Albert Simon

On the inhomogeneous two‐plasmon instability

The two‐plasmon instability in warm inhomogeneous plasma for a normally incident pump is considered. The complex eigenfrequencies of the absolute instability are obtained by reducing the linearized fluid equations to a Schrodinger equation in wavenumber space. These eigenvalues are obtained in several ways. One is by combining a perturbation expansion in powers of the reciprocal scale length with WKB theory. The resulting algebraic equations are solved by three analytical approximations and by direct numerical solution. A second way is by analysis of the Schrodinger equation using an interactive WKB computer code. A third way is by the use of a shooting code. These methods are all used and compared for threshold curves and growth rates above threshold. Some eigenfunction forms are also obtained. The threshold is near (v0/ve)2k0 L =3, and varies weakly with β≂v4e/v20c2, rising from near 2 to about 4 over six decades of variation of β. The corresponding critical value of (ky/k0)2 is near 0.2/β over this ran...

Laser-plasma interactions in long-scale-length plasmas under direct-drive National Ignition Facility conditions

Laser-plasma interaction experiments have been carried out on the OMEGA laser system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] under plasma conditions representative of the peak of a 1.5 MJ direct-drive laser pulse proposed for the National Ignition Facility (NIF). Plasmas have been formed by exploding 18–20 μm thick CH foils and by irradiating solid CH targets from one side, using up to 20 kJ of laser energy with phase plates installed on all beams. These plasmas and the NIF plasmas are predicted to have electron temperatures of 4 keV and density scale lengths close to 0.75 mm at the peak of the laser pulse. The electron temperature and density of the exploding-foil plasmas have been diagnosed using time-resolved x-ray spectroscopy and stimulated Raman scattering, respectively, and are consistent with predictions of the two-dimensional Eulerian hydrodynamics code SAGE [R. S. Craxton and R. L. McCrory, J. Appl. Phys. 56, 108 (1984)]. When the solid-target or exploding-foil plasmas were irradiate...

Effect of plasma noise spectrum on stimulated scattering in inhomogeneous plasma

The flux of radiation emitted from an inhomogeneous plasma by stimulated Raman and Brillouin scattering is calculated with a source that includes both bremsstrahlung and longitudinal plasma wave noise sources. Significant enhancement of the flux above that found for a bremsstrahlung source alone are found for stimulated Brillouin scattering and for nonthermal but stable velocity distributions.

Single‐mode saturation of the bump‐on‐tail instability: Immobile ions

A slightly unstable plasma with only one or a few linear modes unstable is considered. Nonlinear saturation at small amplitudes has been treated by time-asymptotic analysis which is a generalization of the methods of Bogolyubov and co-workers. In this paper the method is applied to instability in a collisionless plasma governed by the vlasov equation. The bump-on-tail instability is considered for a one-dimensional plasma. (MOW)

Nonlinear laser-matter interaction processes in long-scale-length plasmas

This paper reports on nonlinear laser–plasma interaction experiments using long‐scale‐length plasmas produced by the 24‐beam OMEGA laser system operating at 351 nm. The experiments were carried out with distributed phase plates (DPP’s) in all beams and with and without smoothing by spectral dispersion (SSD). Most of the beams were used to create a large preformed plasma, which had gradient scale lengths of ≤800 μm at electron densities below a quarter of the critical density nc and temperatures in excess of 1 keV. One of the beams, the ‘‘interaction beam,’’ was timed independently and tightly focused to intensities ∼1015 W/cm2. All beams had pulse durations of ∼0.6 nsec. The interaction processes studied were mainly Raman scattering and the two‐plasmon decay (TPD) instability as evidenced by its characteristic 3/2‐harmonic emission. Details of the Raman and 3/2‐harmonic spectra are presented. Evidence was found for the TPD instability close to its Landau cutoff density at ∼ 0.2nc. Raman emission was narro...

Raman scattering in inhomogeneous laser‐produced plasma

A model of Raman scattering in inhomogeneous laser‐produced plasma is described, which invokes enhanced Thomson scattering. The enhancement is caused by pulses of hot electrons arising from laser–plasma interactions at the critical or quarter‐critical surfaces. A simple model predicts the locations of two enhanced frequency bands, one between ω0 and ω0/2, and the other between 2ω0 and ω0. The model is shown to fit the results of eight experiments, including a new experiment using the six‐beam Omega laser facility [Phys. Rev. Lett. 48, 1179 (1982); Phys. Fluids 27, 2181 (1984)], converted to 351 nm. In this latter experiment, simultaneous measurement of the upscattered and downscattered bands is carried out, and good agreement is found with the enhanced Thomson model.

Nonlinear Theory of the E x B Instability

A weakly ionized plasma will go unstable in the presence of a density gradient and parallel electric field, both at right angles to a magnetic field, if the electric field is of the right sign and sufficiently large. The nonlinear limit of the first mode to go unstable when E is increased is determined as a function of Δ [Δ ≡ (E ‐ Ec)/Ec]. A general theory of this small amplitude behavior is used. The mode amplitude, frequency shift, density flattening, electric field change, and anomalous plasma transport are obtained as functions of Δ. It is shown that mode coupling corrections are small, and hence that the quasilinear result is sufficient.

Convection of Weakly Ionized Plasma in a Nonuniform Magnetic Field

A weakly ionized plasma in a nonuniform magnetic field exhibits an instability analogous to the well‐known flute instability of a fully ionized plasma. The instability sets in at a critical magnetic field. When this external field is slightly increased above the critical value by the fractional amount Δ, to begin with only one mode goes unstable and it grows in amplitude to a finite limiting value. Using a general theory of the nonlinear behavior derived in an accompanying paper, we evaluate the mode amplitude and frequency shift as functions of Δ, as well as the corresponding enhanced particle transport. The results are first obtained in the quasilinear approximation, but it is then shown that the mode‐coupling contribution is negligible and hence that the result is generally valid for small Δ.

Nonlinear saturation of the absolute stimulated Raman scattering instability in a finite collisional plasma

Using multiple time scale analysis, the nonlinear saturation of the absolute stimulated Raman scattering instability is examined in a finite homogeneous collisional plasma. The amplitude of the incident wave is assumed to exceed the absolute instability threshold by the fractional amount Δ(≪1). A single backscattered wave and a single plasma wave grow until time‐asymptotic saturation occurs. The reflected light intensity is determined analytically and is proportional to Δ. The spatial variation of the saturated wave amplitudes is also obtained. The reflected intensity is compared to the values predicted for the convective instability, for incident intensities which are marginally less than the absolute threshold intensity. In ‘‘short’’ plasmas, i.e., ones which extend over only a few linear convective gain lengths, the reflected light intensity is found to be much higher when the absolute instability is excited.

Nonlinear theory of the collisional drift‐wave instability

Experiments have shown the existence of a steadily oscillating drift mode above a critical magnetic field, Bc, in fully ionized Q machines. The amplitude and frequency shift of this mode are calculated as a function of Δ = (B − Bc)/Bc for small Δ. The self‐consistent analysis includes temperature variation and shows that non‐linear saturation is due to flattening of the density gradient by zero‐frequency harmonic generation. The results are compared with experiment and show good agreement.