No abstract

Based on accurate representation of the He + -He angular differential scattering cross sections consisting of both elastic and charge exchange collisions, we performed detailed numerical simulations of the ion velocity distribution functions (IVDF) byMonte Carlo collision method (MCC). The results of simulations are validated by comparison with the experimental data of the ion mobility and the transverse diffusion. The IVDF simulation study shows that due to significant effect of scattering in elastic collisions IVDF cannot be separated into product of two independent IVDFs in the transverse and parallel to the electric field directions.

The kinetic theory of a low-voltage beam discharge instability in He is developed in the conditions, when the distance between electrodes is of the order of electron–atom collision length and the density of electrons in a primary beam is up to ten percent of the plasma density. The dispersion equation and its numerical and analytical solutions are obtained. The stability loss of this discharge is described in the framework of the problem with initial and boundary conditions. A significant dispersion is found in the propagation of harmonic disturbances in this system. On the basis of the developed theory, the areas of applicability of the hydrodynamic approach to the consideration of the system “cold electron beam - cold plasma” are elucidated. The theory significantly expands the range of parameters of the electron beam–plasma system, where a quantitative description of the spatial and temporal dynamics of wave propagation in such a system is possible. In the well-known special cases, such as a “weak electron beam - cold plasma,” the results obtained coincide with the data of other authors.

The instability of a low-voltage beam discharge in rare gases is studied using He as an example. Attenuation of an electron beam due to collisions of electrons with atoms is taken into account when the electron mean free path is of the order of the inter-electrode distance. It was found that amplification of several waves with different increments and different speeds is possible with the loss of stability. In the case of a linear decrease in the beam intensity, two waves are generated. The wave having a smaller gain increment propagates at a higher speed. The indicated phenomenon is characteristic not only for a low-voltage beam discharge in rare gases, but also for any “cold high-energy electron beam—plasma” system when collisions of beam electrons with atoms should be taken into account.

This Comment is devoted to some mathematical inaccuracies made by the authors of the paper 'Information hidden in the velocity distribution of ions and the exact kinetic Bohm criterion' (Plasma Sources Science and Technology 26 055003). In the Comment, we show that the diapason of plasma parameters for the validity of the theoretical results obtained by the authors was defined incorrectly; we made a more accurate definition of this diapason. As a result, we show that it is impossible to confirm or refute the feasibility of the Bohm kinetic criterion on the basis of the data of the cited paper.