Spectroscopic studies and rotational and vibrational temperature measurements of atmospheric pressure normal glow plasma discharges in air

DC normal glow (NG) discharges were created in atmospheric pressure air for a pin to plate type geometry. The rotational and vibrational temperatures of the discharge were measured by comparing modelled optical emission spectra with spectroscopic measurements from the discharge. The temperatures were measured as a function of discharge current, ranging from 50 µA to 30 mA, and discharge length, ranging from 50 µm to 1 mm. Rotational temperatures from 400 to 2000 K were measured over this range. Vibrational temperatures vary from 2000 K to as high as 5000 K indicating a non-equilibrium plasma discharge. Spectroscopic measurements were compared using several different vibrational bands of the 2nd positive system of N2, the 1st negative system of and the UV transitions of NO. NO and transitions were also used to determine the electronic temperature and density. The discharge temperature appears to be controlled by two cooling mechanisms: (1) radial conductive cooling which results in an increase in temperature with increasing discharge current and (2) axial cooling to the electrodes which results in a temperature saturation with increase in discharge current. The measured discharge temperature initially increases rapidly with discharge current then becomes nearly constant at a higher discharge current. Thus, radial cooling appears to dominate at lower discharge currents and the axial cooling at higher discharge currents. The vibrational temperature decreases with increasing rotational temperature due to increased vibrational to translation relaxation but the discharge remains non-thermal and stable over the range studied. The discharge appears to have a maximum vibrational temperature at the low current limit of the NG regime.