Enhancement Of Density Fluctuations Research Articles

Thermodynamic instability and the critical point of fine particle (dusty) plasmas: enhancement of density fluctuations and experimental conditions for observation

We analyze thermodynamics of fine particle (dusty) plasmas, regarding them as systems composed of charged particles with hard cores interacting via the repulsive Yukawa potential and the ambient plasma (of ions and electrons), taking the contribution of the latter properly into account. When the Coulomb coupling between fine particles becomes sufficiently strong, the isothermal compressibility of the whole system diverges and we have a phase separation and an associated critical point. The enhancement of long-wavelength density fluctuations near the critical point is shown. Experimental conditions of fine particle plasmas, densities and temperatures of components and the fine particle size are obtained corresponding to characteristic parameters around the critical point and the dependence on ion species and other factors is discussed. These phenomena will be observed in the bulk three-dimensional system which may be realized on the ground by somehow canceling the effect of gravity on fine particles. Experiments under the microgravity environment are naturally expected to provide a chance of observation.

Magnetic Reynolds number effects in compressible magnetohydrodynamic turbulence

The effects of the magnetic Reynolds number, Reσ, on decaying two-dimensional compressible magnetohydrodynamic (MHD) turbulence are investigated through direct numerical simulations. The initial relative intensities of, and the correlation between, the fluctuating velocity (u) and magnetic induction (b) fields are also varied, as measured with the respective parameters f and angle θ. The investigations cover the parameter ranges 1⩽Reσ⩽250, 0°⩽θ⩽90°, and 1⩽f⩽3. The results suggest that, at the lowest Reσ investigated, the magnetic field has a negligible impact on the evolution of the turbulence kinetic energy Ek. At higher Reσ values, when magnetic effects are important, the magnetic field tends to accelerate the decay of the turbulence energy relative to non-MHD flows. On the other hand, the magnetic energy Eb shows the opposite trend, being rapidly driven from its initial values to essentially zero very early in the transient at lower Reσ values, while higher Reσ values significantly retard this decay. An enhancement of density fluctuations is noted in the intermediate Reσ range. An interesting observation pertaining to the normalized cross helicity is the fast decay to zero of this quantity when Reσ=1, independent of the values of θ and f. That is, the fluctuating u and b fields tend to be uncorrelated when the magnetic Reynolds number is low. In this case, the role of the magnetic field is passive, and it is merely convected by the velocity field. The conditions required to maintain a high correlation during the evolution are discussed. We have also seen that the Eb decay mode is less sensitive to the value of θ than that of Ek. The relative contribution of Ek, Eb, and the internal energy Ei to the total energy Et is discussed in relation to the values of f, θ, and Reσ.