Turbulence Studies at the Top of the Pedestal

The confinement of tokamak plasmas dramatically improves during a high performance mode of operation (\hbox{H-mode}). An \hbox{H-mode} plasma typically has steep temperature and density gradients (a barrier) near the edge of the plasma, but the question remains as to what limits the gradients in the \hbox{H-mode} barrier. Two possibilities are electron temperature gradient (ETG) and kinetic ballooning mode (KBM) turbulence. A key parameter that drives the ETG mode is $\eta_{\rm e}$, the ratio of the electron density scale length to the electron temperature scale length. It is shown that qualitative changes in $\eta_{\rm e}$ at the top of the pedestal correlate in time with the occurrence of ELM-free phases in some high-$\beta$ \hbox{DIII-D} discharges. The effect this change in $\eta_{\rm e}$ has on turbulent fluxes is presented.