Gasdynamic effects in optical discharges produced by periodic pulse femtosecond laser

Abstract

Quasi-stationary gas streams in argon (10 bar) were observed for the first time being generated by periodic-pulse optical discharge produced by laser pulses of less than 500 fs pulse length with energy up to 200 μJ/pulse and repetition rate 1.66÷10 kHz. Optical discharge was obtained in laser beam focused by off-axis (90°) parabolic mirror. In experiments the shape of the discharge zone was varied accordingly to the laser beam waist shapes varied from astigmatic to non-aberrated ones depending on the parabolic mirror tilt. Intense convective streams flowing out of the discharge volume were observed by schlieren technique. The gas streams produced could be directed normally to the laser beam axis, at some angle to the beam axis or along the beam axis toward the laser or in opposite direction. It was found that the directions of the streams produced, dynamics of their formation and their intensity were governed by the shape of the discharge zone. It was revealed that most intense and fast forming streams produced were directed normally to the laser beam axis. Two opposite streams are induced by the discharge located in astigmatic beam waist in a form of flattened “disk” ∼10 μm thick and ∼100 μm wide. The streams were directed normally to the “disk” surfaces. The energy spent on the gas flow acceleration was estimated to be up to 30% of thermal component of energy dissipated in plasma. When the focusing mirror was aligned to get no astigmatism, the gas flow generated was directed along optical axis toward the laser or backward in some cases. Refraction of the incident laser beam on the refraction index gradients of heated and excited gas injected by backward stream was followed by oscillations of the discharge zone location and generated stream direction. Discharge became stable when the gas streams were co-directional, normal or angled to the laser beam. Further studies are required to define mechanisms and possible applications of the phenomena observed.