TRANSITION TO FILAMENTARY REGIME IN LASER-PLASMA INTERACTION EVIDENCED BY 2-OMEGA-EMISSION

The level of second-harmonic light emitted from a laser-exploded foil plasma at nominal irradiance up to 3.10(13) W/cm2 was found to be extr emely sensitive to both target position and irradiance on target. Eith er a small target displacement or a small increase in irradiance resul ted in a jump of the 2omega level of more than three orders of magnitu de. Correspondingly, a transition was observed from a 2omega source pa ttern clearly signed by the original laser spot pattern to unstable pa tterns of filaments whose size is consistent with the maximum growth o f the instability.