Studies of a low power atmospheric pressure discharge with flowing He#

The mechanisms that create and sustain atmospheric pressure glow discharges (APGD) are still the subject of debate. Here electrical, optical and laser-aided diagnostic techniques are used to characterise a low power atmospheric pressure discharge, operating in air with helium gas flowing through the inter-electrode space. The discharge is created between two copper covered glass plates, separated by 2.6 mm. The outputs of a variable frequency voltage supply are attached to each electrode. At driving frequencies of about 25 kHz what appears visually to be a spatially and temporally uniform glow discharge is established at pk-pk voltages above 180 V. This mode is characterised by discrete current draw, persisting for 2 microsec., twice during the voltage period. The visible spectra are dominated by emission from N2 and N2+ with some evidence of He emission lines. These results are consistent other results1. Preliminary analysis of the N2 second positive series indicates rotational temperatures 400 K and vibrational temperatures 3000 K. Preliminary absorption measurements indicate densities of the triplet and singlet metastable states of Helium are less than 3X10^9 atoms cm-3 and no evidence of long lived nitrogen species. 1. F. Massine et al. J. Appl. Phys. 83, 3411-3420 (1998) #Supported by UK EPSRC