Three-dimensional visualization in laser produced plasmas and nano-particle formation.

摘要

The formation of nano-particles is investigated in neutral-gas-confined laser produced plasma (LPP). High density plasma of electron density of above 4 x 1018 cm-3 is readily achieved when a high power pulse laser, ≈0.7 x 109 W/cm 2, is delivered to the surface of aluminum target that is immersed in a high pressure argon gas. Subsequent expansion and strong thermal diffusion lead the plasma into the neutral gas. This returns to aluminum vapors again but in the form of nano-particles.; For diagnosis, a thin shell of the plume at a fixed distance from the target surface is imaged onto a set of two optical fiber ribbons that are mutually orthogonal. The plasma luminosities are then recorded as a function of times on a streak camera. The 2-D structure of the shell has N x N unknowns and the two streaks provide only 2N measurements. In addition, we take a front view snapshot at an early time to begin 2-D structure reconstruction. The structure is expressed in the form of a cell-by-cell specific emission intensity profile.; A reconstruction method is developed for reconstruction of the structure of arbitrary shaped and self-absorbing LPP plumes. Two power law scaling relations are invoked for a local pressure and temperature. The procedure is iterative, and consists of computing the two luminosity profiles from a proposed structure and comparing them with the measured counterparts. When the agreement is maximized, the constants of scaling relations are determined. The 3-D structure is realized when all other plasma shells are reconstructed.; Each cell temperature is analytically extrapolated to later times according to the measured cooling rates, in order to reach the critical point of aluminum. Aluminum atoms aggregate into large clusters through clustering and growth by condensation. We have found that the computed cluster size distribution at a time exhibits self-similarity for all times when rescaled by maximal size and population. This scaling is exploited to compute the final cluster distribution. The resulting cluster size distribution for the entire LPP plume agrees well with the measured histograms.