Two-region model for positive and negative plasma sheaths and its application to Hall thruster metallic anodes.

An asymptotic presheath/sheath model for positive and negative sheaths in front of a conducting electrode, with a continuous parametric transition at the no-sheath case, is presented. Key aspects of the model are as follows: full hydrodynamics of both species in the presheath; a kinetic formulation with a truncated distribution function for the repelled species within the sheath; and the fulfillment of the marginal Bohm condition at the sheath edge, in order to match the two formulations of the repelled species. The sheath regime depends on the ratios of particle fluxes and sound speeds between the two species. The presheath model includes the effect of a magnetic field parallel to the wall on electrons. An asymptotic, parametric study of the anode presheath is carried out in terms of the local ion-to-electron flux ratio and Hall parameter. The drift-diffusive model of magnetized electrons fails in a parametric region that includes parts of the negative sheath regime. In the case of the Hall parameter vanishing near the electrode and a weakly collisional plasma, a quasisonic, quasineutral plateau forms next to the sheath edge

Toda joia, toda beleza! Finding what is left in the margins or regime collisions: A pluralist take on managerialism

This paper has two authors, two titles and is written in the form of a dialogue,rather than conveying a unitary voice, as one would instead expect of a coauthored paper. The reason for this is that the articulation of the authors' disagreement, despite the identification of each of them with “the left”, is precisely the object of inquiry. After briefly introducing the problem on which the authors’ discussion takes place, namely regime collisions, and the clash of approaches that are available to (decide whether to) deal with them, a dialogue follows, in which the authors’ voices are clearly separated as they discuss the specific issue of the measurement of quality as a (managerialist) proposal to “solve” regime collisions, and contrast that to more openly politicised views of approaching regime collisions. In the end, the main features of such discussion are examined in such a way as to bring forth the peculiar self-consciousness that pluralist spaces give rise to, weakening and downsizing every point of view that appoints itself as the “higher” vantage point from which to describe the world and enumerate problems, and stimulating a constant oscillation between perspectives. This commitment to a pluralistic confrontation and the ensuing hybridization of perspectives is, we argue, at the heart of the idea of “the left” which we both identify with

Two charges on plane in a magnetic field: special trajectories

A classical mechanics of two Coulomb charges on a plane $(e_1, m_1)$ and $(e_2, m_2)$ subject to a constant magnetic field perpendicular to a plane is considered. Special "superintegrable" trajectories (circular and linear) for which the distance between charges remains unchanged are indicated as well as their respectful constants of motion. The number of the independent constants of motion for special trajectories is larger for generic ones. A classification of pairs of charges for which special trajectories occur is given. The special trajectories for three particular cases of two electrons, (electron - positron), (electron - $\alpha$-particle) are described explicitly.Comment: 22 pages, 5 figure