Microwave absorption by a plasma near a critical density

Experimental results are reported on the interaction of S-band microwave radiation with a magnetized plasma whose density varies in two dimensions at intermediate frequencies (ω/sub H/e>ω>√ω/sub H/eω/sub H/i). The wave approached the focus (i.e., the point at which the density at the chamber axis was equal to the critical density n/sub c/) from the high-density side. Narrow plasma channels were observed to form near the focus; they continually contracted toward the axis with distance into the low-density region. The region of maximum energy evolution lies a certain distance (1--10 cm) ahead of the focus on the side with n/sub e/>n/sub c/. A small fraction of the wave energy is liberated beyond the focus (n/sub e/<n/sub c/). This result shows that the wave propagating toward the focus was converted into a plasma wave under these experimental conditions. The experimental results agree with a theory for the propagation of a slow plasma wave in a plasma column with density varying in two dimensions