Microwave absorption and plasma heating due to microwave breakdown in the atmosphere

Abstract

The propagation and absorption of microwaves above the breakdown threshold in the atmosphere with the self-consistent breakdown plasma are investigated. Typical hydrodynamic calculations show that an ionization front is rapidly formed which moves toward the microwave source and consequently decouples the microwaves from the original ionization region. By focusing the microwaves or using a reflector, ionization can be confined to localized regions where the microwave strength is high enough to cause breakdown even though the incoming microwaves are below threshold. In a strongly collisional atmosphere, it is found that the cutoff plasma density, which roughly equals the collisionless cutoff density multiplied by the collisionality, is much higher than the calculated maximum density. This results in high absorption that increases with microwave power, decreases with atmospheric pressure, and is quite independent of other parameters. In a weakly collisional atmosphere, breakdown easily creates a plasma density higher than the cutoff density and causes reflection. It is found that the reflection decreases with the collisionality of the system and is quite independent of the microwave strength. In general, the microwave field strength in the ionization regions is attenuated to the breakdown value at steady state, and the resulting electron temperature is about 2 eV, independent of the incident microwave flux. Limitations of the calculations are due to the availability of experimental data for the rate coefficients, but comparison to results from recent focused microwave experiments shows excellent agreement.