Critical Point for Transition of an Electron Avalanche to a Streamer

Abstract

Using the experimental data on the dependence of the coefficient of ionization by electron impact ${\alpha \mathord{\left/ {\vphantom {\alpha N}} \right. \kern-0em} N}$ as a function of E/N in the gas CF₄, the values of the strongly inhomogeneous electric field at the points located along the force lines are compared with those of the fields of the electron avalanches developing in them. It has been shown that the distributions of the increasing electric field of the avalanche always have the only point of intersection with the constant-field distribution. This is evidence that in the avalanche, a dipole has appeared with charges N_e ≈ N_i located at a distance of about α^–1 between them. The opposite direction of the electric fields created by the power supply and the avalanche charges neutralizes the field inside the dipole down to zero. Emergence of such a region (point) in the development of electron avalanche radically changes the character of further development of the gas discharge. In fact, it is a final point in the development of the initial electron avalanche. Since the drift current stops at this point (the avalanche current circuit is broken), the electron avalanche is transformed to a plasma state; ionization by electron impacts passes due to the energy accumulated in the interelectrode capacitance of the discharge gap. In other words, the avalanche continues to develop only due to internal sources and, as a result, acquires a new spatial structure in the form of a double charge layer. As a result, the charge system transforms to a self-organizing and evolving system, which, after a series of sequential transformations, becomes the base for forming a streamer. The dynamics of a sequential transformation of the charge system to new stable structural formations is seen in the experimental data.