Browsing by Author "Østgaard, Nikolai"

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    Evaluation of monte carlo tools for high energy atmospheric physics
    (Katlenburg-Lindau : Copernicus, 2016) Rutjes, Casper; Sarria, David; Skeltved, Alexander Broberg; Luque, Alejandro; Diniz, Gabriel; Østgaard, Nikolai; Ebert, Ute
    The emerging field of high energy atmospheric physics (HEAP) includes terrestrial gamma-ray flashes, electron-positron beams and gamma-ray glows from thunderstorms. Similar emissions of high energy particles occur in pulsed high voltage discharges. Understanding these phenomena requires appropriate models for the interaction of electrons, positrons and photons of up to 40MeV energy with atmospheric air. In this paper, we benchmark the performance of the Monte Carlo codes Geant4, EGS5 and FLUKA developed in other fields of physics and of the custom-made codes GRRR and MC-PEPTITA against each other within the parameter regime relevant for high energy atmospheric physics. We focus on basic tests, namely on the evolution of monoenergetic and directed beams of electrons, positrons and photons with kinetic energies between 100keV and 40MeV through homogeneous air in the absence of electric and magnetic fields, using a low energy cutoff of 50keV. We discuss important differences between the results of the different codes and provide plausible explanations. We also test the computational performance of the codes. The Supplement contains all results, providing a first benchmark for present and future custom-made codes that are more flexible in including electrodynamic interactions.
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    X-ray Emissions in a Multiscale Fluid Model of a Streamer Discharge
    (Hoboken, NJ : Wiley, 2018) Lehtinen, Nikolai G.; Østgaard, Nikolai
    We use a three-specie fluid model of electric discharge in air to simulate streamer evolution from the avalanche-to-streamer transition to the collision of opposite-polarity streamers. We estimate the upper limit on the production of thermal runaway electrons, which is dominant during the second of these processes. More thermal runaways are produced if the ionization due to natural background and photoionization is reduced, due to possibility of creation of higher electric fields at streamer tips. The test-particle simulation shows, however, that these thermal runaway electrons have insufficient energies to become relativistic runaways. The simulations are done in constant uniform background fields of E0=4 and 6 MV/m. A simulation was also performed in E0=2 MV/m after formation of streamers in 4-MV/m field, in order to approximate the average background field created by ∼1-MV voltage over a ∼1-m electrode gap used in laboratory spark experiments. We conclude that the used fluid model is insufficient to explain X-ray observations during such experiments. We discuss the possible role of mechanisms which were not included in this or previous modeling but may play the deciding role in the electron acceleration and X-ray production during a streamer collision.