Transport and turbulence studies in the linear ohmic

Transport in ohmically heated plasmas in Alcator C-Mod was studied in both the linear (LOC) and saturated (SOC) ohmic confinement regimes and the importance of turbulent transport in the region r/a = 0.5-0.8, was established. After extensive analysis with TGLF and GYRO, it is found that using an effective impurity ion species with Zi = 8, and moderately high Zeff (2.0-5.6), in the LOC regime electron transport becomes dominant due to TEM turbulence. The key ingredient in the present results is the observation that dilution of the main ion species (deuterium) by impurity species of moderate charge state reduces the dominant ITG turbulence, in contrast to the SOC regime with little, if any dilution. The turbulent spectrum measured with the Phase Contrast Imaging (PCI) diagnostic is in good agreement with predictions of a synthetic PCI diagnostic adopted to Global GYRO. The toroidal rotation in the low density LOC regime is in the co-current direction but as the density is raised, in the SOC regime the rotation reverses to the counter current drive direction. Recently we have measured the impurity content of the plasma and found an effective Zi of 9.