Gyrokinetic analysis of the linear ohmic confinement regime in Alcator C-Mod

In earlier studies of ohmically heated low density C-Mod plasmas, TRANSP results indicated ion heat diffusivities significantly smaller than those predicted by GYRO [1]. In the present work we present new experimental data where the ion temperature profiles have been measured by x-ray crystal spectroscopy, and the radial electric field is deduced from toroidal flow measurements. TGLF, the trapped gyro-Landau fluid model code was used to model the turbulent transport as a function of density, Z$_{i}$ and Z$_{eff}$, and the results indicate that the second (impurity) ion species with moderate Z$_{i}$ ($\le $ 8) reproduce the measured ion and electron thermal diffusivities in the range 0.5 $<$r/a$<$ 0.8. The nonlinear flux tube code GYRO predicts similar results [2]. The global GYRO based synthetic PCI diagnostic predicts Doppler shifted frequencies in agreement with observations. \\[4pt] [1] L. Lin, et al, Plasma Phys. Contr. Fusion 51, 065006 (2009).\\[0pt] [2] M. Porkolab, et al, EPS 2011 , Strasbourg, France, P5.109.