Application of a particle simulation to modeling commutation in a linear thyratron

Abstract

Commutation is the period during which a thyratron makes the transition between an open state and a conducting state. During commutation the voltage across the thyratron decreases from the holdoffvoltage to the conduction value; 1O's ofkV to < 100 V. The time for commutation, as measured by the anode voltage fall time, is usually 10-100 ns, a value whi.ch depends on the holdoffvoltage, internal gas pressure, and grid geometry. In this paper, a model for commutation in a thyratron is described and its results are compared to experiment for a thyratron having a linear geometry. The model uses a Monte Carlo particle simulation for electrons and a continuum fluid representation for ions. A particle mUltiplication and renormalization scheme is used in the model to simulate electron avalanche so that only a moderate number of particles (4000-12000) need to be used. A modified null cross-section technique is used to account for large changes in the density of electron collision partners as a function of position or time. A model for the external circuit enables simulation of current and voltage. Results from the model for these quantities agree wen with experiment, and indicate that commutation occurs in two stages. A survey study contrasts the tradeoff between highvoltage holdoff and switching speed.

Effect of anisotropic scattering for rotational collisions on electron transport parameters in CO

“…Once again, significant differences among calculated reduced mobilities can be observed for E/N < 3 Td, when different angular scattering models are used in MC simulations. In particular, deviations exceeding 50% are found between results from MC simulations with isotropic scattering and with a dipole-Born model for rotational processes at 77 K, these differences decreasing to about 30% at 300 K. However, contrarily to what is observed for results obtained with Biagi's cross sections, MC simulations assuming Kushner's anisotropic model with m = 1 [29] lead to the best agreement with measurements. The use of other scattering models, like the one by Longo and Capitelli [59] or the dipole-Born DCS, leads to strong overestimations of μN for E/N < 3 Td, when compared with experimental data.…”

“…The advantage of using this formulation is that one might keep the same ICS, while varying the angular scattering model [27,58]. The considered angular distributions are based on isotropic scattering, empirical formulas given by Longo and Capitelli [59] and Kushner [29], and the dipole-Born theory [31]. The scattering angles in electron-molecule collisions can be sampled according to a theorem of probability theory [60], by inverting the following expression:…”

“…where α = 2Θ m /π. In [29], Kushner proposes an empirical expression for the angular distribution of the form:…”

“…In fact, the description of different types of rotational interactions requires different angular scattering models that are consistent with the net momentum exchange. This aspect is not addressed in the work of Casey and co-authors [28], where simple empirical formulas are used for anisotropic rotational scattering, similar to the ones proposed by Kushner [29] and by Reid [25].…”

Evaluation of angular scattering models for electron-neutral collisions in Monte Carlo simulations

“…In Monte Carlo simulations of electron transport in low temper ature plasmas, it is commonly assumed that the elastic momentum transfer cross section and the total cross sections for the inelastic scattering processes and for ionization provide sufficient information about electron interactions with the neutral background gas. The approach to forward scattering with increasing energy in electron-neutral collisions is usually not taken into account although some authors have done so by using simplified models for the angular scattering based on classical theory using a screened Coulomb potential or purely empirical formulae (see, for example, Boeuf & Marode [1]; Kushner [2]; Surendra et al [3]; Belenguer & Pitchford [4]). The absence of detailed knowledge about the differential scattering cross sections (DCS) for electron-neutral collisions generally precludes more precise calculations.…”