Electron-beam generated air plasma: Ozone and electron density measurements

A 100-keV electron beam is used to ionize air within a test cell in which the pressure is varied from 1 mT to 600 T. In these experiments, the electron beam is operated with a single pulse of 10 -ms duration at a current of 5.6 mA. The beam in injected into the tank through a 12.7-mum aluminum foil window that separates the electron source from the test cell. Byproducts of volumetric ionization and dissociation include plasma electrons, ozone produced by the rapid reaction of atomic oxygen with oxygen in the air, and excitation of the N₂ Second Positive line at 337.1 nm. The N₂ emissions were observed 2 cm from the foil window using a fiber optic system. Ozone was detected by optical absorption at 255 nm and electrons were detected by an rf phase measurement system operating at 10 GHz. The ozone and rf measurements were made on the test-cell midplane, 25 cm from the foil window. An air chemistry code1 is used to estimate ozone production and rf phase shift as a function of volumetric ionization rate which was subsequently compared with the experimental data. These measurements are discussed as well as an estimate of the power required to generate and sustain the plasma density.