Spectroscopic investigation of the equilibrium state in the electric arc cathode region

Several thin layers of an arc plasma column were observed side-on at different distances from the cathode. The arc was operated in argon at atmospheric pressure with the arc current equal to 200 A. As the basic diagnostic method we used the Olsen - Richter diagrams. For determination of temperature the Larenz - Fowler - Milne and Boltzmann plot methods were applied. Electron density was measured from the Stark width of the ArII line and continuum intensity. The plasma near the cathode tip shows axial and radial dependence of its physical equilibrium state. In the hot core, the plasma is in LTE. Toward outer zones, the plasma state deviates from LTE. For plasma layers closer to the cathode, this deviation begins at smaller distances from the axis and for higher temperatures it vanishes for h > 3 mm. It is caused after all by the ArI ground state overpopulation. Theoretical estimations of the ArI ground state overpopulation give results consistent with experiment. Calculations show that this overpopulation is caused by inward transport of ground state atoms into the plasma column.