Electromagnetic modeling of the effect of plasma sheath on radar signatures of hypersonic targets

Abstract

Hypersonic missiles have been suggested to have significantly different radar signa tures compared to similar slower vehicles. This has been attributed to the plasma sheath that forms when high-temperature air in the hypersonic shock front partially ionizes air molecules. Under certain conditions, the free electrons in this plasma can absorb electromagnetic radiation. It is unclear whether this effect could dampen the scattered radiation from targeted hypersonic missiles, effectively making them stealthy to radar. The physics of plasma-induced stealth of hypersonic vehicles is quite complex, involving the spatial distributions of plasma and electron–neutral col lision frequencies as well as the microwave frequency. For this reason, broad claims about the stealth of hypersonic vehicles must be supported by accurate numerical simulations. In this thesis, the effects of a thin plasma sheath have been studied in various settings and geometries with the aid of electromagnetic as well as hydrodynamic simulations. The ability of a plasma to shield rough surfaces, which otherwise have a large radar signature, was investigated by simulating the electromagnetic wave scattering of a plasma-covered metal slab with surface roughness in a 2D geometry. Somewhat unexpectedly, the plasma did not behave like a mirror for low frequencies and when the radiation was expected to be attenuated, it was instead amplified. Additionally, the ability of the plasma sheath to distort scattered electromagnetic waves was studied analytically. The distortion was exaggerated in this simple model compared to a real-world scenario, but even so, it was negligible for almost every frequency and can therefore be neglected. Finally, to study the effect of a plasma sheath with a realistic plasma density, the plasma frequency and collision frequency distributions were computed for a metal sphere flying at speeds of 3–5 km s−1 in an ambient pressure representative of a 40 km altitude. While the calculated radar signature has a limited accuracy below 1 GHz, we can conclude that in that frequency range the presence of the plasma sheath can either increase or reduce the radar signal, depending on the frequency. At some frequencies the decrease was as large as 10 dB. Our results also indicate that the plasma sheath only has minor effects on the radar signatures of hypersonic vehicles traveling below 3 km s−1 , for the microwave bands above 1 GHz.