Nonlinear impulse current spreading and electrical breakdown in soil

The results of laboratory experiments on the impulse current spreading from the spherical electrodes and the breakdown occurring in quartz sand of various humidity at the impulse voltages of $20$–$40$ kV are presented. It is shown that, at the current densities on the electrode above some critical value, a sharp nonlinear decrease of the impulse grounding resistance occurs, an ionization-superheating instability develops, and thus the current contraction takes place and a plasma channel in the soil occurs. A method is proposed to determine the critical strength of the ionization electric field. It was discovered that, at long discharge gaps in humid sand, the breakdown develops with a long time delay, similarly to thermal breakdown.