Spectroscopic study of ionizing and recombining plasma in a stationary plasma source

A hot cathode, which is composed of La2O3–W needles, has been constructed and tested in a modified arc discharge source in order to produce a high-density plasma by large discharge current. No damage of needles was found after 100 h at a discharge current of 300 A. Characteristics of generated helium plasmas have been investigated by spectroscopic observations. The electron density has been found to be ≧2×1014 cm−3 at a discharge current 100 A with this cathode. Experimental results were analyzed with calculations based on a collisional radiative model including an effect of radiation trapping. As a result, generated plasmas were shown to be classified into two groups by the pressure of neutral helium, that is, an ionizing plasma and recombining plasma. In low temperature recombining plasma, population inversions between He+ ion n=2 and the higher excited levels have been obtained.