A method to determine the conduction profile in a Tokamak plasma from the response of a toroidal electric field pulse is described. The conductivity profile was determined so that an observed current pulse was fitted best by the solution of the current diffusion equation with an appropriate conductivity profile and a boundary condition relating to the observed electric field at the plasma surface. The dependences of the conductivity profile on the pulse amplitude and width were examined. It is concluded that the Tokamak plasma behaves as a classical conductive medium for a wide range of pulse amplitudes and rise times. Deviation from the classical response was observed in two cases: (A) When the safety factor q(a) is reduced to lower than 3 by the pulse, the plasma is disrupted by a double tearing mode having m=3. (B) When a fast rise time pulse with a large amplitude was applied, a two-stream instability with a large growth rate (Buneman instability) was excited by the local enhanced electron drift velocity.
Enhancement of the ISTTOK plasma confinement and stability by negative limiter biasing