Delay time reduction of high power microwave surface flashover using metallic initiators

Abstract

High power microwave (HPM) surface flashover can be rapidly induced by introducing metallic points on to the dielectric surface with negligible effect on the transmission properties. An experimental setup comprised of a magnetron operating at 2.85 GHz to produce a 4.5 MW, 3 μs pulse is used for observing surface flashover in various atmospheric conditions. An active pulse sharpening mechanism is used to reduce the pulse rise time in order to apply the electric field in tens of nanoseconds. For a system in which HPM transmission must be quickly suppressed, field enhancing geometries can provide a way for flashover to develop rapidly while keeping insertion loss at a minimum (<;0.01 dB). Initial experiments utilizing 0.2 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> aluminum points with a spatial density of 25/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> have increased the global effective electric field by a factor of ~1.5. This increase in electric field has sharply reduced delay times for surface flashover (i.e. the time between the application of the HPM pulse and a sharp drop in transmitted power). For an environment consisting of air at 155 torr, for instance, the delay time is reduced from 455 ns to 101 ns. Presented in this paper is a comparison of various field enhancing geometries and how they relate to flashover development. Also, an analysis of time resolved images will be given along with an estimation of field enhancement factors.