Investigation of Lower Hybrid Wave Damping
Authors: S. Bernabei, A. Cardinali, G. Giruzzi, et al
Lower Hybrid (LH) current drive experiments on PBX-M 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 2-D 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. A model is proposed to explain how first-pass damping is possible in such plasmas. The radio frequency (RF) power can form a tail of energetic electrons, and waves with moderate phase velocity can damp on them. For waves with very fast phase velocity, there must be an upshift of the nö spectrum for any damping to occur. These hypotheses are supported by ray tracing results which were coupled to relativistic Fokker-Planck calculations of the electron distribution function.