Lower Hybrid Current Drive Experiments on Alcator C-Mod: Comparison with Theory and Simulation

Recently, lower hybrid current drive (LHCD) experiments have been carried out on Alcator C-Mod using an RF system consisting of 12 klystrons at 4.6 GHz, feeding a 4 $\times $ 22 waveguide array. Up to 900 kW of LH power has been coupled in the range1.6 $\le $ n$_{//} \quad \le $ 4), where n$_{// }$is the parallel refractive index. Driven LH currents have been inferred from magnetic measurements by extrapolating to zero loop voltage, yielding an efficiency of n$_{20}$I$_{LH}$R/P$_{LH} \quad \approx $ 0.3 [1]. We have simulated the LH current drive in these discharges using the combined ray tracing / 3D (r, v$_{\bot }$, v$_{//})$ Fokker Planck code GENRAY -- CQL3D [2] and found similar current drive efficiencies. Measurements of nonthermal x-ray emission and electron cyclotron emission (ECE) confirm the presence of a significant fast electron population that varies with waveguide phasing and plasma density. Studies are currently underway to investigate the role of fast electron diffusion and full-wave effects such as diffractional broadening in determining the spatial and velocity space structure of the nonthermal electrons. The 3D (r, v$_{\bot }$, v$_{//})$ electron distribution function from CQL3D has been used in synthetic diagnostic codes to simulate the measured hard x-ray and ECE emissions. Fast electron diffusion times have been inferred from x-ray data by employing a radial diffusion operator in CQL3D and determining the fast electron diffusivities that are required to reproduce the experimentally observed profiles of hard x-ray emission. Finally, we have been performing full-wave LH field simulations using the massively parallel TORIC --LH solver [3] in order to assess spatial and spectral broadening of the incident wave front that can result from diffraction and wave focusing effects. \newline \newline [1] R. Parker, Bull. Am. Phys. Soc. \textbf{51}, 20 (2006). \newline [2] R.W. Harvey and M. McCoy, ``The CQL3D Fokker Planck Code,'' \textit{Proc. IAEA Tech. Comm. Meeting on Simulation and Modeling of Thermonuclear Plasmas}, Montreal, Canada, 1992. \newline [3] J. C. Wright \textit{et al.}, Nucl. Fusion \textbf{45}, 1411 (2005).