Bremstrahlung-continuum diagnostics of the plasma of an externally sustained discharge

The emission spectrum of the nitrogen plasma of an atmospheric-pressure discharge initiated by an electron beam has been studied theoretically and experimentally. The visible and near-IR parts of the spectrum are dominated by the bremsstrahlung continuum which results from the scattering of plasma electrons by nitrogen molecules. The electron density and the reduced field E/N can be determined from the bremsstrahlung continuum by fitting the theory to experiment. The electron density has also been found by measuring the discharge current and the properties of the electron beam. The ionization balance was calculated by solving the Boltzmann kinetic equation for the energy distribution of the fast electrons. An analytic expression is found for the rate at which ion--electron pairs are formed; this expression takes cascade ionization into account. A good agreement is found between the calculated and measured bremsstrahlung spectra with a total cross section of (5 +- 1) x 10-16 cm2 for the excitation of the first eight levels of nitrogen. This cross section agrees with the data found by Schulz