<title>Designing a high-power array for a target in the upper atmosphere</title>

Abstract

High-power microwave pulses can destroy electronics of targets at altitudes of 100 km or higher, and preliminary designs of microwave antennas driven by Relativistic Klystron Ampiffiers have been sketched.1 This paper discusses the susceptibility of the atmosphere to microwave breakdown, and the constraint on the design of a microwave weapon imposed by the need to avoid breakdown. The discussion consists of two parts.Part I outlines some relevant mechanisms of breakdown and ofthe knowledge base on which their description rests. For the interesting case of the first pulse of a train of L-band pulses, with a pulse duration of 160 ns, the condition to avoid breakdown is summarized by a bound on the energy flux of the pulse as it propagates upward across a layer at an altitude near 50 km. Subsequent pulses are subject to a more stringent bound, not yet determined.Part II presents a general model for a converging coherent microwave beam that concentrates energy flux on a target at or above 100 km while maintaining flux at 50 km below a bound expressed as a parameter. For the value of the 50-km flux bound worked out in Part I for a first L-band pulse, it is shown that, for a Gaussian current distribution with a cutoff to be described, to deliver 1 mJ/cm2 to a target at 100 km, the minimum array diameter is 730 m. This array is dense, not sparse. The minimum array diameter is achieved for a converging beam focused narrowly on the target, so that the radius of the spot of intense radiation is 20 m for this case. If a larger spot is required, then in order to avoid breakdown, the antenna array must also be larger. Other cases are calculated.Precise criteria by which to optimize current distributions for the array are not yet determined. The Gaussian current distribution is not optimal by any standard, and a better distribution would permit a smaller diameter array without atmospheric breakdown. How much improvement is possible awaits further investigation.PART I: INTERACTION BETWEEN MICROWAVES AND THE ATMOSPHERE 1. OVERVIEW At sufficiently high power, microwave pulses propagating through the atmosphere shake the free electrons enough to ionize the surrounding air, making more free electrons, so that a plasma is created. If the ionization rate multiplied by the pulse duration is much greater than 1, the density of free electrons increases greatly during a single pulse. Equivalently, the plasma frequency increases. Depending on the density of neutral species and other factors to be discussed, this increase threatens severe attenuation of the microwave pulse, even with the plasma frequency below the carrier frequency. If the plasma frequency exceeds the carrier frequency, the latter part of the microwave pulse cannot propagate at all; it is reflected. Additional newly found features that will not be discussed here are mentioned by Goldstein.2 In addition to these effects, a target entering the atmosphere at high velocity can induce a plasma frequency above the carrier frequency, excluding the microwave beam from part or a...

The influence of random variations in the tropospheric refractive index on the focusing of an intense microwave beam from a phased array

“…Preliminary designs of microwave termas driven by relativistic klystron amplifiers (P•KA's) have been described by Small et al [1989], Blank [1990], Goldstein [1990], and M•ters and Wu [1993]. It has been shown by Goldstein [1990] and Myers and Wu [1993] that at the altitude of 50 km, the electron momentum collision frequency approaches the carrier frequency. It follows that the air might suffer breakdown in the intense microwave beam and that this will be absorbed and reflected by the plasma.…”