The Lens Effects of Hypersonic Plasma Sheath on Terahertz Signals

Plasma sheaths enveloping reentry and hypersonic vehicles in near space could yield communication blackout. Terahertz (THz) communications are considered as a potential approach to solve the blackout problem. Many previous studies have been carried out to investigate the propagation characteristics of THz signals in plasma sheaths. Yet, the impacts of plasma sheath on the signal transmission direction are not clear enough so far. In this study, three atmospheric window frequencies in the THz band, which are 94, 140, and 220 GHz, are taken, for example, to model the transmission directions for obliquely propagating THz signals in a blunt-coned plasma sheath. The change of signal transmission directions is investigated. Based on the study, it is found that the THz signals are focalized by the plasma sheath. Further analysis shows that the plasma sheath acts as a "lens system" to the transmitting THz signals. The plasma sheath defocuses the transmitting signals at the beginning and then focuses the signals. The mechanism for that behavior is analyzed. The impacts of the lens effects on the THz relay communication for hypersonic vehicles are qualitatively discussed based on the study.