Effect of electron number densities on the radio signal attenuation in an inductively coupled plasma facility
Creators: name:  D. LUIS 
name:   A. VILADEGUT 
name:   A. CAMPS 
name:   O. CHAZOT 
Publisher: name: Proceedings of  the 9th European Conference for Aerospace Sciences. Lille, France, 27 June - 1 July, 2022
Year: 2022
Subjects: FLIGHT PHYSICS
PLASMA FLOWS
ENERGY DEPOSITION

description: Spacecraft enter planetary atmospheres at hypersonic velocities generating shock waves ahead of the vehicle and consequent extreme aerodynamic heating. The high post-shock temperatures dissociate and ionize the gas creating a plasma layer that surrounds the spacecraft. High ionization degrees affect the propagation of the electromagnetic waves emitted by the on-board antennas, leading to a communication blackout for several minutes. A radio wave propagating through an ionized plasma can experience changes in amplitude, phase and/or polarization. For instance, the presence of charged particles slows down the propagation of the signal along the path, causing a signal delay; and a linearly polarized wave also suffers a gradual rotation of its plane of polarization (Faraday rotation) due to the presence of the geomagnetic field and the anisotropic nature of the plasma. Quantitatively, the radio wave propagation properties in ionized media is characterized by the refractive index, which can be described by the Appleton-Hartree equation. At high frequencies ( f 100 MHz), the refractive index depends mainly on the electron density, and the strength and direction of the geomagnetic field in relation to the ray path. Ultimately, the radio communication blackout occurs when the characteristic frequency of the plasma layer around the vehicle exceeds the radio frequency used for communications. The plasma frequency, fp, depends on the ionization degree of the gas, and it is related to the electron number density, ne, by fp = 1/(2*pi)*sqrt((qe^2*ne)/(me*e0) ~ 9*sqrt(ne) where qe and me are, respectively, the electron charge and mass, and e0 the free space permittivity. The Horizon 2020 MEESST (Magnetohydrodynamic Enhanced Entry System for Space Transportation) project aims at designing and testing a proof-of-concept magnetic shielding device for mitigation of radio blackout during space entry. As part of the project, this work proposes investigating the effects of the ionized medium on the propagation of the radio signal through the plasma flow in a plasma wind tunnel facility. A representative communication system is being designed to be implemented in the test chamber of the Plasmatron facility at VKI. The operating frequency of the antennas is chosen based on simulations using an in-house inductively coupled plasma magnetohydrodynamics solver integrated into the Computational Object-Oriented Library for Fluid Dyanmics (COOLFluiD) and varies between 33 and 38.5 GHz. The transmitter antenna is placed in the stagnation point of a quartz probe. To study the influence of the quartz on the antenna radiation diagram, a characterization of the full antenna setup is conducted at the anechoic chamber at UPC. Posteriorly, the radio signal propagation through the plasma, from emitter to receiver, is tracked with a Vector Network Analyzer (VNA), allowing the determination of signal attenuation and phase change between the two antennas. Simultaneously, characterization of the electron number densities on the flow is conducted using high resolution optical emission spectroscopy. These measurements are going to allow correlating the electron number densities with the signal changes and attenuation, and to characterize the plasma flow.