Investigation on edge cooling threshold of density limit in the J-TEXT tokamak with limiter and divertor configurations

Abstract The cooling of the plasma edge is widely considered to be a key element in the density limit of tokamak. This paper investigates the critical edge cooling threshold of the density limit, exploring various plasma configurations in the J-TEXT tokamak. Notably, significant differences in edge electron temperature in the vicinity of last closed flux surface (LCFS) were observed between limiter and divertor configuration. However, the electron temperature dropped to a similar level in the vicinity of q = 3 surface close to density limit, independent of the magnetic field configurations. In addition, to evaluate the reliability of critical edge cooling threshold, experiments were implemented by increasing the carbon impurity content for enhancing 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 surface remains consistent even with the stronger edge cooling rate. The consistency observed in the electron temperature threshold near the q=3 surface, regardless of magnetic configuration and edge cooling rate, could help to refine existing theoretical and simulation work and improve the prediction accuracy of the density limit disruption.