The laser induced fluorescence(LIF) method has been used to measure density of He $2^1$S metastable state in He plasma. He plasma has been generated by transfomer coupled plasma(TCP) source contained in multipurpose plasma chamber(MPC). The discharge condition was 10~60 mTorr pressure range and 100~1000 W rf power range. The laser system for this experiment was composed by Nd:YAG pumping laser and optical parametric oscillator (OPO), which can tune the laser wavelength over very broad range.
The depletion of He matastable state is observed about 50~60 mTorr at 800 W rf power. We have confirmed this phenomenon also using optical emission spectroscopy (OES) and Langmuir probe measurement.
