Abstract
Possible forbidden transition in helium is suggested for the measurement of fluctuations in plasmas. Instead of the three-level system, four-level system is proposed; one of the lower levels is a metastable state 21S, from which the resonant absorption is performed by using a high-power dye-laser to populate an upper level. A result calculated quantum mechanically is presented. The advantages of this method as compared with the usual technique of the plasma satellite line are discussed.