等离子体中X射线透过率分析及潜在通信应用研究

摘要

When a supersonic spacecraft enters into the atmosphere of earth, part of the spacecraft's kinetic energy changes into thermal energy, thus causing the air surrounding the craft to be heated and compressed. As a result, the temperature near the surface may reach several thousands of kelvins, which leads the surface materials to be ionized and form a plasma sheath around the vehicle. This plasma layer has an electron density ranging from 1015m-3 to 1020m-3, and may interrupt the radio communication signal between the re-entry vehicle and ground-based stations, which is known as ' communication blackout'. According to the radio attenuation measurement (RAM) experiments carried out by NASA(National Aeronautics and Space Administration) in the 1970s, the duration time of communication blackout ranges from 4 to 10 minutes in an altitude range from 40 km to 100 km. Communication blackout has puzzled aerospace industry for several decades, and has not yet been completely resolved. Due to this, it becomes necessary to understand the causes of communication blackout and the methods for its mitigation. Compared with other communication methods, x-ray communication(XCOM) has the advantages of short carrier wavelength and high photon energy, as well as strong ability to resist anti-interference, thus being able to open a novel way to solve this long-lasting unresolved problem. In this paper, to begin with, we analyze the transmission coefficiencies under different plasma electron densities and collision frequencies based on Wentzel Kramers Brillouin (WKB) approximation method. The simulation results indicate that the x-ray carrier is not influenced by the reentry plasma sheath. After that, a plasma source based on glow discharge is used to verify the mathematical model. The nonmagnetized unobstructed plasma region is Ф200 mm × 180 mm, which can be used for simulating plasma sheath near the reenter spacecraft. Then the transmission coefficiency, energy spectrum similarity and energy spectrum peak offset under different x-ray energy, x-ray flow and plasma electron density are firstly analyzed. Experimental results indicate that plasma can lead the x-ray signal to be attenuated to a certain extent, the increase of plasma electron density will cause higher attenuation. However, with a higher signal x-ray energy and x-ray flow, the XCOM could achieve less attenuation in the re-enter plasma layer. When the plasma electron density ranges from 6 × 1016/m3 to 1.2 × 1017/m3, 1.34 Mcps signal x-ray photons' flow with 20 kV anode voltage would achieve more than a 95％ transmission efficiency. Also, the spectrum of x-ray signal can obtain more than 95.5％ similarity and the peak offset is less than 1.3％ after passing the plasma sheath. Subsequently, based on the original mathematic model and experimental results, considering the free-free absorption, free-bound absorption, bound-bound absorption and scattering effect of x-ray photons in plasma, the x-ray transmission characteristics are optimized to make simulation results well consistent with the experiment results. Finally, an MCNP (Monte Carlo N Particle) transport simulation is used to analyze the feasibility of XCOM in blackout region, which indicates that the energy range 15 —25 keV is the suitable to achieve the XCOM in adjacent space, and the relation of potential transmitting speed with bit error is calculated. Theoretically, the XCOM can achieve about 1.3 Mbps communication speed in blackout region. In summary, these theoretical and experimental results indicate that the XCOM is a potential and novel method to solve the blackout communication problems.%X射线具有波长短、光子能量高等特点,有望在等离子体环境中实现信息的有效传输.本文首先采用基于连续介质中的WKB分层法,研究了黑障条件下,X射线在非均匀等离子体鞘套中的透过率特性,仿真了不同等离子体电子密度和碰撞频率下X射线信号的透过率,理论上证明了X射线可用于黑障区信息传输的可行性.其次通过搭建环形扩散辉光放电等离子体发生器及实验验证系统,进行了国内外首次X射线穿过等离子体鞘套的验证实验.实验结果表明,等离子体对X射线信号的透过率存在一定程度的衰减,透过等离子体前后的X射线信号能谱轮廓相似度优于95.5％,能谱峰值点的偏移量小于1.3％.此外,在原有理论模型的基础上,考虑等离子中的粒子与X射线的碰撞、吸收效应,优化了X射线在等离子体中的透过率模型,与传统的理论方法相比,该模型可对实验现象进行更好的解释.同时计算了X射线在临近空间的透过率,并分析了X射线通信所能达到的潜在指标.这些结果有望为解决黑障区信号传输提供一定的理论与实验依据.