Time- and space-resolved spectroscopic characterization of a laser carbon plasma plume in an argon background

Abstract

We present time-and space-resolved spectroscopic observations of a laser-produced carbon plasma, in an argon background. An Nd : YAG laser pulse, 370 mJ, 3.5 ns, at 1.06 µm, with a fluence of 6.9 J cm −2 , is used to produce a plasma from a solid graphite target in a 0.5 to 415 mTorr argon background. The spectral emission in the visible is recorded with 15 ns time resolution. We use 20 ns time resolution plasma imaging, filtered at characteristic carbon species emission wavelengths, to study the dynamics of the expanding plasma. The carbon plasma emission is found to evolve from the characteristic of single ionized carbon, to a more complex one, where C 2 and C 3 molecular bands dominate. Several plasma fronts, with either ionic or molecular composition, are seen to detach from the laser target plasma. The temporal and spatial features of the molecular carbon species evolution are found to be dependent on the actual argon background pressure.

Effects of a static inhomogeneous magnetic field acting on a laser-produced carbon plasma plume

“…Without magnetic field, figure 4, the plasma plume is seen to expand along the radial and axial directions. A characteristic plasma front, as reported previously for laser produced carbon plasmas 5,17,19 detaches from the laser plasma at the target surface, and propagates forward along the axis. In a time scale of less than 0.5 µs no significant visible plasma emission is seen in the region beyond the target.…”

“…When the laser-produced carbon plasma expands in a low pressure neutral gas backgrounds, the main effect of the background gas is to favor C 2 molecule formation through three-body collisions involving a neutral atom or molecule. 5,20,35,37 As in this case no significant gas background effect is to be expected to promote C 2 molecules formation, a different mechanism should be accounted. In this case C 2 molecules formation might be explained by magnetic field induced enhanced collisionality, as a result of reduced radial diffusion.…”

“…Another characteristic phenomenon observed in laser-produced plasma expanding in low pressure background gas is the generation of successive plasma fronts or plume splitting. 5,19,[38][39][40][41] This is due either to double vaporization of target material from a relatively long duration laser pulse or to the formation of a double layer at the initially expanding plasma resulting from inter-penetration of the plasma species and background gas, which leads to plume splitting and sharpening. 39,40 As plume splitting is not observed in vacuum background, our observation of two successive plasma fronts in the presence of the magnetic field, seen in figure 5, might be ascribed to the existence of a strong field gradient very close to the target surface, which reduces plasma expansion rate due a reduction in the diffusion coefficient, thus providing similar interaction conditions to those in low pressure background.…”

“…[1][2][3][4][5] In this context, the application of either static or transient magnetic fields, parallel or transverse to the forward direction defined by the free expanding laser plasma plume, is known to have a strong effect on plasma dynamics and composition. Early investigations focused on measurements of plasma parameters and in the identification of magnetic field induced kinetic effects on plasma behavior.…”

“…In particular, our results show that in a vacuum background the presence of the magnetic field plays an equivalent role to that of a low pressure neutral background gas, in order to favor the emission of successive plasma fronts and C 2 molecules formation, as reported previously in investigations of laser produced carbon plasmas. 4,5,[17][18][19][20] …”