The linear stability of an ablating plasma is investigated as an eigenvalue problem by assuming the plasma to be at the stationary state. For various structures of the ablating plasma, the growth rate is found to be expressed well in the form γ=α(kg)1/2 −βkVa, where α=0.9, β≂3–4, and Va is the flow velocity across the ablation front, and is found to agree well with recent two-dimensional simulations in a classical transport regime. Short-wavelength lasers inducing enhanced mass ablation are suggested to be advantageous to stable implosion because of the ablative stabilization.

A mathematical method for a fully self-consistent treatment of the Rayleigh–Taylor instability is developed by solving the linearized fluid equations as an eigenvalue problem. The method is applied to analyze the instability in stationary ablating plasmas with strong inhomogeneity. A reduction of growth rate compared to the classical value is found. The importance of a self-consistent treatment of the Rayleigh–Taylor instability is shown by comparing the result with the growth rate estimated by approximate theoretical arguments.

Simulations of the Rayleigh-Taylor instability of ablatively accelerated thin-shell fusion targets show that the nonlinear evolution exhibits spike amplitude saturation due to ablative mass removal; the shell anterior surface evolves to a laminar (nonturbulent) quasistationary distorted state. The perturbed flow causes a significant departure from spherically symmetric behavior, but the laminar shell interior structure makes it appear possible to retain some of the advantages of larger-aspect-ratio fusion targets.PACS numbers: 52.50.Jm, 47.20.+ m, 52.35.Py, 52.65,+ z Spherically symmetric calculations of the behavior of laser-driven fusion targets have demonstrated major advantages of employing ablatively imploded spherical shells for obtaining optimum performance. The use of shells, as opposed to solid spheres, has been shown to reduce significantly the peak laser power required to drive successfully a target of fixed mass. In general, it is predicted that performance improves with increasing shell aspect ratio, A (=r/Ar) 9 where r and Ar are the initial radius and thickness of the shell, 1 " 3 and the useful range of A is approximately 5

Calculations are shown of the structure of plasmas in equilibrium with solid surfaces that reemit incident plasma ions as relatively cold neutral gas. A numerical transport model that includes a Fokker–Planck treatment of ion–ion collisions obtains the distribution function for ions in a phase space of one spatial coordinate and two velocities. This is done self-consistently with an electrostatic potential, a Maxwell–Boltzmann description of electrons, and electron impact ionization of the reemited neutrals. Solutions are obtained from a higher temperature kinetic regime where Coulomb collisions are nearly negligible to a lower temperature regime where plasma behavior is approximately fluidlike. A result of these calculations is the resolution of an ambiguity posed by previous kinetic regime calculations that omitted ion–ion collisions and obtained a family of solutions for each set of physical parameters [Phys. Rev. Lett. 49, 650 (1982); Phys. Fluids B 1, 448 (1989)]. The physically correct solution for semi-infinite surface plasmas is shown to be the member of each family that maximizes the ion thermal conduction to the surface and the magnitude of a maximum in the electrostatic potential that is found in these and the previous calculations. Further results are in agreement at lower temperatures with solutions obtained from a fluid model and the identification of the correct boundary condition on normal flow velocity to be used in fluid models.

A numerical study is done of a plasma in contact with a solid surface that reemits some fraction of the incident plasma as neutral gas. The calculation uses a steady-state, kinetic treatment of the transport equations in one space dimension and one or two velocity dimensions to determine self-consistently the distribution functions of the interacting species and the electrostatic potential. The dominant phenomena are the ionization of the neutral gas and the acceleration of the resulting ions away from a potential maximum that is predicted to form in the ionization region. Other effects involved are a Debye sheath structure between the solid surface and the potential maximum, and collisional trapping and untrapping of electrons in the well represented by the potential maximum. Results are presented from a nondimensional model with a monatomic returning neutral species, and for diatomic molecular deuterium returning from the surface. For each set of physical parameters chosen, a one parameter family of solutions is obtained. A hypothesis is presented for the choice from this family of solutions that would be found experimentally.