Effect of plasma temperature and plasma pulsation frequency on atomic nitrogen production

An electromagnetic surface wave launched by a waveguide (type surfaguide [M. Moisan, Z. Zakrzewski, J. Phys., D., Appl. Phys. 24 (1991) 1025]) powered by a microwave generator (2.45 GHz) generates a plasma which produces atomic nitrogen by dissociation of molecular nitrogen. This plasma takes place in a quartz tube where an argon/nitrogen mixture is flowing. The dissociation rate depends directly on the power density injected into the discharge. The microwave power is therefore pulsated in order to be able to work with high instantaneous power while preserving a low mean power. The frequency of pulsation has a great influence on the effectiveness of the dissociation process: an optimal frequency has been be determined which depends on various parameters of the discharge such as power and tube diameter. The optimal conditions of pulsation seems to be due to a thermal effect and more precisely can be correlated with the radial profile of the gas temperature in the discharge. The dissociation fraction of the molecular nitrogen in atomic nitrogen is determined by following the decrease of the molecular nitrogen signal when the plasma is ignited. The latter is measured by mass spectrometry and by optical emission in the post-discharge. In order to understand the thermal effect inducing an optimum pulse frequency, the discharge is also characterized by optical spectroscopy: the change in concentration of species like argon atoms, neutral and ionized molecular nitrogen has been followed as a function of the pulse frequency and the gas discharge temperature measured by means of the rotational bands of N2+.