Measurement of the rotational transform at the axis of a tokamak

Measurements of the internal magnetic field structure of Texas Experimental Tokamak discharges are made by use of laser-induced fluorescence of an injected ${\mathrm{Li}}^0$ beam. From measurements near the magnetic axis, we obtain the axial safety factor, $q_0$ (or equivalently the rotational transform, ι=2π/q) for various discharge conditions. In particular, for low-$q_a$ sawtoothing discharges, we find time-averaged values of $q_0$ significantly less than 1 (∼0.7 to 0.8), in contrast with models used to describe the nonlinear internal relaxation process commonly known as sawteeth.