No abstract available

[en] We present a simple analytic model for ablative stablization of the Rayleigh-Taylor instability. In this model the effect of ablation is to move the peak of the perturbations to the location of peak pressure. This mechanism enhances the density-gradient stabilization, which is effective at short wavelengths, and it also enhances the stabilization of long-wavelength perturbations due to finite shell thickness. We consider the following density profile: exponential blowoff plasma with a density gradient β, followed by a constant-density shell of thickness δt. For perturbations of arbitrary wave number k, we present an explicit expression for the growth rate γ as a function of k, β, and δt. We find that ''thick'' shells defined by β δt≥1 have γ²≥0 for any k, while ''thin'' shells defined by β δt<1 can have γ²<0 for small k, reflecting stability by proximity to the back side of the shell. We also present LASNEX simulations that are in good agreement with our analytic formulas

[en] Using a new electron gun, a number of measurements bearing on the generation of beam--plasma discharge (BPD) in WOMBAT (waves on magnetized beams and turbulence) [R. W. Boswell and P. J. Kellogg, Geophys. Res. Lett. 10, 565 (1983)] have been made. A beam--plasma discharge is an rf discharge in which the rf fields are provided by instabilities [W. D. Getty and L. D. Smullin, J. Appl. Phys. 34, 3421 (1963)]. The new gun has a narrower divergence angle than the old, and comparison of the BPD thresholds for the two guns verifies that the BPD ignition current is proportional to the cross-sectional area of the plasma. The high-frequency instabilities, precursors to the BPD, are identified with the two Trivelpiece--Gould modes [A. W. Trivelpiece and R. W. Gould, J. Appl. Phys. 30, 1784 (1959)]. Which frequency appears depends on the neutral pressure. The measured frequencies are not consistent with the simple interpretation of the lower frequency as a Cerenkov resonance with the low-Trivelpiece--Gould mode; it must be a cyclotron resonance. As is generally true in such beam--plasma interaction experiments, strong low-frequency waves appear at currents far below those necessary for BPD ignition. These low-frequency waves are shown to control the onset of the high-frequency precursors to the BPD. A mechanism for this control is suggested, which involves the conversion of a convective instability to an absolute one by trapping of the unstable waves in the density perturbations of the low-frequency waves. This process greatly reduces the current necessary for BPD ignition

[en] When the resonance condition of the particle-wave interaction is varied adiabatically, the particles trapped in a wave are found to form phase space holes or clumps that enhance the particle-wave energy exchange. This mechanism can cause increased saturation levels of instabilities and even allow the free energy associated with instability to be tapped in a system in which background dissipation suppresses linear instability

[en] OAK-B135 The effect of a Gaussian prepulse (picket pulse) before a ''drive'' pulse on the Rayleigh-Taylor (RT) instability growth rate was measured for single-mode, 20-, 30-, and 60-mm-wavelength mass perturbations. These data, from the OMEGA [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] laser system, show that the measured RT growth of mass perturbations was reduced when a picket pulse was used. The picket pulse and subsequent relaxation period, before the drive pulse, cause the foil to expand and rarefy, resulting in higher ablation velocities during the drive pulse and greater ablative stabilization. This effect was examined both computationally and experimentally for different picket-pulse intensities

No abstract available

[en] The propagation of a large-amplitude lower-hybrid wave is considered. Modulational instabilities arising from its interaction with low-frequency electrostatic perturbations are investigated. The growth lengths of the convective instabilities are obtained and compared with previous results for adiabatic perturbations. (author)

[en] Dust grain growth and settling time inside initial gaseous giant protoplanets in the mass range 0.3 to 5 Jovian masses, formed by gravitational instability, have been investigated. We have determined the distribution of thermodynamic and physical variables inside the protoplanets solving the structure equations assuming their gas blobs to be fully convective and with this distribution we have calculated growth and settling time of grains with different initial sizes (10⁻² cm ≤ r₀ ≤ 1 cm). The results of our calculations are found to be in good agreement with those obtained by different approaches.

[en] The Jeans instabilities in an unmagnetized, collisionless, isotropic self-gravitating matter system are investigated in the context of $\mathrm{\kappa <!--\; ??\; -->}$-deformed Kaniadakis distribution based on kinetic theory. The result shows that both the growth rates and critical wave numbers of Jeans instability are lower in the $\mathrm{\kappa <!--\; ??\; -->}$-deformed Kaniadakis distributed self-gravitating matter systems than the Maxwellian case. The standard Jeans instability for a Maxwellian case is recovered under the limitation $\mathrm{\kappa <!--\; ??\; -->}=0$. (paper)

[en] Capabilities of stationary states in a system of weakly coupled waves one of which is instable relative to a decay into a half subharmonic, are analyzed taking beam-plasma instability as an example. It is shown that under certain values of system parameters three-wave interaction conditioning decay instability becomes notable. In the case when three-wave decay in a beam-plasma system is absent the nonlinear dynamics of instability progress will be defined by a nonlinear frequency shift. It turns out that the maximum amplitudes of coupled waves under the presence of decay instability are lower than the amplitudes of set waves in the absence of decay