Quasi-stationary convection in a periodic-pulsed optical discharge in high pressure rare gas

Abstract

Unusual convection flows were observed in stabilized pre-breakdown phase of the periodic-pulsed optical discharge (POD) called “quiet” POD. The discharge was a relatively weakly glowing plasma filament sustained by focused λ = 1.064 μm laser pulses with repetition rate of f_r = 50÷100 kHz at the intensity several times below than that required for the optical breakdown to occur. No strong shock waves or irregular turbulence around the discharge were observed, in contrast to breakdown types of POD. Significant laser beam refraction measured in the beam cross-section behind the discharge zone was explained by the gas heating in the discharge up to 10 kK, providing high gradients of gas density and refraction index. Intense convective flow was detected on the schlieren images as thermal traces of the laser-induced gas streams flowing from the discharge zone, directed mainly normally to the optical axis. Repeated relaxation of the gas expanding after being rapidly heated by the laser pulse is proposed to explain the effect. The periodic-pulsed discharge located in the elongated beam waist generates an anisotropic heated region with gas streams and vortices, which may form the observed regular convective flow at the late stages of expanding.