Abstract
Streamers are ionization waves in electric discharges. One of the key ingredients of streamer propagation is an ambient gas that serves as a source of free electrons. Here, we explore the dependence of streamer dynamics on different spatial distributions of ambient air molecules. We vary the spatial profile of air parallel and perpendicular to the ambient electric field. We consider local sinusoidal perturbations of 5%–100%, as induced from discharge shock waves. We use a cylindrically symmetric particle-in-cell code to simulate the evolution of bidirectional streamers and compare the electron density, electric field, streamer velocity and electron energy of streamers in uniform air and in perturbed air. In all considered cases, the motion is driven along in decreasing air density and damped along increasing air density. Perturbations of at most 5%–10% change the velocity differences by up to approximately 40%. Perturbations perpendicular to the electric field additionally squeeze or branch streamers. Air variations can thus partly explain the difference of velocities and morphologies of streamer discharges. In cases with large perturbations, electrons gain energies of up to 30 keV compared to 100 eV in uniformly distributed air. For such perturbations parallel to the ambient electric field, we see the spontaneous initiation of a negative streamer; for perpendicular perturbations, x-rays with energies of up to 20 keV are emitted within 0.17 ns.
Highlights
Streamers, small, filamentary plasma channels of negative or positive polarity moving through air, form the early stages of lightning leaders [1,2,3,4,5] and of transient luminous events [6,7,8,9,10,11,12,13]
In cases with large perturbations, electrons gain energies of up to 30 keV compared to 100 eV in uniformly distributed air. For such perturbations parallel to the ambient electric field, we see the spontaneous initiation of a negative streamer; for perpendicular perturbations, x-rays with energies of up to 20 keV are emitted within 0.17 ns
Their propagation mechanisms depend on their polarity: positive streamers propagating along electric field lines require an additional electron source, by background ionization [14, 15] or ionization through UV photons [16,17,18,19,20], to support their motion, whilst negative streamers move through the acceleration of electrons against the electric field lines out of the streamer head and the subsequent ionization of air molecules ahead facilitated
Summary
Small, filamentary plasma channels of negative or positive polarity moving through air, form the early stages of lightning leaders [1,2,3,4,5] and of transient luminous events [6,7,8,9,10,11,12,13]. Briels et al [28] have investigated the similarity laws of positive streamers in air and in nitrogen with a purity of approximately 99.9% in laboratory experiments They altered the different ambient pressures between 0.013 to 1 bar, being equivalent to changing the density of ambient air molecules. Since the rate of electrons colliding with air molecules depends on the mean free path and the energy gained through the available electric field, streamers in the same ratio of the ambient field and density behave Some early discussions of density perturbations in connection with discharges are found in Marode et al [32], suggesting that streamers, before the formation of a hot conductive leader channel, induce a radial flow of neutral air molecules through thermal expansion, which reduces the air density by up to 50% and facilitates the electron motion and the spark breakdown.
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