Filamentation of laser light in laser-produced plasmas

Filamentation of laser light in laser-produced plasmas has been studied via a series of experiments using the Essex High Power Laser System. Thin metallic freestanding foil targets were irradiated by laser pulses of 2-5 ns at intensities on the order of 10(exp 13) to 10(exp 14) W cm(exp -2). Various time-integrated and time-resolved diagnostics were utilized and a wide range of results were obtained. The major task of the experiments was the imaging of the rear surface of the target foil into the recording cameras with appropriate spatial and temporal resolutions. The time-integrated results presented comprise the image of the rear surface of the target foils and side on second harmonic shadowgrams and interferograms. From these results, clear evidence for the formation of filaments was obtained. Time-resolved results revealed several features of filamentation. A pulsation in the light transmitted through the target was observed in all streak camera records. It was concluded that these pulsations are a result of the self-focusing of the whole laser beam followed by diffraction leading to a repeated foci behavior. An unexpected and anomalous transmission of laser light through the target foil was observed and was concluded to be a result of the target preheating by a long low-level prepulse which makes the target foil transparent during the early stages of the laser pulse. Wherever appropriate, the results were compared with predictions from the hydrodynamic code (MEDUSA) and with the results of other workers.