Biased H-mode and long-distance correlation of edge fluctuations on HBT-EP

Confinement changes consistent with a biasing induced H-mode have been observed on HBT-EP by using an electrode inserted into the plasma edge to create a local radial electric field well. During the biased H-mode phase, there is a clear reduction in density, temperature, and floating potential fluctuations measured by triple probes, as well as a concomitant rise in the global soft X-ray (SXR) profile, initiating at the edge region. Both positive and negative electrode polarizations were capable of inducing a transition with corresponding signs of the radial electric field. Using a double electrode configuration to confine the biasing region to an area between the two electrodes (both well within the last closed flux surface), an electric field well, SXR increases, and fast MHD rotation could still be achieved out to the edge without bifurcation in the biased electrode current. Additionally, long distance correlation of low frequency oscillations in the floating potentials and ion saturation currents measured by two toroidally separated triple probes during the biased H-mode reveals an n=0 structure with mesoscale radial wavelength that may indicate the presence of zonal flows.