Echoes at Upper Hybrid Resonance. I. Measurements on Afterglow Plasma

The two‐pulse stimulated radiation from nonuniform rare gas (Ar, Ne, He) afterglow plasma columns longitudinally immersed in a uniform (one part in 10 4 ) magnetic field was studied. For microwave excitation pulses with a center frequency near the maximum upper hybrid frequency of the spatially nonuniform plasma columns, strong two‐pulse echoes are observed. A detailed study of these upper hybrid echoes, using an S‐band microwave apparatus, is given. At high densities strong echoes are observed shifted from free electron cyclotron resonance by as much as 20%. The conditions for maximum echo follow maximum upper hybrid resonance as the electron density is varied, but are also found to be dependent on the power and time separation τ of excitation pulses. The power and time width of the first echo are shown to have a complex, yet systematic dependence on four major parameters: electron density, magnetic field, excitation power, and τ. The observed behavior is found to be in general qualitative agreement with calculations involving a one‐dimensional slab of cold nonuniform collisionless plasma. A study of the effects of neutral gas pressure on certain echo properties indicates that electron neutral collisions do not play any essential role in the echo formation process. Also, some anomalous observations at X band are reported.