The cause and the evolution of “lens” structure for terahertz signals in hypersonic plasma sheaths

Reentry vehicles in near space used to suffer from communication blackout, which is led by the plasma sheath enveloping the whole reentry vehicle. Terahertz (THz) communication is believed to be a potential solution to mitigate the communication blackout effectively. Previous studies have investigated the attenuation characteristics of THz signals in plasma sheaths. However, the offset of THz signal transmission direction by the plasma sheath has rarely been a concern. In this study, the auto-evolution of the plasma sheath is taken into account. The consequent evolution of the refraction index distribution of the plasma sheath is investigated. It is found that the plasma sheath acts as a gradient index lens to the propagating THz signals. The lens structure keeps evolving due to the evolution of the plasma sheath itself. The main mechanism that dominates the evolution of the lens structure is the convection of electrons. By analyzing the offset of the transmission direction yielded by the lens structure, it is suggested that the onboard THz antenna could be installed close to the bottom of the vehicle in order to stabilize the transmission direction of THz signals in an evolving plasma sheath.