Density Measurement of Helium Metastable Atoms in a Plasma by the Laser-Induced Fluorescence Method

Density profiles of helium metastable atoms in 23S and 21S levels have been measured in a plasma (n e ∼1012 cm-3, T e ∼7 eV, radius of the plasma column ∼1 cm) by the Laser-Induced Fluorescence Method (LIFM), where both quenching effects of fluorescence by plasma and characteristics of laser radiation are taken into account. The He(23S) metastable atoms are distributed in inverse proportion to the radius in the periphery of the plasma column (r>1 cm), while the profile of He(21S) metastable atoms has a gentle slope compared with that of He(23S) atoms. The difference in the density distributions is thought to be caused by the production of He(21S) atoms in the plasma periphery through the n1P (n≧2) levels due to optical excitation processes of He(11S) by resonance lines emitted from the plasma column.