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Scattering of Electromagnetic Waves in the Presence of Wave Turbulence Excited by a Flow With Velocity Shear

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3 Author(s)
Sotnikov, V.I. ; Sensors Directorate, Air Force Res. Lab., Hanscom AFB, MA, USA ; Leboeuf, J.N. ; Mudaliar, S.

It is well known that an incompressible sheared flow with an inflection point in the velocity profile will result in the formation of turbulent vortices. In the case of compressible plasma flow with velocity shear, ion-acoustic fluctuations in addition to vortices will be generated. We present detailed analysis of the excitation of such low-frequency oscillations in a compressible plasma flow with velocity shear. To examine the process of the excitation and nonlinear saturation of low-frequency oscillations in the presence of a flow shear, a nonlinear system of equations was derived. We employ a predictor-corrector method to solve this system numerically. Spectral analysis of obtained numerical solutions allows to calculate the turbulent density spectra for different velocity profiles. We find that the impact of this turbulence associated with ion-acoustic wave fluctuations is considerably more significant and dominant than that due to the turbulent vortices. Another goal for our research is to understand the influence of this turbulent flow on electromagnetic (EM) signals. Since the thickness of the flow can be fairly small in the case of hypersonic vehicles, we employed a single-scattering perturbation theory to study the scattering of the EM signals from the plasma sheath. We observe that the EM scattering from the turbulent density fluctuations of the flow results in shifted signal spectra above and below that of the source. Such shifts can have rather adverse effects on the sensor performance. For instance, shifts in temporal spectra will result in channel interference and crosstalk in communication systems. Because of the nature of the dispersion characteristics of the ion-acoustic waves, the shifts in spatial spectra of scattered EM waves can be very large. This will lead to large fluctuations in integrated phase shifts and hence, results in significant signal distortion. We carried out detailed theoretical analyses and numerical calculations to understand th- - e nature of the influence of such hypersonic turbulent flow on EM signals. The complete loss of the EM signal because of the overdense condition of the plasma sheath is well known. We find that even in underdense conditions, GPS-based navigation can be significantly impaired because of the GPS signal distortion by the turbulence caused by such ion-acoustic wave fluctuations.

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Plasma Science, IEEE Transactions on  (Volume:38 ,  Issue: 9 )