Study of the cathode region of an electric arc

The plasma in the neighbourhood of the cathode tip of an electric ape shows spatial dependence on its physical equilibrium state. Near the axis the pl asma is in the local thermodynamic equilibrium (LTE) state. Toward the oute r zones, the plasma state deviates from the LTE equilibrium. This deviation is related to the distance from the cathode tip rather than to the electro n temperature and density. In our opinion, it is caused mainly by the ArI g round state overpopulation. Results of our experiments support this conclus ion. This overpopulation is caused by inward transport of ground state atom s into the plasma column due to the fast flow of the plasma near the tip. O ur theoretical estimations of the ArI ground state overpopulation give resu lts consistent with the experiment. (C) Academie des sciences/Elsevier, Par is.