Observation of Energy Transfer between Identical-Frequency Laser Beams in a Flowing Plasma

We report enhanced transmission of a low intensity laser beam when crossed with an identical-frequency beam in a plasma with a flow velocity near the ion sound speed. The time history of the enhancement and the dependence on the flow velocity strongly suggest that this is due to energy transfer between the beams via a resonant ion wave with zero frequency in the laboratory frame. The maximum energy transfer has been observed when the beams cross in a region with Mach-1 flow.