ABSTRACT
The use of high-frequency power to produce plasmas in air and high-pressure gases is a relatively new development. These methods span the regimes of seed gas ionization via carbon monoxide (CO) and ultraviolet flash tubes and lasers, seed gas ionization and optical pumping via carbon monoxide lasers and ionization sustainment by rf plasma torches and microwave plasma sources. Their advantage is that power can be spatially focused away from electrodes or wall materials by means of antennas or optical lenses. In addition, since the focus is adjustable, large, three-dimensional volumes of plasma can be created in space without the need for electrodes that can degrade. Historically, rf air plasma torches in air were the first to be investigated. Then microwave and later flash-tube and laser sources became of interest. Recently, electron beams propagated through a vacuum window to protect the cathode and short-pulse high-voltage plasma sources in air have been investigated. Much of the recent research presented in this chapter was supported by a Defense Department Research and Engineering multi-university research initiative (MURI) entitled 'Air Plasma Ramparts' and AFOSR grants administered by Dr Robert Barker.
