Study On Plasma Sheath Reproducing Technology And Related Issues In EM-Wave Propagation Experiment

When a spacecraft leaves orbit and reenters the atmosphere as it travels to a landing site, there is a critical period of time that all communications between spacecraft and ground control are lost. This period is also known as ‘radio blackout’ or ‘reentry blackout’, which is caused by an envelope of ionized air surrounding the craft and created by the tremendous heat from air compressing around the craft. The ionized envelope, which is called the ‘plasma sheath’, shields the electromagnetic(EM) signals, and ultimately causes communication failure. Radio blackout has puzzled the aerospace industry for decades, and has not yet been completely resolved. In recent years, as the development of manned space engineering, space shuttle vehicles, planetary exploration, novel strategies equipments and especially near-space-vehicles(NSV), the “blackout” issues become a research focus again. However, there are many shortages in such as real flying testing and experimental method, left many unresolved issues in wave propagation, such as thin layer phenomena and low frequency band propagation window problems.According to these problems, a novel method is proposed for simulation plasma sheath near by the antenna windows. The method uses glow discharge with principle of diffusion which be able to produce a large-scaled plasma layer with certain equivalence to actual plasma sheath. The propagation experiment system has been built and then plasma diagnosis, calibration, and similarity analysis were also conducted. Based on these works further EM wave propagation researches in plasma were carried out, including UHF/L/S-band radio blackout reproducing, EM-wave propagation in thin plasma layer, and low-frequency EM-wave propagation characteristics. The main innovations and contributions of this paper include:1. A novel plasma generator is presented. The generator is able to offer large scale, continuous, non-magnetized plasma with a hollow structure, which provides a path for an electromagnetic wave. The plasma is excited by a low-pressure glow discharge, with wide electron density range. The presented method is suitable in simulating a plasma sheath, and researching propagation related issues.2. The EM-wave propagation experiment system was built using the proposed plasma generator. An electromagnetic wave propagation experiment was also reproduced a continuous radio blackout in UHF, L- and S-bands, and the results are consistent with theoretical expectations. The illustrated system could provides a general testing environment with certain similarity to actual plasma sheath near by antenna window. The system also is pexpected to used for developing or testing of reentry radio equipments, such as communication, navigation and telemetry system.3. The ‘thin layer phonomenon’ was inspected experimentally. The ‘thin layer phonomenon’ is an exceptional EM-wave propagation phenomenon inconsistent with the classical theory which remains controversial. This phenomenon is observed during the 1980 s in the reentry plasma shock tube experiments, and subject to the conditions of those years, no other means can be chosen for further verification or comparison. In this paper, the proposed plasma propagation experiments system was modified to carry out the relevant experiment, and the results clearly tend to the classical theory, support of the ‘Thin-layer phenomenon’ is a experiment-specific errors of shock tube, and the possible reasons for the error was analyzed.4. Although a ‘low-frequency window’ in plasma propagation was theoretically predicted as early as 1960 s, the experimantal reports were rarely presented so far.. The low-frequency radio wave propagation characteristic was experimentally inspected in this paper. An unusual pehonomena was found- the attenuation of low frequency wave emitted from a small-loop antenna(near field) is far below that of plane wave theory expectations. The theoretical explanation was given, which may be a potential novel method to suppress the ‘blackout’.The achievements of this paper will support further theoritical researches and reentry vehicle engineering. the protential theoretical value and application is proposed as following: :1. On theoretical value: firstly, we clarified the possible reasons of anomalous ‘Thin-layer phenomenon’, support the classical propagation theory remaining applicable even in thin plasma, which also supports the applicability of multi-layered-media numerical method(SMM/WKB) in inhomogeneous plasma, since the accuracy of overall result relies on the correctness of each thin layer. Secondly, we found that low-frequency electromagnetic waves with low- impedance has a higher transmittance in plasma, and low impedance waves can be generated simply by a magnetic dipole antenna near-field region which may result in a novel potential theory to mitigation ‘blackout’.2. On engineering application prospects: a novel plasma sheath simulation method and wave propagation experimental platform was proposed, which provides an important supplementary mean for reentry related researches. This provides a simple and effective physical verification environment for reentry electronic information devices, and supports further research or development of EM wave propagation, radar and image sensing, antenna characteristics, ‘blackout’ mitigation, and etc.