine existing theoretical and simulation works and improve the prediction accuracy of the DL disruption.
Overview of the first Wendelstein 7-X long pulse campaign with fully water-cooled plasma facing components