Monte Carlo Simulation of Surface Flashover

Surface flashover is an electrical breakdown that occurs on or just above the surface of an insulator when a sufficiently high voltage stress is applied. This phenomenon is distinct from bulk breakdown which occurs within the insulating material. It is believed that the following mechanisms are involved in surface flashover. First, electrons are emitted at the triple junction of cathode, insulator, and vacuum due to field enhancement at microscopic irregularities and dielectric mismatch. The electrons are accelerated in the field between the cathode and anode. Some of these electrons impact the surface of the insulator where---depending on the particular material, the angle of incidence, and the impact energy---0,1,2, or possibly more secondary electrons are emitted with a distribution of angles and kinetic energies. Clearly, if more than one secondary electron is emitted, then that part of the insulator surface will acquire a positive charge.

It is believed that such a positively charged surface does develop on some (if not most) of the insulator. Once the surface acquires a positive charge, the trajectories of the emitted secondary electrons are likely to reimpact the surface in such a way that more than one additional secondary is emitted for each impacting electron. Thus, an avalanche of electrons hopping along the surface of the insulator toward the anode occurs. This avalanche knocks off gas molecules that were adsorbed on the insulator surface. When the pressure in this layer of gas reaches a critical value, ordinary gas phase breakdown due to electron impact ionization of the gas occurs.

In order to study the charging phase that occurs prior to flashover, we developed a Monte Carlo simulation of the process for a simplified geometry. Since a simulation would be followed for millions of avalanches, large quantities of suitably random numbers were needed. Appropriate probability distributions for the important physical quantities were developed. However, not all of these distributions were invertible, thus we made extensive use of the Von Neumann rejection technique. A set of results showing the surface charging at different times in the simulation (top/early to bottom/late) and for different electric field strengths (left/weaker to right/stronger) from the M.S. thesis of Dr. Gregory B. Osterman is shown in the figure.

The Paper Chase

If you are interested in obtaining more information about this topic, try the following:
  1. G.B. Osterman, "Monte Carlo Simulation of Surface Charging Prior to Surface Flashover," M.S. Thesis, Texas Tech University (1990).
  2. G.B. Osterman and T.L. Gibson, "Numerical Simulation of Surface Charging Prior to Flashover in Lexan and Lucite," Proceedings of the 14th International Symposium on Discharges and Electrical Insulation in Vacuum, Santa Fe, N.M. 368-71 (1990).

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