Pulsed positive discharges in air at moderate pressures near a dielectric rod

Abstract

We study pulsed positive discharges in air in a cylindrically symmetric setup with an electrode needle close (about 1 mm) above the top of a dielectric cylindrical rod of 4 mm in diameter mounted at its bottom on a grounded plate electrode. We present ICCD (intensified chargecoupled device) pictures and evaluations of experiments as well as simulations with a fluid discharge model; the simulations use cylindrical symmetry. In the experiments, there is an initial inception cloud phase, where the cylindrical symmetry is maintained, and later a streamer phase, where it is broken spontaneously. At 75-150 mbar, discharges with cylindrical symmetry are not attracted to the dielectric rod, but move away from it. The dielectric rod plays the sole role of an obstacle that shades (in the context of photoionization) a cone-shaped part of the inception cloud; the cone size is determined by the geometry of the setup. The material properties of the dielectric rod, such as its dielectric permittivity and the efficiency of the photon induced secondary electron emission do not have a noticeable effect. This is due to the abundance of photoionization in air, which supplies a positive discharge with free electrons and allows it to propagate along the electric field lines. Using some simple field calculations, we show that field enhancement due to dielectric polarization does not play a significant role in our geometry as long as the discharge maintains its cylindrical symmetry. The field component towards the rod is insufficiently enhanced to cause the discharge to move towards the rod. Any additional electrons produced by the dielectric surface do not influence this discharge morphology. This interpretation is supported by both experiments and simulations. At higher pressures (400-600 mbar) or for larger gaps between the needle and the dielectric rod, the inception cloud reaches its maximal radius within the gap between needle and rod and destabilizes there. In those cases, streamer channels are more likely to turn into a surface streamer. All our experiments and simulations were performed at moderate pressures (75-600 mbar), but we expect that the results will be the same for other pressures assuming that all the lengths scales (including the rod) in the setup are rescaled according to the Townsend scaling of the discharge.

An adaptive Cartesian embedded boundary approach for fluid simulations of two- and three-dimensional low temperature plasma filaments in complex geometries

“…Other researchers have shown interest in engineering and atmospheric applications. For example, by using a static mesh, Dubinova et al [28] has investigated streamer propagation along a cylindrical rod by using the Plasimo software [29], and the results qualitatively agree with experiments. Inception criteria for streamers near large ice particles have also been simulated [30], as well as various investigations of dielectric barrier discharges [31,32,33,34].…”

A computational study of positive streamers interacting with dielectrics

“…This affinity for a dielectric surface was reported to depend on the discharge gap geometry [6], gas composition, pressure [7], and dielectric properties [8,9].…”

“…To gain more insight into the physics of surface discharges, different types of numerical simulations have been performed, see e.g. [7,10,11,12,13,14,15]. Studies on the interaction between plasmas and dielectrics have often been performed at lower pressure and in noble gases, where the discharge mechanisms are relatively well understood [16,17].…”

Accurate and efficient calculation of photoionization in streamer discharges using fast multipole method

“…the approximation errors might be significant in some cases. On the other hand, despite the high computational cost [12], the results calculated from the integral form are free of further approximations, therefore, these results are often used as reference solutions [6,38,8]. Moreover, the integral form can be easily extended to stochastic versions [9,41].…”

“…The modified Helmholtz equation ( 6) can be solved efficiently by numerous fast elliptic solvers like multigrid-preconditioned FGMRES method [23]. N E = 2 was used in [27], and the constants λ j and C j were chosen to fit the lowpressure experimental data from [35] (the misprint of these constants is corrected in [12]). N E = 3 was suggested in [6] for a better fitting for the range 1 < p O 2 r < 150 Torr•cm, and the constants λ j and C j are chosen to fit the function in Eq.…”