A Plasma Microwave Barrier That Opens in Microseconds

Abstract

Summary form only given. Plasma barriers are used to protect sensitive microwave apparatus from potentially damaging electronic warfare signals. Unfortunately, the characteristic decay time of the plasma after power turn-of is typically many milliseconds, so the opening time of such a barrier generally is predicted also to be many milliseconds. However, we have found both experimentally and theoretically that we can open such a barrier on a time scale of microseconds. We do this by increasing the plasma density rather than waiting for it to decay. We have two layers of plasma. We produce a standing wave between the two layers that results in microwave transmission, analogous to the transmission found in an optical Fabry-Perot Resonator. The secret lies in the boundary layer behavior of the plasma. Once microwave cutoff occurs, one would expect the plasma behavior to be static. What actually occurs is that at microwave cut-off, the reflection is in phase with the incident wave, in analogy to an open coaxial line. (The electron and displacement currents are equal, but out-of-phase.) As the plasma density further increases, the reflection smoothly changes from in-phase to 180 degrees out-of-phase, in analogy to a shorted coaxial line. (The electron current is much greater than the displacement current.).