Frequency conversion, "superluminal" propagation, and compression of a powerful microwave pulse in propagating ionization front

Frequency up-conversion (-10%) and compression (almost twofold) of a powerful (<= 250 MW) microwave pulse in the propagating ionization front produced by the pulse itself in a gas-filled waveguide, is investigated experimentally and analyzed theoretically. Pulse envelope reshaping and group velocity increase manifest themselves in a propagation of the pulse faster than in the empty waveguide. A simple one-dimensional mathematical model allows the adequate interpretation of the experimental results.