Atmospheric pressure collisional plasma absorber: theory, experiments, and issues relating to efficient production

Summary form only given. The dispersion relation for a cold collisional plasma is known to predict absorption of radio frequency waves due to energy transfer from the wave to plasma electrons, and then to a neutral species in a momentum-transfer collision. For a plasma source that generates a smooth decreasing electron density as a function of decreasing distance from the source, theoretical models predict that the reflection coefficient from such plasma distributions can be very small. Hence, a collisional plasma with can be very small. Hence, a collisional plasma with smooth density gradients can attenuate radio frequency waves without significant reflection. Calculations for a collisional plasma with a smooth gradient predict broadband absorption of 40 dB or more from 100 MHz to 10 GHz. Experiments using electron-beam impact ionization and photon sources for photoionization have quantified laboratory-scale plasma absorbers in waveguides and cubic-meter-size systems operating at atmospheric pressure. Simple but efficient means of plasma production in air are needed, but have been slow in development. Modern approaches will be discussed, as well as fundamental processes in a plasma made of ambient air that affects the power budget for plasma production.