The effect of easily ionized additives on mercury arc plasmas: transition from the ionizing to the recombining phase

The emission properties of a plasma are determined from the distribution of the population density over the excited states. In lighting arcs, where the electron density is high enough and the resonance radiation is significantly absorbed, the Griem criterion is fulfilled so that the influence of the radiative decay on the excited state populations can be neglected. In this case the actual density of an excited state normalized to the Saha density at the electron temperature and density prevailing in the discharge expresses the deviation of this excited state from the local Saha equilibrium. In ionizing-phase plasmas the normalized density of the ground state is much higher than one, whereas in recombining-phase plasmas it is lower than one. We have found that at the moment of the maximum current phase of ac high-pressure discharges the plasma in pure mercury is in the ionizing phase, whereas in the presence of a small amount of an easily ionized additive (NaI) the mercury plasma is in a recombining-phase. The method used for determining the electron temperature, the electron density, and the actual densities of the excited states is based on the mercury self-reversed emission lines and is independent from equilibrium assumptions. The actual density of the ground state was obtained from the electron temperature determination at the minimum current phase assuming that the heavy particle temperature at this moment equals the electron temperature and does not undergo a modulation over the ac period. In pure mercury plasmas the number of ionization reactions has to be larger than that of recombination due to the continuous removal of the charged particles from the arc core by convection and diffusion. Thus, a Saha imbalance (net ionization) is created and the actual densities of the excited states have to be higher than the corresponding Saha densities. In presence of sodium iodide the freed sodium atoms in the high-ionization central region of the arc supplies a source- of electrons. Thus, in spite of the outward transport of the charged particles, a net recombination is created, the Saha imbalance is inversed and the actual densities of the excited states have to be lower than the corresponding Saha densities.