An experimental study of plasma waves generated by co-linear optical mixing

Longitudinal plasma waves produced by the resonant heating of two transverse electromagnetic waves are of interest for several reasons. First, it has been predicted theoretically and demonstrated in simulations that the exceedingly large electric field of such a wave may be used to accelerate particles to high energies. Secondly, because the plasma wave necessarily obeys the plasma dispersion relation, it may serve as a versatile and accurate plasma density probe. Furthermore, several schemes have been proposed to locally heat plasmas using the damping of these waves. The latter application is particularly relevant to the study of ionospheric plasma physics and cross-field transport. We present here results appropriate to all of these research avenues. These investigations also raise some interesting new questions concerning plasma wave-wave interactions. In co-linear optical mixing, two electromagnetic waves with slightly differing frequencies are injected parallel into a plasma. If the plasma density is such that the plasma frequency approximately matches the difference frequency of the EM waves, then a longitudinal electrostatic wave will be resonantly excited. The two EM waves in this experiment are the 9.6 μm line (4J) and the 10.6 μ line (12J) of a CO2 laser. An arc discharge preionizes hydrogen gas to provide the initial plasma. The resonant plasma density corresponding to these wavelengths, 1.1 x 1017 cm-3, is diagnosed by means of ruby Thomson scattering and tuned by varying the arc parameters. The authors observe at this resonant density ruby light, both red- and blue-shifted by ωp, at an angle of 70