In his comment on our paper 'Information hidden in the velocity distribution of ions and the exact kinetic Bohm criterion' (2017 Plasma Sources Sci. Technol. 26 055003) Benilov objects some of the ...interpretations we give to the Bohm criterion in general and to its kinetic form in particular. In this reply we elaborate further the discussions from the original paper and show that actually most of our views are well aligned with those expressed by Benilov, whereas the disagreements seem to be mostly a consequence of ambiguous terminologies and definitions.
Abstract
Particle-in-cell simulations are used to study how neutral pressure influences plasma properties at the sheath edge. The high rate of ion–neutral collisions at pressures above several mTorr ...are found to cause a decrease in the ion velocity at the sheath edge (collisional Bohm criterion), a decrease in the edge-to-center density ratio (
h
l
factor), and an increase in the sheath width and sheath potential drop. A comparison with existing analytic models generally indicates favorable agreement, but with some distinctions. One is that models for the
h
l
factor need to be made consistent with the collisional Bohm criterion. With this and similar corrections, a comprehensive fluid-based model of the plasma boundary transition is constructed that compares well with the simulation results.
Abstract In laboratory and space plasmas, the emission of electrons from the surface significantly affects the characteristics of the plasma sheath that forms at that surface, which is crucial to ...understanding the overall plasma-wall interaction mechanism. In this work, the collisional fluid model is used for laboratory dusty plasma, whereas the collisionless model is used for lunar dusty plasma. We have extended the Bohm sheath criterion for the formation of the stable plasma sheath due to electron emission from the surface, loss of ion flux, and the gas pressure of the collisional laboratory dusty plasmas. It is found that ion flow at the sheath boundary is considerably influenced by the concentration of electron emission, the ion loss term, and gas pressure. The evolution of the dust charge explicitly determines the magnitude of the ion flow at the sheath boundary. The plasma parameters adopted in the present case are reliable in laboratory and space dusty plasmas, especially the dusty plasma environment on the lunar surface. The lunar surface and dust grains on the Moon become electrically charged as a result of the interaction between solar wind plasma and photoemission electrons emitted from the lunar surface. In addition, the lunar plasma sheath characteristics, dust-charging process, and stable dust levitation in the sheath region have been studied.
The plasma exit flow speed at the sheath entrance is constrained by the Bohm criterion. The so-called Bohm speed regulates the plasma particle and power exhaust fluxes to the wall, and it is commonly ...deployed as a boundary condition to exclude the sheath region in quasineutral plasma modeling. Here the Bohm criterion analysis is performed in the intermediate plasma regime away from the previously known limiting cases of adiabatic laws and the asymptotic limit of infinitesimal Debye length in a finite-size system, using the transport equations of an anisotropic plasma. The resulting Bohm speed has explicit dependence on local plasma heat flux, temperature isotropization, and thermal force. Comparison with kinetic simulations demonstrates its accuracy over the plasma-sheath transition region in which quasineutrality is weakly perturbed and the Bohm criterion applies.
Sheath formation and its characteristics are studied in collisional magnetized plasma consisting of nonextensive electrons and thermal ions. Using two-fluid model, the Bohm criterion for ion velocity ...is deduced as a function of plasma parameters and shown that ion-neutral collisions impose an upper limit for the Bohm criterion. It is also found that deviation from the standard Maxwellian distribution significantly affects the sheath characteristics. Sheath potential and ion density are considered under the effects of magnetic field, ion temperature, nonextensivity and ion-neutral collision frequency. The results indicate that by applying an external magnetic field, it is possible to control the sheath thickness.
Tsankov and Czarnetzki (2017 Plasma Sources Sci. Technol. 055003) showed that measurements with a wall-mounted plasma monitor can allow noninvasive access to spatially resolved plasma parameters, ...including the ion velocity distribution. On the basis of experimental data and numerical modeling, they proposed a new form of collisionally modified kinetic Bohm criterion. In this work, we point out that the kinetic Bohm criterion may be written in a form that is exactly equivalent to the original criterion but does not pose the much-debated problem of divergence for slow ions. This form enables one to show that the kinetic Bohm criterion, as well as the Bohm criterion for monoenergetic ions, has a distinct mathematical meaning. Collisionally modified Bohm criteria do not possess the same, or any other, mathematical meaning. Hence, there are no mathematical grounds for speaking of a collisionally-modified Bohm criterion and no non-arbitrary way to introduce it. Therefore, the use of the term 'exact kinetic Bohm criterion' in the paper by Tsankov and Czarnetzki is hardly justified.
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