Electromagnetic Effects on the Transmission of Radio Waves Through Plasma

Abstract

At speeds exceeding Mach 5, vehicles reentering Earth's atmosphere generate enough heat to ionize the atmosphere as they move through it, creating a layer of plasma around the leading surface of the vehicle. Free electrons in the plasma interact with electromagnetic waves, such as radio wave transmissions sent from a ground communication station, and can lead to attenuation. In the most extreme cases, the radio wave attenuation is so severe that it leads to a total loss of communication, often referred to as blackout.

There have been many proposed techniques to mitigate this problem, from antenna and vehicle design to electromagnetic manipulation of the plasma itself. The Auburn L3Harris Investigation in Signal Scattering and Attenuation (ALISSA) experiment was built to study the effect of using a static external magnetic field to reduce the amount of attenuation caused by plasma on the transmission of 1-2 GHz radio waves. Experiments performed in ALISSA showed that applying a 0.2 T (2000 Gauss) field at the surface of the antenna, perpendicular to the radio wave propagation vector, reduced the attenuation caused by the plasma by 20 dB (factor of 100 in signal strength).

Another proposed mitigation technique is electromagnetic manipulation of the plasma to reduce the electron density. The cutoff frequency of a plasma is directly related to its electron density. Lowering the electron density would increase the amount of radio wave frequencies that could transmit through the plasma with little attenuation. The Auburn Linear Experiment for Instability Studies (ALEXIS) was used to study the potential of biasing a hollow cylindrical copper electrode to reduce the electron density of a plasma. Experiments performed in ALEXIS show that supplying a sufficiently large negative bias (with respect to chamber ground) to the electrode lowered the electron density in the downstream sections of ALEXIS by an order of magnitude.