ELECTRON AND POPULATION TEMPERATURES IN A NON-LTE OPTICALLY THICK PLASMA

The distribution temperature between excited atomic states in an arc plasma can be determined using self-reversed lines. This technique was applied to a high-pressure mercury arc. The electron temperature was deduced from the distribution temperature between the 6(3)P2,0 metastable levels. The population temperatures of the 6(3)P1, 7(3)S1 and 6(3)D3 levels were also determined as well as the lowering of the population of the 6(3)P1 resonance level caused by resonance radiation. The electron density was deduced from the Saha law, the known electron temperature and the measurement of the density of the 6(3)D2 level. At the arc axis the difference between the electron temperature and the 'equilibrium temperature' deduced from the optically thin line technique assuming local thermodynamic equilibrium (LTE), is of the order of 1000 K and 450 K at the moments of maximum and minimum current phase, respectively. The difference found between the population temperatures of the different levels indicates that collisionally dominated plasma is not a sufficient condition for the presence of LTE in the central region of an inhomogeneous arc where the diffusion of charged particles can disturb the Saha balance.