Microparticle injection effects on microwave transmission through an overly dense plasma layer

摘要

Summary form only given. Vehicles traveling at hypersonic velocities within the Earth's atmosphere, such as spacecraft during reentry and other hypersonic vehicles, are enveloped by a dense plasma layer. This plasma layer reflects and significantly attenuates GPS and S-band signals for vehicle navigation, telemetry, and voice communications, resulting in radio blackout.Injecting microparticles into a plasma discharge will reduce the free electron density via electron attachment to particles. Reducing the free electron density lowers the plasma cutoff frequency, and may allow lower frequency bands of electromagnetic signals to penetrate the plasma layer. In these studies, a linear hollow cathode produces an electron beam that is accelerated into a low pressure (50 to 150 mTorr) background of Argon gas, producing an electron beam discharge. A 170 Gauss axial magnetic field produced by two electromagnet coils in a Helmholtz configuration results in a well-collimated electron beam, producing a 2dimensional Argon plasma discharge. This discharge sheet is approximately 100 cm long by 30 cm wide by 2 cm thick, at densities as high as 1012 cm-3. The plasma sheet is intended to mimic the intense plasma layer produced and experienced by vehicles traveling at hypersonic velocities. A shaker device with fine mesh on the bottom is filled with alumina powder and fitted with a vibrating motor. When supplied with a modest voltage, the vibration drops alumina microparticles from the mesh openings, into the plasma sheet discharge, creating a dusty plasma. Varying the voltage supplied to the vibrating motor varies the flux rate and density of powder dropped into the plasma. A transmitting microwave horn is oriented normal to the dense plasma sheet while the receiving horn is mounted on a stage that can be rotated up to 180 degrees azimuthally. Results from these experiments measuring the cutoff and transmission of S-band microwaves incident on a dusty plasma layer, as well as Langmu- r probe measurements assessing microparticle effects on plasma density and transparency are reported.