Laser diagnostics for the electron density of helium low temperature plasmas using saturated absorption spectroscopy

Abstract

Helium (He) low temperature plasma with an electron density higher than 10(11) cm(-3) was produced using an inductively coupled plasma source, and the electron density of this plasma was determined using a high-resolution laser spectroscopic technique. To remove the Doppler broadening which is the most dominant line broadening in low temperature plasmas, a saturated absorption spectroscopy system was configured. The Lamb dip was successfully separated from the Doppler broadening for the 2(1)S-4(1)P transition of the He plasma using a high-resolution laser beam with a narrow spectral line width (below 1 MHz) at 396.5 nm. The estimated spectral line widths of natural broadening, van der Waals broadening, resonance broadening, and Stark broadening were 40.14 MHz, 0.93 MHz, 0.15 MHz, and 12.60 MHz, respectively, when the He gas pressure was 30 mTorr and 1 kW RF power was applied. The electron density of the He plasmas was determined to be similar to 3 x 10(11) cm(-3), which was comparable with those determined by a Langmuir probe and optical emission spectroscopy combined with a collisional-radiative model.