Breakdown wave, shock wave, and radiation wave in laser created plasmas

Abstract

The various phenomena occurring during the growth of laser generated plasmas are both theoretically and experimentally presented. The experimental results obtained with short and long laser pulses, and short and long focal lengths are described. Their interpretation is based on three different models, depending on experimental condition. The existence of an ionization threshold implies the possibility of a breakdown wave, especially in the case of short pulses, or the formation of a radiation supported shock wave. For experiment with high-density targets, the opacity may be so high that the thermal radiation conductivity is the principal reason for propagation of ionization and heating. Nomenclature a = thermal sound velocity c = speed of light D = velocity of waves EQ = energy of radiation F -free diffusion coefficient of electrons h = Planck constant / = flux density of laser light K = absorption coefficient k = Boltzmann constant n = number density for unit volume, cm~3 P = laser beam power p = pressure r,x = positions T -absolute temperature t = time u = material velocity Z = atomic number 18 = recombination coefficient v = frequency of creation of electron X = ionization potential 12 = geometrical parameter (generally a solid angle) T = delay to ionization p = mass density •y = polytropic coefficient AJ£o = band width of radiation X = mean free path Subscripts 0 = initial or neutral B = related to Bremstrahlung C = related to breakdown E = expansion fan e = associated to electrons i -related to ionization by collisions P = Planck R = radiation wave RO = Rosseland S = shock wave t -related to threshold or direct ionization Presented as Paper 68-678 at the AIAA Fluid and Plasma