Investigation of the high density discharges on the J-TEXT Tokamak

Recently, the operation region has been explored for J-TEXT Ohmically heated discharges by means of gas puffing. The results showed that the maximum achievable density has been significantly expanded, from 0.43 $n_{G} $ (Greenwald limit) to 0.85 $n_{G}$, after the stainless steel vacuum wall was covered by the graphite tiles. Nevertheless, the J-TEXT high density discharges were frequently terminated by a disruption. The investigation concluded that the maximum achievable density strongly relied on the total plasma current $I_{P} $, but very weakly depended on the edge safety factor $q_{a}$. Some features of such disruption have been identified by analyzing the measured data from the 17-channel (covering $-0.94a\sim 0.94a$ of the cross-section) FIR polarimeter-interferometer. For example, in the density ramp-up phase, asymmetry of density profile between the LFS (low field side) edge ($r>0.8a$) and the HFS (high field side) edge ($r<-0.8a$) would appear gradually. In addition, a reversed density gradient on the HFS edge occurred. Before the disruption, edge density on the HFS suddenly dropped to a lower level and tended to restore the symmetry of density profile between the LFS and HFS edge. Simultaneously, the radiation measurements, including CIII impurity radiations, soft X-ray emissions and so on, indicated that plasma radiation increased dramatically. Such lower density level at the HFS edge can maintain for $\sim 100ms$ when $q_{a} >5$, but less (or even null) for $q_{a} <3.5$