Effect of pulsed-laser deposition parameters on plasma expansion studied by fast-framing photography

摘要

Fast-framing photography was used to study the effect of various parameters on the expansion of the visible plasma plume produced by KrF laser irradiation of a YBa$-2$/Cu$- 3$/O$-7$MIN@x$/ target. The ambient gas pressure has a considerable effect on the evolution and structure of the plasma. In vacuum, the visible emission is strongly forward- directed and has a relatively short duration. In the presence of an oxygen atmosphere, a long-lived, highly luminous front is formed, whose expansion may be well described by a drag model or a shock model (spherical or plane shock). Ahead of the luminous front there is weak emission, which for pressures over about 700 mTorr evolves into a highly emissive `tip'. The decrease of energy density on the target by increase of the laser spot (defocusing) leads to a more forward-directed motion of the visible species. Larger energy densities obtained by increasing the energy/pulse lead to a sharper, `V' shaped front. The effect of a biased ring electrode placed 0.5 cm to 2 cm in front of the target was also studied. The emission characteristics in the electrode's presence change greatly. The effect of ring- target distance, voltage magnitude and polarity, and pressure were observed.