Confinement and the safety factor profile

The conjecture that the safety factor profile, q(r), controls the improvement in tokamak plasmas from poor confinement in the Low‐ (L‐) mode regime to improved confinement in the supershot regime has been tested in two experiments on the Tokamak Fusion Test Reactor (TFTR) [Plasma Phys. Controlled Nucl. Fusion Res. 1, 51 (1987)]. First, helium was puffed into the beam‐heated phase of a supershot discharge, which induced a degradation from supershot to L‐mode confinement in about 100 ms, far less than the current relaxation time. The q and shear profiles measured by a motional Stark effect polarimeter showed little change during the confinement degradation. Second, rapid current ramps in supershot plasmas altered the q profile, but were observed not to change significantly the energy confinement. Thus, enhanced confinement in supershot plasmas is not due to a particular q profile, which has enhanced stability or transport properties. The discharges making a continuous transition between supershot and L‐mode confinement were also used to test the critical‐electron‐temperature‐gradient transport model. It was found that this model could not reproduce the large changes in electron and ion temperature caused by the change in confinement.