In-situ Measurements of the Temperature Gradient in Complex Plasmas

Abstract

Complex or dusty plasma is a medium containing ionized gas and micron-sized solid particles. The microparticles are sensitive to the thermophoretic force. Thermophoresis is often a disturbing factor in microgravity complex plasma experiments or a way to control the microparticle suspensions. In spite of that, no attempts to measure the temperature gradient in-situ in complex plasma are known in the literature. We present such measurements performed in Ar rf discharge used for complex plasma experiments with the help of laser spectroscopy. The experiments were performed in the PK-3 Plus chamber, where we could control the axial temperature gradient up to 20 K/cm by heating the bottom electrode. We used tunable diode laser absorption spectroscopy (TDLAS) for the temperature measurement from Doppler profiles. Two parallel laser beams passed through the discharge with a height difference of 1.5 cm. The absorption is proportional to the concentration of excited atoms with a Doppler shift corresponded to the difference between the laser wavelength and the atom spectral line. This allowed us to measure line-integrated temperatures at those two positions. We investigate absorption of 772,38 nm spectral line. The temperature differences could be measured with accuracy better than 0.5 K.