Abstract
The nonlinear stage of the evolution of unstable Langmuir waves excited by two oppositely-propagating beams is studied experimentally. When a pump at frequency omega p is applied, the two monochromatic Langmuir waves thus generated ( omega p, k) and ( omega p, -k) form a standing wave. The ponderomotive force and the generation of cavitons lead to the process of plasma wave collapse to a spatial width of 7 lambda D and an intensity E2/4 pi nT approximately=0.4. Experimental evidence showing that transit time damping is the limiting mechanism for the collapse process is also discussed.