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, \ensuremath{\iota}=2\ensuremath{\pi}/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 (\ensuremath{\sim}0.7 to 0.8), in contrast with models used to describe the nonlinear internal relaxation process commonly known as sawteeth.