Radial Distributions of Active Species in Plasmas and Afterglows

Abstract

The radial distributions of plasma and afterglow radiative species have been analysed with optical fibers connected to an optical spectrometer.

With an optical fiber of  dia. giving a detected light resolution of about  at a  distance, the  long time afterglow (LA-10-2 s) was studied in a large volume reactor (5 liters). At pressure of 6-8 Torr, flow rate  slpm, the LA-  was a jet of dia.  with densities of -atoms and  metastable molecules of  and , respectively.

By putting a horizontal slit of  in front of the dia.  optical fiber, a light beam of  dia. was detected across an afterglow tube of  dia. In a  RF plasma at 8 Torr, , Bessel typed profiles were recorded for all the radiative bands in the pink afterglow and flat profiles of the  band in the late afterglow. The  band is produced in the late afterglow by -atoms rebounding on the tube wall, explaining its flat profile. In the pink afterglow, the  band is mainly produced by collisions of the  and  is destroyed on the tube wall, which explains its Bessel profile. With a small  content into , the  and N₂(A) states in the pink afterglow are destroyed, lowering the  emission that were only produced by the  atoms, yielding a flat profile.

A  radial profile was also obtained with a collimator set on the  optical fiber and moved along the radial axis of a surface wave (SW) plasma. With a SW plasma, the electric field (E) reaches a maximum value on the tube wall giving high energy to electrons. As a consequence, the Ar metastable atoms produced by high energy electrons and destroyed by low energy electrons ( unsensitive to E) will show radial profiles of which the shapes were analysed to determine the metastable excitation  ) and de-excitation ( ) rates. In Ar SW plasmas at  Torr, , it was determined that  and  as for a DC positive column at same electron density : .

3D spatial distributions of the intensity of the optical thin ArI549.6 nm line was measured in DC and SW plasma columns at 150 mTorr inside tubes of  dia. and  long. Bessel-type radial distributions were found in the Ar DC column and more rounded profiles in the Ar SW plasmas at 600-900 MHz, with hollow shapes at the plasma end.