Microwave-Induced Plasma Shield Propagation in Rare Gases

It has been observed that microwave irradiation of localized weak ionization in heavy rare gases can cause an ionization wave (“plasma shield”) to form and propagate toward the rf source. This ionization wave is preceded by an electron precursor, with the electron density at the ionization wave front increasing very rapidly to a maximum followed by a relatively slow plasma decay. At rf power levels well below normal breakdown, the ionization wave will form and then propagate at velocities from about 2 × 103 cm/sec to 1 × 107 cm/sec. Discontinuous changes of ionization wave velocities with changes in rf power and gas pressure indicate the existence of three different velocity-controlling mechanisms in the gas pressure and rf power ranges investigated. These mechanisms for ionization wave formation and propagation have been investigated in xenon, krypton, and argon gases, and are discussed.