Abstract
Streamers are ionization filaments of electric gas discharges. Negative polarity streamers propagate primarily through electron impact ionization, whereas positive streamers in air develop through ionization of oxygen by UV photons emitted by excited nitrogen; however, experiments show that positive streamers may develop even for low oxygen concentrations. Here we explore if bremsstrahlung ionization facilitates positive streamer propagation. To discriminate between effects of UV and bremsstrahlung ionization, we simulate the formation of a double headed streamer at three different oxygen concentrations: no oxygen, 1 ppm O2 and 20% O2, as in air. At these oxygen levels, UV-relative to bremsstrahlung ionization is zero, small, and large. The simulations are conducted with a particle-in-cell code in a cylindrically symmetric configuration at ambient electric field magnitudes three times the conventional breakdown field. We find that bremsstrahlung induced ionization in air, contrary to expectations, reduces the propagation velocity of both positive and negative streamers by about 15%. At low oxygen levels, positive streamers stall; however, bremsstrahlung creates branching sub-streamers emerging from the streamer front that allow propagation of the streamer. Negative streamers propagate more readily forming branching sub-streamers. These results are in agreement with experiments. At both polarities, ionization patches are created ahead of the streamer front. Electrons with the highest energies are in the sub-streamer tips and the patches.
Highlights
Streamers are thin, ionized plasma channels formed by ioniz ation waves in an electrified gas [1,2,3,4,5,6,7,8]
Negative polarity streamers propagate primarily through electron impact ionization, whereas positive streamers in air develop through ionization of oxygen by UV photons emitted by excited nitrogen; experiments show that positive streamers may develop even for low oxygen concentrations
We explore an alternative ionization mechanism: when electrons are accelerated in the electric field of a streamer they may emit bremsstrahlung photons when they are scattered off the air nuclei [29,30,31] or shell electrons [32, 33], and the photons may ionize the neutral molecules provided their energy is above the ionization energy [34]
Summary
Streamers are thin, ionized plasma channels formed by ioniz ation waves in an electrified gas [1,2,3,4,5,6,7,8]. Since we use a particle code, updating the position of electrons and photons as well as the collision with air molecules is independent of the actual grid. The ambient electric field is pointing towards −z and has a magnitude of 3Ek, where Ek ≈ 3.2 MVm−1 is the conventional break down field at STP This field magnitude is a compromise between a need to reduce computational time by acceler ating electrons fast into the energy regime relevant for the bremsstrahlung process, and the fields expected in stages of laboratory or atmospheric discharges [37, 38]. It is the time step of particle position and velocity updates, and collision processes It is defined by the Nambu scheme:. The second time step is smaller than the first one, which allows N = ∆tE /∆tc collisions and particle updates between two updates of the electric field. To reduce computa tional noise we have implemented a splitting scheme for computer-electrons solely populating one grid cell yielding maximal 100 new computer-electrons with reduced weight
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