Rayleigh scattering on a microwave surfatron plasma to obtain axial profiles of the atom density and temperature

The axial dependency of the central-axis value of the heavy particle density and temperature of surface wave plasmas is studied using Rayleigh scattering (RyS). The plasma is generated at a frequency of 2.45 GHz in argon by a surfatron operating under the standard settings of a power of 45 W, a flow rate of 50 sccm and a pressure of 20 mbar. To investigate the effect of the pressure on the gas temperature we also investigated 6 and 10 mbar plasmas while keeping the other control parameters unchanged. By using a two dimensional intensified CCD array we could determine and eliminate the influence of false stray light; a mayor disturbing factor in the determination of the Rayleigh signal. In order to trace the energy fluxes that determine the gas temperature we performed Thomson scattering so that the properties of the electron gas are known. It is found that the gas temperature, Ta, depends on the wall temperature and the product of the gas pressure and the electron pressure. The later implies that Ta follows the electron density axially, meaning that it is highest at the launcher and decreases monotonically in the wave propagation direction. The maximum gas temperature of around Ta = 800 K is found close to the launcher for the highest pressure of 20 mbar. For lower pressures we find lower Ta values. In all cases, the extrapolation of Ta toward the end of the plasma column leads to a temperature of about 310 K which is close to room temperature. This study reveals that, for the argon plasmas under study, the central-axis values of the gas temperature is determined by the balance between the heating of the gas by means of elastic electron collisions and the cooling due to heat conduction from the centre to the wall.