Characterization of laser ablated Al-plasma in ambient atmosphere of nitrogen

Summary form only given. We report on the characterization of pulsed laser ablation of aluminum in ambient pressure of nitrogen varying from 0.01 to 70 Torr using ICCD images of the expanding plume. Q-switched Nd:YAG (DCR-4G, Spectra Physics) laser with a pulse width of 8 ns at full width half maximum (FWHM) at a repetition rate of 10 Hz, operating in the fundamental mode (/spl lambda/=1.064 /spl mu/m) was used to create plasma in vacuum and in presence of nitrogen gas. The images of the expanding plasma were recorded using a gated ICCD (DH 720-18F-03, Andor Technology) at various time delays with respect to the ablating laser pulse. ICCD consisted of 696/spl times/256 active CCD array. The images were captured with a gate width of 10 ns. At pressures 0.01 Torr to 1 Torr plasma expansion followed shock model whereas at 10 and 70 Torr plasma expansion followed drag model. At pressures /spl ges/1 Torr and later times plasma-gas interface showed instability in the leading edge of the expanding plume. The pressure gradients at the expanding plume front in nitrogen ambient resulting in the self-generated magnetic field is responsible for the instability and gives rise to what is called Rayleigh-Taylor instability. Information from ICCD images has been used to estimate the initial strength of self-generated magnetic field of around 2600 Gauss at 1 Torr ambient pressure. The effect of background gas on polarization in the ablated plasma is reported. The measurement of the degree of polarization of Al III transition 4s /sup 2/S/sub 1/2/-4/sub p/ /sup 2/p/sub 3/2//sup 0/ at 569.6 nm is reported. To study the degree of polarization in the plasma a Wollaston prism was inserted in the path between the plasma emission and the monochromator (HRS-2, Jobin Yvon) slit to record the time-resolved spectrum of two different polarized states simultaneously. The output was detected using ICCD as the detector at the exit slit of the monochromator with spatial response in the region 190-8- 0 nm. The result shows that polarization could have been induced due to the self-generated magnetic field.