This paper presents an investigation of the transient setting of ponderomotive nonlinearity in a collisionless plasma by a Gaussian pulse (in space and time). When the pulse duration is comparable to the diffusion time of carriers across the beam, the modification in the background electron concentration depends not only on the intensity of the pulse at that time but also on the back history of the pulse. Because of the modification of the background electron density, the plasma wave and the EM pulse are nonlinearly coupled and the plasmahyphen;wave excitation becomes a transient phenomena. If the initial power of the pulse is more than critical power for selfhyphen;focusing the pulse is selfhyphen;focused and the plasmahyphen;wave excitation is accordingly modified.
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