No abstract

Presented are simulations in one dimension of laser pulses with a pre-ionized thin target. The duration of the laser pulse is between 10 and 200 fs, the laser power is between 1018 and 1022 W/cm2, and the wavelength is 400 nm. The targets were foils of either aluminum at normal density, or in a few cases plastic (chemical formula CH) at a density of 1.1 g/cm3, and varied from 20 to 4000 nm in thickness. The simulation results show that at these intensities the absorption mechanism for a normally incident pulse changes to a collisionless mechanism first proposed by Kruer and Estabrook [W. L. Kruer and K. Estabrook, Phys. Fluids 28, 430 (1985)]. Use of a thin target ensures that the energy imparted to the electrons will be electrostatically transferred to the ions rather than transported for large distances into the target.

The nonuniform (nonlinear) equilibria of the classical (short circuit) Pierce diode and the extended (series RLC external circuit) Pierce diode are described, and the spectrum of oscillations (stable and unstable) about these equilibria are worked out. It is found that only the external capacitance alters the equilibria, though all elements alter the spectrum. In particular, the introduction of an external capacitor destabilizes some equilibria that are marginally stable without the capacitor. Computer simulations are performed to test the theoretical predictions for the case of an external capacitor only. It is found that most equilibria are correctly predicted by theory, but that the continuous set of equilibria of the classical Pierce diode at Pierce parameters (α=ωpL/v0) that are multiples of 2π are not observed. This appears to be a failure of the simulation method under the rather singular conditions rather than a failure of the theory.

A Kelvin-Helmholtz instability has been identified numerically on an azimuthally symmetric Alfvdn resonant layer in an axially bounded, straight cylindrical coronal loop. The physical model employed is an incompressible, reduced magnetohydrodynamic (MHD) model including resistivity, viscosity, and density variation. The set of equations is solved numerically as an initial value problem. The linear growth rate of this instability is shown to be approximately proportional to the Alfv6n driving amplitude and inversely proportional to the width of the Alfv6n resonant layer. It is also shown that the linear growth rate increases linearly with m -1 up to a certain m, reaches its maximum value for the mode whose half wavelength is comparable to the Alfv6n resonant layer width, and decreases at higher m's. (m is the azimuthal mode number.)