Cathode Spot Injection of Dielectric Particles with Applications for Communications Blackout Plasma Depletion.

Abstract

Vehicles traveling at hypersonic velocities in the atmosphere are readily enveloped by a dense plasma layer that blocks, reflects, and significantly attenuates radio signals typically used for communications. This problem is known as 'radio blackout' and persists for spacecraft reentering the atmosphere, hypersonic test vehicles, and hypersonic missile/defense systems.

The goal of this study is to create a dusty plasma by injecting dielectric particles that will collect many electrons, thereby lowering the electron density. If the electron density of the plasma layer can be significantly lowered, then the cutoff frequency will be lowered, allowing for the transmission of radio signals through the plasma layer.

In this work, the dusty plasma was created by injecting particles into the plasma using cathode spots. Cathode spots form a dense, high flow velocity plume of plasma that emanates from the electrode surface, ejecting dielectric particles into an overhead plasma used to simulate the plasma layer. In the process of clearing powder from the electrode surface, apparent cathode spot motion in the presence of a static magnetic field follows the `retrograde' or -JxB motion. Particle tracking studies found that particles were ejected with a greater component of velocity in the horizontal plane in the direction of cathode spot motion, likely due to multiple boosts as the cathode spots and particles traveled together in the same direction on the electrode surface. However, the primary component of velocity was in the vertical direction. The particle ejection velocity distribution had an average and median vertical velocity of 0.66 and 0.5 m/s, respectively, with a maximum ejection velocity of 1.5 m/s. The total particulate mass necessary to achieve depletion of up to 90 percent was determined by directly dropping microparticles into the discharge. Electron depletion followed an approximately linear relationship, increasing with powder mass flux. Cathode spot injection of particles also resulted in electron depletion greater than 95 percent as measured by electron saturation current. Depletion may be increased by using particles of optimum size and increasing particle mass flux. It has been shown that this method is capable of depleting the reentry plasma layer.