Non-linear MHD calculations of the m/n = 2/1 free boundary kink mode in a cylindrical tokamak are carried out by taking account of the parallel diffusion of resistivity. 'Vacuum bubbles' are formed inside the plasma for qa somewhat smaller than 2. When 1.75 < qa < 1.9 for q0/qa = 0.5 (q0 is the safety factor at the magnetic axis and qa is that at the plasma surface), the plasma column shrinks with elliptic deformation, the value of qa decreases in time, the plasma becomes stable against the m/n = 2/1 mode, and finally, a damped oscillation is observed. Interaction between the plasma and the material limiter causes the shrinkage for all unstable values of qa. When q0 is nearly equal to or larger than unity, the plasma shrinks rapidly, and qa can be reduced to values below unity, upon which the m/n = 1/1 kink mode becomes unstable. The plasma shrinkage is a candidate for major disruptions in a tokamak discharge with qa less than or equal to 2.
Nonlinear MHD simulations of Quiescent H-mode plasmas in DIII-D