Use of a helicon source for development of a re-entry blackout amelioration system.

摘要

During atmospheric re-entry and hypersonic flight, a bow shock forms around the vehicle leading edge. The air becomes super-heated as it passes through the shock, ionizes and forms a plasma sheath. This sheath prevents transmission of electromagnetic waves with frequencies similar to those used for radio communications. This phenomenon is referred to as the "communications blackout." In this dissertation, hypersonic communications blackout is studied, and a method for ameliorating the blackout is presented.;A plasma source was designed and built for the purpose of simulating a re-entry plasma sheath. The plasma number density in a re-entry plasma sheath ranges from 1014 m--3 to 1018 m--3. A helicon source was chosen to simulate the conditions during atmospheric re-entry because it produces high-density plasma while maintaining that density downstream of the source. For this reason, and because the electron temperature downstream of the source (1 eV to 6.5 eV) is of a similar order of magnitude as that found during re-entry (0.4 eV to 1 eV), the helicon source was deemed appropriate. The Plasmadynamics and Electric Propulsion Laboratory helicon source was found to produce an upstream ion number density of 2.5 x 1019 m--3. Downstream, where experiments with the plasma amelioration system were performed, the number density ranged from 0.55 x 1017 m--3 to 3.3 x 1017 m--3, which represent altitudes between 65 km and 75 km.;After characterizing the helicon plasma source, the amelioration system was placed downstream. The re-entry and hypersonic vehicle plasma communications (ReComm) system consists of a single solenoid electromagnet with two electrodes perpendicular to the magnetic field. The crossed fields direct plasma away from a region surrounding an antenna, creating a "window" in the sheath through which radio signals can pass. Langmuir probe, hairpin resonance probe and signal attenuation measurements show that the system is effective at reducing the number density to 80% of that measured when no fields are present. However, the system did not perform as expected. The majority of the reduction occurred with only the presence of the magnetic field. Possible explanations were studied using both analytical methods and COMSOL to model the fields. The shape of the magnetic field itself contributed greatly to the plasma number density reduction.