A new d.c. machine arrangement for an electronically assisted commutation

The d.c. machine is the cheapest way to realize variable speed drives. However its performances are limited by the commutator operation. Indeed, the latter wears out very rapidly because of the sparks, even arcings, which appear between the segments and the brushes, in spite of use of commutating poles. Over the last fifteen years, our laboratory has implemented external circuits to assist commutation and alleviate, or even suppress, the main difficulties. This paper presents a new arrangement which seems very promising.La machine à courant continu est le moyen le plus économique pour réaliser des entraînements à vitesse variable. La machine classique voit néanmoins ses performances limitées par la présence du collecteur. En effet, ce dernier, siège d'étincelles voire d'arcs d'origine, électromagnétique s'use vite et constitue de ce fait le point faible de la machine à courant continu. Depuis quelques années, notre laboratoire a mis l'accent sur la recherche de dispositifs externes d'aide à la commutation de ces machines qui remplaceraient avantageusement les pôles auxiliaires. Mais c'est la première fois qu'il expérimente une machine à courant continu de technologie classique mais sans pôles auxiliaires avec un système d'aide à la commutation électronique externe

Modeling of a new electron-streamer acceleration mechanism

International audienceLightning stepped leaders and laboratory spark discharges in air are known to produce X-rays [e.g., Dwyer et al., Geophys. Res. lett., 32, L20809, 2005; Kochkin et al., J. Phys. D: Appl. Phys., 45, 425202, 2012]. However, the processes behind the production of these X-rays are still not very well understood. During discharges, encounters between streamers of different polarities are very common. For example, during the formation of a new leader step, the negative streamer zone around the tip of a negative leader and the positive streamers initiated from the posiive part of a bidirectional space leader strongly interact. In laboratory experiments, when streamers are approaching a sharp electrode, streamers with the opposite polarity are initiated from the electrode and collide with the former streamers. Recently, the encounter between negative and positive streamers has been proposed as a plausible mechanism for the production of X-rays by spark discharges [Cooray et la., JASTP, 71, 1890, 2009; Kochkin et al., J. Phys. D: Appl. Phys., 45, 425202, 2012], but modeling results have shown later that the increase of the electric field involved in this process, which is above the conventional breakdown threshold field, is accompanied by a strong increase of the electron density. The resulting increase in the conductivity, in turn, causes this electric field to collapse over a few tens of picoseconds, preventing the electrons reaching high energies and producing significant X-ray emissions [e.g., Ihaddadene and Celestin, Geophys. Res. Lett., 45, 5644, 2015]. In this work, we will present simulation results of a new electron acceleration mechanism for producing runaway electron energies above hundred keV. The mechanism couples multiple single streamers and streamer head-on collisions, similar to a laboratory discharge, and is suitable for explaining the high-energy X-rays produced by discharges in air and by lightning stepped leaders

Modeling of a new electron-streamer acceleration mechanism

International audienceLightning stepped leaders and laboratory spark discharges in air are known to produce X-rays [e.g., Dwyer et al., Geophys. Res. lett., 32, L20809, 2005; Kochkin et al., J. Phys. D: Appl. Phys., 45, 425202, 2012]. However, the processes behind the production of these X-rays are still not very well understood. During discharges, encounters between streamers of different polarities are very common. For example, during the formation of a new leader step, the negative streamer zone around the tip of a negative leader and the positive streamers initiated from the posiive part of a bidirectional space leader strongly interact. In laboratory experiments, when streamers are approaching a sharp electrode, streamers with the opposite polarity are initiated from the electrode and collide with the former streamers. Recently, the encounter between negative and positive streamers has been proposed as a plausible mechanism for the production of X-rays by spark discharges [Cooray et la., JASTP, 71, 1890, 2009; Kochkin et al., J. Phys. D: Appl. Phys., 45, 425202, 2012], but modeling results have shown later that the increase of the electric field involved in this process, which is above the conventional breakdown threshold field, is accompanied by a strong increase of the electron density. The resulting increase in the conductivity, in turn, causes this electric field to collapse over a few tens of picoseconds, preventing the electrons reaching high energies and producing significant X-ray emissions [e.g., Ihaddadene and Celestin, Geophys. Res. Lett., 45, 5644, 2015]. In this work, we will present simulation results of a new electron acceleration mechanism for producing runaway electron energies above hundred keV. The mechanism couples multiple single streamers and streamer head-on collisions, similar to a laboratory discharge, and is suitable for explaining the high-energy X-rays produced by discharges in air and by lightning stepped leaders

Modeling of a New Electron Acceleration Mechanism Ahead of Streamers

International audienceHead‐on collisions between negative and positive streamers have been proposed as a mechanism behind X‐ray emissions by laboratory spark discharges. Recent simulations using plasma fluid and PIC models of a single head‐on collision of two streamers of opposite polarities in ground pressure air predicted an insignificant number of thermal runaway electrons >1 keV and hence weak undetectable X‐ray emissions. Because the current available models of a single streamer collision failed to explain the observations, we first use a Monte Carlo model coupled with multiple static dielectric ellipsoids immersed in a sub‐breakdown ambient electric field as a description of multiple streamer environment and we investigate the ability of multiple streamer‐streamer head‐on collisions to accelerate runaway electrons >1 keV up to energies ~200‐300 keV instead of just one single head‐on collision. The results of simulations show that the streamer head‐on collision mechanism fails to accelerate electrons, instead they decelerate in the positive streamer channel. In a second part, we use a streamer plasma fluid model to simulate a new streamer‐electron acceleration mechanism based on a collision of a large negative streamer with a small neutral plasma patch in different Laplacian electric fields|E0|=(35, 40, 45) kV/cm, respectively. We observe the formation of a secondary short propagating negative streamer with a strong peak electric field >250 kV/cm up to 378 kV/cm over a time duration of ~0.16 ns at the moment of the collision. The mechanism produces up to 106 runaway electrons with an upper energy limit of 24 keV

Evaluation of Monte Carlo tools for high-energy atmospheric physics II: Relativistic runaway electron avalanches

This work is distributed underthe Creative Commons Attribution 4.0 License (https://creativecommons.org/licenses/by/4.0/)The emerging field of high-energy atmospheric physics studies how high-energy particles are produced in thunderstorms, in the form of terrestrial γ-ray flashes and γ-ray glows (also referred to as thunderstorm ground enhancements). Understanding these phenomena requires appropriate models of the interaction of electrons, positrons and photons with air molecules and electric fields. We investigated the results of three codes used in the community-Geant4, GRanada Relativistic Runaway simulator (GRRR) and Runaway Electron Avalanche Model (REAM)-to simulate relativistic runaway electron avalanches (RREAs). This work continues the study of Rutjes et al. (2016), now also including the effects of uniform electric fields, up to the classical breakdown field, which is about 3.0 MV m at standard temperature and pressure. We first present our theoretical description of the RREA process, which is based on and incremented over previous published works. This analysis confirmed that the avalanche is mainly driven by electric fields and the ionisation and scattering processes determining the minimum energy of electrons that can run away, which was found to be above ≈ 10 keV for any fields up to the classical breakdown field. To investigate this point further, we then evaluated the probability to produce a RREA as a function of the initial electron energy and of the magnitude of the electric field. We found that the stepping methodology in the particle simulation has to be set up very carefully in Geant4. For example, a too-large step size can lead to an avalanche probability reduced by a factor of 10 or to a 40 % overestimation of the average electron energy. When properly set up, both Geant4 models show an overall good agreement (within ≈ 10 %) with REAM and GRRR. Furthermore, the probability that particles below 10 keV accelerate and participate in the high-energy radiation is found to be negligible for electric fields below the classical breakdown value. The added value of accurately tracking low-energy particles (< 10 keV) is minor and mainly visible for fields above 2 MV m. In a second simulation set-up, we compared the physical characteristics of the avalanches produced by the four models: Avalanche (time and length) scales, convergence time to a self-similar state and energy spectra of photons and electrons. The two Geant4 models and REAM showed good agreement on all parameters we tested. GRRR was also found to be consistent with the other codes, except for the electron energy spectra. That is probably because GRRR does not include straggling for the radiative and ionisation energy losses; hence, implementing these two processes is of primary importance to produce accurate RREA spectra. Including precise modelling of the interactions of particles below 10 keV (e.g. by taking into account molecular binding energy of secondary electrons for impact ionisation) also produced only small differences in the recorded spectra. © 2018 Author(s).This work was supported by the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement no. 320839 and the Research Council of Norway under contracts 208028/F50 and 223252/F50 (CoE). For part of the results of this work, it was necessary to use the Fram computer cluster of the UNINETT Sigma2 AS, under project no. NN9526K. Gabriel Diniz is supported by the Brazilian agency CAPES. Casper Rutjes acknowledges funding by FOM project no. 12PR3041, which also supported Gabriel Diniz's 12-month stay in the Netherlands. Ivan S. Ferreira thanks CNPqs grant PDE(234529/2014-08) and also FAPDF grant no. 0193.000868/2015, 03/2015. This material is based in part upon work supported by the Air Force Office of Scientific Research under award no. FA9550-16-1-0396. The authors would like to thank the two referees, Ashot Chilingarian and an anonymous referee, for their valuable comments and suggestions that helped to improve the quality of this work

Evaluation of Monte Carlo tools for high-energy atmospheric physics II:Relativistic runaway electron avalanches

\u3cp\u3eThe emerging field of high-energy atmospheric physics studies how high-energy particles are produced in thunderstorms, in the form of terrestrial γ-ray flashes and γ-ray glows (also referred to as thunderstorm ground enhancements). Understanding these phenomena requires appropriate models of the interaction of electrons, positrons and photons with air molecules and electric fields. We investigated the results of three codes used in the community-Geant4, GRanada Relativistic Runaway simulator (GRRR) and Runaway Electron Avalanche Model (REAM)-to simulate relativistic runaway electron avalanches (RREAs). This work continues the study of Rutjes et al. (2016), now also including the effects of uniform electric fields, up to the classical breakdown field, which is about 3.0 MV m\u3csup\u3e-1\u3c/sup\u3e at standard temperature and pressure. We first present our theoretical description of the RREA process, which is based on and incremented over previous published works. This analysis confirmed that the avalanche is mainly driven by electric fields and the ionisation and scattering processes determining the minimum energy of electrons that can run away, which was found to be above ≈ 10 keV for any fields up to the classical breakdown field. To investigate this point further, we then evaluated the probability to produce a RREA as a function of the initial electron energy and of the magnitude of the electric field. We found that the stepping methodology in the particle simulation has to be set up very carefully in Geant4. For example, a too-large step size can lead to an avalanche probability reduced by a factor of 10 or to a 40 % overestimation of the average electron energy. When properly set up, both Geant4 models show an overall good agreement (within ≈ 10 %) with REAM and GRRR. Furthermore, the probability that particles below 10 keV accelerate and participate in the high-energy radiation is found to be negligible for electric fields below the classical breakdown value. The added value of accurately tracking low-energy particles (&lt; 10 keV) is minor and mainly visible for fields above 2 MV m\u3csup\u3e-1\u3c/sup\u3e. In a second simulation set-up, we compared the physical characteristics of the avalanches produced by the four models: Avalanche (time and length) scales, convergence time to a self-similar state and energy spectra of photons and electrons. The two Geant4 models and REAM showed good agreement on all parameters we tested. GRRR was also found to be consistent with the other codes, except for the electron energy spectra. That is probably because GRRR does not include straggling for the radiative and ionisation energy losses; hence, implementing these two processes is of primary importance to produce accurate RREA spectra. Including precise modelling of the interactions of particles below 10 keV (e.g. by taking into account molecular binding energy of secondary electrons for impact ionisation) also produced only small differences in the recorded spectra.\u3c/p\u3

Evaluation of Monte Carlo tools for high-energy atmospheric physics II: Relativistic runaway electron avalanches

The emerging field of high-energy atmospheric physics studies how high-energy particles are produced in thunderstorms, in the form of terrestrial γ-ray flashes and γ-ray glows (also referred to as thunderstorm ground enhancements). Understanding these phenomena requires appropriate models of the interaction of electrons, positrons and photons with air molecules and electric fields. We investigated the results of three codes used in the community-Geant4, GRanada Relativistic Runaway simulator (GRRR) and Runaway Electron Avalanche Model (REAM)-to simulate relativistic runaway electron avalanches (RREAs). This work continues the study of Rutjes et al. (2016), now also including the effects of uniform electric fields, up to the classical breakdown field, which is about 3.0 MV m-1 at standard temperature and pressure. We first present our theoretical description of the RREA process, which is based on and incremented over previous published works. This analysis confirmed that the avalanche is mainly driven by electric fields and the ionisation and scattering processes determining the minimum energy of electrons that can run away, which was found to be above ≈ 10 keV for any fields up to the classical breakdown field. To investigate this point further, we then evaluated the probability to produce a RREA as a function of the initial electron energy and of the magnitude of the electric field. We found that the stepping methodology in the particle simulation has to be set up very carefully in Geant4. For example, a too-large step size can lead to an avalanche probability reduced by a factor of 10 or to a 40 % overestimation of the average electron energy. When properly set up, both Geant4 models show an overall good agreement (within ≈ 10 %) with REAM and GRRR. Furthermore, the probability that particles below 10 keV accelerate and participate in the high-energy radiation is found to be negligible for electric fields below the classical breakdown value. The added value of accurately tracking low-energy particles (< 10 keV) is minor and mainly visible for fields above 2 MV m-1. In a second simulation set-up, we compared the physical characteristics of the avalanches produced by the four models: Avalanche (time and length) scales, convergence time to a self-similar state and energy spectra of photons and electrons. The two Geant4 models and REAM showed good agreement on all parameters we tested. GRRR was also found to be consistent with the other codes, except for the electron energy spectra. That is probably because GRRR does not include straggling for the radiative and ionisation energy losses; hence, implementing these two processes is of primary importance to produce accurate RREA spectra. Including precise modelling of the interactions of particles below 10 keV (e.g. by taking into account molecular binding energy of secondary electrons for impact ionisation) also produced only small differences in the recorded spectra