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Abstract:

Spectral profiles of optically thick line radiation emitted by an expanding cylindrical laser-produced plasma have been recorded in high resolution using a vertical dispersion variant of the double-crystal spectrometer. By uniformly irradiating a narrow wire with a 1 ns pulse a quasi-steady plasma expansion is generated with a velocity gradient in excess of 10(9) s(-1). Such velocity fields strongly influence the transfer of radiation through a plasma, resulting in a complex distortion of the lineshapes of optically thick lines. We have computed theoretical profiles of the Al XIII Ly-alpha resonance line. Simulation of the laser-plasma interaction is performed using the 1D Lagrangian hydrocode MED103 which incorporates a non-LTE time-dependent excitation and ionisation package based on the average-atom model. Line emission profiles are calculated by the method of multi-frequency line transfer in the co-moving frame. We investigate the sensitivity of the lineshape on plasma parameters such as the velocity profile, comparing our theoretical predictions with experiment. (C) 1997 Published by Elsevier Science Ltd. All rights reserved.

Abstract:

The transfer of radiation through plasmas with large velocity gradients is of relevance to several astrophysical situations, such as supernova explosions, maser operation, and stellar winds. Similar conditions often prevail in laser-produced plasmas, with velocity gradients of order 10(9) s(-1) significantly altering the effective optical depth and line shape. Some of the simplest cases to study experimentally are the hydrogenic resonance lines. Experiments performed in both planar and cylindrical geometry, comparing the observed line profiles with those modeled using a one-dimensional Lagrangian hydrocode, incorporating average-atom nonlocal thermodynamic equilibrium (non-LTE) atomic physics are described. The opacity effects on the ion populations an treated within the escape factor approximation, taking into account the effects of the velocity gradient. The hydrocode gives time- and space-dependent values of the electron and ion densities, excited state fractions, electron and ion temperatures, and velocities, The hydrodynamic output is post-processed with a radiative transfer routine to construct the simulated line shape. Details of the experiments and results are presented, and relevance to the astrophysical situations discussed. (C) 1997 American Institute of Physics.