Major disruption characteristics with a toroidal limiter in PDX

The PDX tokamak provides an experimental facility for the comparison of a variety of limiter configurations. A large data base of disruption characteristics has been obtained during operation with (a) an inner wall toroidal limiter with its limiting surface at 85 cm, giving a = 40 cm for R = 125 cm, (b) top and bottom carbon rail limiter giving R = 143 cm, a = 40 cm, and (c) an outboard scoop limiter giving R = 153 cm, a = 40 cm. The results show that the average current decay rate ΔI ΔT (kA/ms) for disruptions on the inner wall limiter is about four times faster than for those on the rail or scoop limiters. In addition, the achievement of discharges with q_ψ less than 3 (q_ψ is the equilibrium safety factor calculated by a MHD code) with the inner wall toroidal limiter is extremely difficult, due to the rapid growth of an m = 3, n = 1 mode as q approaches three. However, the disruption is often triggered by the growth of an m = 2, n = 1 mode after the m = 3, n = 1 mode has saturated in amplitude. The transport phase of the disruption phenomena has the same time scale regardless of limiter configuration. Numerical calculations of resistive MHD stability have been used to explain some of these disruption characteristics.