Aspects of Ultrafast Spark Gap Switching

J.M. Lehr, C.E. Baum, W.D. Prather and F.J. Agee High Energy Sources Division, Phillips Laboratory Kirtland AFB, New Mexico 87117 The Air Force is interested in compact ultra-wideband systems which utilize a minimum volume of high pressure gas. These desires lead us to look closely at single channel spark gaps, because both the size and volume of gas, for example, hydrogen, under pressure can be much less than needed for sources containing ring gap switches. While single channel spark gap switches are desirable, the intrinsic inductance of the spark gap is prohibitively high to achieve large rates of voltage rise. For future applications, the limit of spark gap technology for ultrafast switching is explored. Of primary interest is the fastest possible risetime achievable with a single channel spark gap. Thus far we have calculated the limit on the achievable risetime with spark gap technology, using three different approaches, which are all in good agreement. The first examines the excitation rates in gases to determine its limitations. The second assumes a streamer mechanism and uses the velocity of propagation to estimate the achievable risetime. The third utilizes an equivalent circuit model. It is commonly believed that the impedance mismatch in the spark region, caused by the additional spark gap inductance, is unavoidable. To reduce the effect of the intrinsic inductance of the channel, the High Energy Source Division has devised a simple geometrical alteration to the spark gap geometry which reduced the inductance per unit length of the spark gap to that of its transmission line feed. This is anticipated to permit the realization of picosecond risetime UWB HPM sources.