Transition from Townsend to glow discharge: subcritical, mixed or supercritical

The full parameter space of the transition from Townsend to glow discharge is investigated numerically in one space dimension in the classical model: with electrons and positive ions drifting in the local electric field, impact ionization by electrons ($\alpha$ process), secondary electron emission from the cathode ($\gamma$ process) and space charge effects. We also perform a systematic analytical small current expansion about the Townsend limit up to third order in the total current that fits our numerical data very well. Depending on $\gamma$ and system size pd, the transition from Townsend to glow discharge can show the textbook subcritical behavior, but for smaller values of pd, we also find supercritical or some intermediate ``mixed'' behavior. The analysis in particular lays the basis for understanding the complex spatio-temporal patterns in planar barrier discharge systems.Comment: 12 pages, 10 figures, submitted to Phys. Rev.

On the use of Monte Carlo simulations to model transport of positrons in gases and liquids

7 pags.; 5 figs.In this paper we make a parallel between the swarm method in physics of ionized gases and modeling of positrons in radiation therapy and diagnostics. The basic idea is to take advantage of the experience gained in the past with electron swarms and to use it in establishing procedures of modeling positron diagnostics and therapy based on the well-established experimental binary collision data. In doing so we discuss the application of Monte Carlo technique for positrons in the same manner as used previously for electron swarms, we discuss the role of complete cross section sets (complete in terms of number, momentum and energy balance and tested against measured swarm parameters), we discuss the role of benchmarks and how to choose benchmarks for electrons that may perhaps be a subject to experimental verification. Finally we show some samples of positron trajectories together with secondary electrons that were established solely on the basis of accurate binary cross sections and also how those may be used in modeling of both gas filled traps and living organisms. © 2013 Elsevier Ltd.This work was supported by the MNTR, Serbia, under Contracts ON171037, III41011 and SANU155.Peer Reviewe

Electron swarm transport in THF and water mixtures

6 pags.; 7 figs. Contribution to the Topical Issue “Electron and Positron Induced Processes”, edited by Michael Brunger, Radu Campeanu, Masamitsu Hoshino, Oddur Ing´olfsson, Paulo Lim˜ao-Vieira, Nigel Mason, Yasuyuki Nagashima and Hajime Tanuma.The transport coefficients of electrons in mixtures of gaseous water and tetrahydrofuran (THF) are calculated using a multi-term solution of the Boltzmann equation. Electron transport coefficients at room temperature are presented over a range of reduced electric fields from 0.1-1000 Td, with significant differences between the behaviour in pure water and pure THF being found. The influence of the water to THF mixture ratio on the calculated transport coefficients is also presented. © 2014 EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg.Spanish Ministerio de Economia y Competitividad (Project FIS2012-31230) and MONTRS Projects ON171037 (and III41011).Peer Reviewe