Local X-ray emissivity structure in the density limit in the TJ-I tokamak

Operation of Tokamak devices shows that plasma density can not be increased as much as desired due to the existence of a limit value above which a disruption occurs. Several models have been developed to explain this behaviour. Some of them are based on the idea of higher the plasma density, bigger are the radiation losses that generate a contraction and steepening of plasma current profile making unstable the 2/1 mode. Results from JET seem to confirm this model. A different and complementary approach, put the emphasis on convection assuming the density limit generated by the existence of poloidal asymmetries at the plasma edge. In this paper, we are presenting results from density limit experiments at the small TJ-I Tokamak (R0=0.3m, a=0.1m, BT≤1.9T, Ip≤70KA, O.H. only) by using soft x-ray arrays to study instability development and possible existence of poloidal asymmetries prior to the disruption. (author) 3 refs., 5 figs