Transverse diffusion of particles and energy in a laser-heated plasma column

In a laser-heated magnetically confined column of plasma separated from material walls by a vacuum field, laser energy absorption near the center leads to a ’’light pipe’’ effect essential to the concept of heating long columns to fusion temperatures. Outward diffusion of particles at the vacuum-plasma boundary tends to move the position of maximum density inward, and hence to narrow the light pipe, while over-all expansion of the heated column against the field tends to widen it. A model is formulated to describe these effects and is treated analytically and numerically. The results indicate that for conditions of interest, the light pipe will persist long enough to permit heating of reactor-scale columns.