The Influence of Thermal Motion of Particles on the Formation of Cavities in Plasma

Abstract

The effect of the thermal motion of ions and electrons on the formation of plasma density cavities which appear due to inhomogeneous stochastic electric fields is considered. Using the equation of motion of plasma particles in a constant magnetic field and in an inhomogeneous stochastic electric field with a frequency of the order of lower hybrid oscillations, taking into account the thermal motion of particles, the diffusion coefficients and drift velocities of ions and electrons are obtained. These values are used in the Fokker-Planck equation to determine the stationary distribution of the plasma density due to the effect of an inhomogeneous stochastic electric field. The conditions for the thermal velocities of particles under which the formation of a cavity is possible are obtained.

Temporal Evolution of the Plasma Density Cavity Caused by Inhomogeneous Stochastic Electric Fields

“…During the formation of the electron cavity, ions due to drift in inhomogeneous stochastic oscillations are displaced by a distance much less than electrons [16][17][18]. Therefore, in the region of the electron cavity a stationary electric field is formed, which, in turn, accelerates ions from this cavity and, as a result, a neutral plasma density cavity is formed.…”

“…where Δω is the width of the spectrum of stochastic oscillations which is the order of ω lh [16,17]. However, the effect of the stochastic electric field on diffusion along the magnetic field can be neglected in comparison with thermal motion, so…”

“…Let us study the evolution of the development of electron density cavity in time, assuming that the size of the region with an increased level of oscillations along the magnetic field is large enough, so that the electrons do not have time to leave this region before the cavity is formed. Substituting the diffusion coefficient (17) and drift velocity (26) into equation (1) yields…”

“…It is assumed that at the initial time t = 0 the plasma density has a uniform distribution n(x) = n 0 = const. It was shown [17,18] that due to the action of localized stochastic electric fields with an envelope F(x) at the end of evolution in a stationary state, i.e., at t = ∞ the dependence of the electron density on the x-axis is defined by the relation…”

“…It was shown that the movement of electrons is much faster than that of ions, so that the formation of an electron cavity occurs much earlier than an ionic one. In [17,18], the influence of the thermal motion of plasma particles on the conditions for the formation of a cavity was studied. In particular, it was shown that an electron density cavity can form if, during the formation of the cavity, the electrons do not leave the region with an increased level of oscillations along the magnetic field.…”