Large-amplitude Density Research Articles

Fully relativistic filamentation instability of strong electromagnetic radiation in unmagnetized plasmas

Accounting for fully relativistic electron quiver velocity and large-amplitude density fluctuations driven by a fully relativistic ponderomotive force, filamentation instability of an intense circularly polarized electromagnetic wave has been investigated. Thus, the nonlinearities arising from the relativistic electron mass variation and the relativistic ponderomotive force are treated exactly, so that our theory is valid for arbitrarily large values of the radiation intensity. A novel filamentation instability is shown to exist at high pump power.

Stimulated Raman scattering in the presence of stimulated Brillouin scattering

Particle simulations of stimulated Raman scattering (SRS) are presented in which stimulated Brillouin scattering (SBS) also occurs. Frequency spectra of forward and backscattered light are compared for various ratios of ion mass to electron mass. SBS is seen to weaken the SRS backscattering in all cases in which the plasma density profile includes the quarter-critical density. For cases in which the plasma density remains below quarter-critical, the SRS backscattering is not strongly affected by the SBS. In long-scale length, quarter-critical density cases for which mi/me=400, large amplitude density perturbations grow and persist.

Radiative instabilities in a sheared magnetic field

The structure and growth rate of the radiative instability in a sheared magnetic field B have been calculated analytically using the Braginskii fluid equations. In a shear layer, temperature and density perturbations are linked by the propagation of sound waves parallel to the local magnetic field. As a consequence, density clumping or condensation plays an important role in driving the instability. Parallel thermal conduction localizes the mode to a narrow layer where k∥ =k⋅B/‖B‖ is small and stabilizes short wavelengths k>kc, where kc depends on the local radiation and conduction rates. Thermal coupling to ions also limits the width of the unstable spectrum. It is shown that a broad spectrum of modes is typically unstable in tokamak edge plasmas and it is argued that this instability is sufficiently robust to drive the large-amplitude density fluctuations often measured there.

Catapult mechanism for fast particle emission in fission and heavy ion reactions

The fission process of slabs of nuclear matter is modelled in TDHF approximation by adding an initial collective velocity field to the static self-consistent solution. In dependence on its amplitude either large amplitude density oscillations are excited or fission occurs. The final disintegration of the slab proceeds on a time scale of 10−22s and is characterized by a sharp peak in the actual velocity field in the region of the “snatching” inner low density tails. A characteristic time later corresponding to the transit time of a nucleon across the fragment with mean velocity being the Fermi velocity plus twice the maximum “snatching” velocity, a low density lump correlated with a peak in the velocity field emerges in front of the fragments. We call these particles “catapult particles”. Recent experimental results possibly provide evidence for catapult neutrons in low-energy fission. We also speculate on the significance of the catapult mechanism for fast particle emission in the exit channel of heavy ion reactions.

A study of the liquid–vapor interface of mercury: Computer simulation results

This paper extends our recently proposed pseudoatom theory for the liquid–vapor interface of simple metals to mercury. An important difference between mercury and the alkali metals is that in the former the metal–nonmetal transition takes place over a range of density positioned at much higher density than the critical density. A modified pseudoatom model, with an improved treatment of the metal–nonmetal transition, is presented. Using the modified effective Hamiltonian for the ions, Monte Carlo simulations of the liquid mercury surface have been carried out. The simulations reveal a highly structured liquid–vapor transition zone, with large amplitude density oscillations. The metal–nonmetal transition is found to play a critical role in determining the liquid–vapor interface structure, and it gives rise to a partial monolayer of nonmetallic atoms adsorbed on the metal surface. A detailed comparison of our results with the existing experimental and theoretical work, particularly the x-ray reflectance data of Lu and Rice, is presented.

Suppression of Density Waves in Clarifiers

Experiments on a 4-ft (1.2-m) diameter model circular clarifier operating at overflow rates and suspension concentrations corresponding to municipal treatment practice show that baffles designed to function as wave traps suppress low-frequency, large-amplitude density waves observed in these units. Simultaneous measurements of solids removal show substantial improvement. This work establishes the relationship between density waves and the deviation from ideal clarifier performance that earlier papers on internal waves by our research group suggested. The power spectrum of the internal waves and the effect of operating variables such as overflow rate, scraper speed, and suspended solid on the spectrum and solids removal are discussed.

Observation of classical diffusion in the presence of large amplitude density fluctuations

Density fluctuations, δn n ≲ 20%, were observed to have no effect on the classical diffusion of a collisional helium plasma across a poloidal octupole field with an additional weak toroidal field. The implication of these results on tokamak scattering experiments is discussed.

A possible mechanism for mass transfer in X-ray binary systems with OB supergiant companions

We have studied the ''beginning Roche lobe overflow'' phase of mass transfer. We find that before the primary fills its Roche lobe, radiation-driven density waves generated in the atmosphere produce a sufficient mass outflow rate to power a compact X-ray source. In particular, for a model of Cygnus X-1, if the radius of the photosphere is between 82 and 86 percent of the radius of the Roche equipotential, large-amplitude density waves with irregular periods of a few days are observed. These density waves, which are generated by radiation pressure effects, pass through the inner Lagrangian point with velocities approx.100 km s/sup -1/, and can easily provide sufficient material to power the compact X-ray source. (AIP)

Observation of a stationary shock-like structure and enhanced diffusion in a toroidal plasma

Electrons are injected into an afterglow plasma confined in an ℓ = 3 stellarator in order to raise the velocity of poloidal rotation. A steady shock-like structure characterized by a steep density gradient is observed near the critical rotational speed. Strong enhancement of the diffusion rate occurs at higher radial electric fields, accompanied by large-amplitude density fluctuations. Vortices are generated at the three corners of the triangular magnetic surfaces when the radial electric field is reversed inside the plasma.