Investigation of lower hybrid wave damping in the PBX-M tokamak

Lower hybrid (LH) current drive experiments on the Princeton Beta Experiment-Modified (PBX-M) [Phys. Fluids B 2, 1271 (1990)] have shown that the current profile can be changed by varying the phase velocity of the waves. The radial profile of the current carrying electrons was deduced from two-dimensional hard x-ray tomography. For a certain range of phase velocities, there is a correlation between the peak of the fast electron profile and the launched wave spectrum, despite the presence of a wide spectral gap between the phase velocity and the thermal electron energy distribution. A new model is proposed to explain how first-pass wave damping is possible in such plasmas. The rf power can form a tail of energetic electrons, and subsequently waves with moderate phase velocity can damp on them. For waves with very fast phase velocity, there must be an upshift of the n∥ spectrum for any damping to occur. These hypotheses are supported by ray tracing results which are coupled to relativistic Fokker–Planck calculations of the electron distribution function.