The uniform, steady-state atmospheric pressure DC plasma

Summary form only given, as follows. We have developed and diagnosed a steady-state DC plasma discharge apparatus that operates at atmospheric pressure. The device also works well with low frequency AC at 60 Hz. The new feature is a ceramic discharge electrode that prevents the diffuse discharge from contracting into an arc. The apparatus works well in air, but for large volume plasma generation of several liters, helium is used. The maximum DC power applied is 300 Watts. The apparatus has been used successfully in the destruction of microorganisms (1). Recent results from our plasma discharge in helium include separate measurements of the electron and the ion densities. These numbers differ, as the plasma contains negative ions as well as electrons. The most convincing electron density measurement is a microwave attenuation measurement of 10 exp 11 per cc. Conventional plasma resonance measurements do not work, as the high electron-gas atom collision rate suppresses the resonance effects. Although the attenuation is small-about one percent-the attenuation is readily observable by a chopping technique. As a byproduct of the density measurement, the lifetime of the free electrons before being converted into negative ions is observed to be tens of microseconds. A second plasma electron density measurement is made by measuring the electron drift current, which also yields 10 exp 11 per cc. Plasma ion density measurements have been made with a variety of single and double Langmuir probes, and yield densities on the order of 10 exp 12 per cc.