OBSERVATIONS OF RELATIVISTIC PLASMA-WAVES EXCITED BY A 1.064 MU-M AND1.053 MU-M LASER BEAT

We report observations of a relativistic plasma wave excited by two co propagating laser beams with frequency difference close to the plasma frequency in a fully ionized hydrogen plasma. The laser beams were at 1.064 mu m (Nd:YAG) and 1.053 mu m (Nd:YLF) of 200 ps duration and foc used to a approximate to 7x diffraction limit spot with peak irradianc e of 2 x 10(14) W cm(-2) The plasma wave was detected by monitoring th e forward-scattered spectrum of a third copropagating laser beam at 0. 526 mu m. A novel use of Moire deflectrometry ensured the pump beams a nd probe beam were copropagating at focus; The inferred plasma wave de nsity modulation delta n/n is about 1-2% and corresponds to a longitud inal electric field of about 10(9) V m(-1). The plasma wave was observ ed to exist for a time tau(p) less than or equal to 50 ps. These resul ts are not in agreement with theoretical predictions for relativistic detuning which give delta n/n approximate to 8%, tau(p) approximate to 200 ps and suggest that there is some mechanism limiting the growth o f the plasma wave. Simulations indicate that this is due to either pon deromotive blowoff taking the plasma off resonant density or to the mo dulational instability.