A steerable plasma microwave mirror

Abstract

We have developed a steerable plasma microwave mirror based on our previous work on the reflection of microwaves on a plasma slab. We have discovered that for plasma densities above microwave cutoff, the phase of the reflection varies continuously from a voltage antinode at the plasma surface when the plasma density just reaches microwave cutoff, to a voltage node at the plasma surface when the plasma density is much larger than that required for microwave cutoff. An analogy to the reflection at the end of a coaxial cable is that the first case corresponds to an open line (displacement and conduction currents equal and opposite), and the second case corresponds to a shorted line (conduction current much greater than displacement current). The operating plasma mirror as presently designed comprises a surface of plasma tubes. By programming the plasma density in the various tubes, the reflected microwaves experience a varying phase shift from tube to tube and the reflected beam is steered in analogy to the way an emitted microwave beam is steered by a phased transmission antenna array. The time required for reconfiguring the beam direction corresponds to the time required for plasma modification-in this case, a few milliseconds.