Experimental investigation of the initial condition of laser-induced plasmas by collective Laser Thomson scattering*

Temporal evolutions of two-dimensional (2D) plasma profiles produced by a Nd:YAG laser ( 5.9×10⁹ W/cm², 9 ns FWHM) were investigated at 0–20 ns, during and after the laser peak at 0.13–0.5 mm from the initial target surface by measuring the electron density (n_e), temperature (T_e), and drift velocity (V_d), using collective laser Thomson scattering (CTS). The n_e peak was close to the target surfuce. The profile of T_e was nonuniform and showed a peak at 0.3 to 0.4 mm above the target surface, and such high-temperature region of the plasma moved outward at the same speed as that of the expansion of the plasma.  This indicated that the isothermal expansion ceased immediately after the peak of the ablation pulse. Even during the ablation pulse, the laser produced plasmas had considerable V_d, which was measured from the Doppler shift of the spectrum of CTS. The 2D profiles of V_d and pressure also show the plasma anisotropically expanded, strongly perferred the direction perpendicular to the target surface during the ablation pulse.