Spreading of lower hybrid wave driven currents in PBX-M

Lower hybrid current drive (LHCD) in the tokamak Princeton Beta Experiment-Modification (PBX-M) is computed with a dynamic model in order to understand an actual discharge aimed at raising the central q above unity. Such configurations offer advantages for steady state operation and plasma stability. For the particular parameters of this PBX-M experiment, the calculation found singular profiles of plasma current density J and safety factor q developing soon after LHCD begins. Smoothing the lower hybrid wave driven current and power using a diffusion-like equation and a velocity-independent diffusivity for fast-electron current brought the model into reasonable agreement with the measurements if D-fast approximate to 1.0 m(2)/s. Such a value for D-fast is in the range suggested by other work.