Topic/Type: 1. Plasma Simulation, Oral

Scattering of electromagnetic waves in the presence of wave turbulence excited by a flow with velocity shear

V.I. Sotnikov1, J.N. Leboeuf2, S. Mudaliar3

1 Department of Physics, University of Nevada at Reno, NV 89557, USA
2 JNL Scientific, Casa Grande, AZ 85296, USA
3 Sensors Directorate, Air Force Research Laboratory, Hanscom AFB, MA 01731, USA

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 excitation and nonlinear saturation of low frequency oscillations in the presence of a flow shear, a nonlinear system of equations was derived. We employed a predictor-corrector method to solve this system numerically. Spectral analysis of the numerical solutions hence obtained allowed us to calculate the turbulent density spectra for different types of velocity profiles. We find that the impact of the turbulence associated with ion-acoustic wave fluctuations is considerably more significant and dominant than that due to turbulent vortices.

Another goal of our research is to understand the influence of this turbulent flow on electromagnetic signals. Noting that the thickness of such flow can be fairly small in the case of hypersonic vehicles we employed a single scattering perturbation theory to study the scattering of the electromagnetic signals from the plasma sheath. We observe that the electromagnetic 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 sensor performances. For instance, shifts in temporal spectra will result in channel interference and cross talk in communication systems. Further, because of the nature of the dispersion characteristics of the ion acoustic waves, the shifts in spatial spectra of scattered electromagnetic waves can be very large. This will lead to large fluctuations in integrated phase shifts and hence result in significant signal distortion. We carried out detailed theoretical analyses and numerical calculations to understand the nature of the influence of such hypersonic turbulent flow on GPS signals. The complete loss of GPS signal because of over-dense condition of plasma sheath is well known. We find that even in under-dense conditions GPS-based navigation can be significantly impaired because of GPS signal distortion by turbulence generated by such ion-acoustic wave fluctuations.