Experimental Investigation on the Dynamic Propagation Characteristics of Low-Frequency Electromagnetic Waves in Cylindrical Time-Varying Enveloping Plasma Generated by a Shock Tube

A feasible solution for mitigating radio blackout has been proposed using the magnetic field component of low-frequency (LF) electromagnetic waves (referred to as the LF magnetic field) to transmit critical low-rate data because of the low attenuation of the LF magnetic field in the electrically small plasma. Previous works have modeled the hypersonic plasma flow field as spherical or cylindrical steady-state plasma, ignoring the time-varying properties of the plasma. This article presents the dynamic propagation of the LF magnetic field in a hypersonic plasma flow field generated by a shock tube, introducing a colored-Gaussian time-varying electron density model. Analytical expressions for the propagation characteristics of the LF magnetic field in cylindrical time-varying plasma are derived using an equivalent circuit. Meanwhile, a frequency-division-multiplex experimental method for a shock tube is presented to quantitatively measure the propagation characteristics of a multifrequency LF magnetic field. The experimental results are in good agreement with the theoretical results. As the fluctuation intensity of the electron density increases, the fluctuation intensities of the time-varying envelope and phase shift increase, and the power spectral density broadens. As the frequency of the LF magnetic field increases, the effects of the time-varying plasma on the propagation characteristics strengthen. These results are important to the design of LF communication systems and modulation algorithms in the scenario of shock-tube-generated plasma.