Transport Changes Near $q$=1 Surfaces in the DIII-D Tokamak*

Spontaneous improvement in electron energy transport is routinely seen in the core of DIII-D discharges as the safety factor $q$ approaches $1$. For a range of discharge types with constant heating conditions, core $\chi_e$ is seen to decrease just before the first sawtooth, as evidenced by a sharp rise in central electron temperature. The behavior is similar to barriers observed in reverse shear plasmas near $q_{min}=2,3$; however, the picture is made more complicated by the onset and decay of a variety of MHD modes. Changes in turbulent fluctuation amplitudes are noted as well as the presence of high frequency coherent modes. Further evidence of $q=1$ transport barriers is exhibited in an off-axis EC-heated discharge where $q_{min}$ is driven above 1 and unusual hollow $T_e$ profiles with sharp changes in gradients are observed. We compare the data with models of transport barriers near low-order rational $q$ surfaces.