Propagation of laser radiation through a long plasma column

Calculations are presented on the propagation of laser radiation parallel to the magnetic field in a pinch plasma column in reference to recent proposals for plasma heating. It is shown that waveguide type modes with no longitudinal dependence may be necessary because of radiation pressure considerations. The size of these modes is calculated self-consistently assuming classical plasma transport coefficients and inverse Bremsstrahlung absorption. In all cases of interest, the area occupied by the laser radiation is small compared to the cross sectional area of the plasma. The solutions are stable against self-focusing but unstable to a firehose instability at high radiation densities. From this viewpoint, a system in which the radiation can make many passes through the plasma is worse than a single pass system.