Diagnostics of the spatial profile of atomic oxygen in the flowing afterglow of a microwave plasma as a result of variable gas flow conditions

Abstract The spatial profile of atomic oxygen in a cylindrical afterglow chamber with a height of 41 cm and an inner diameter of 30 cm was measured. The source of oxygen atoms was a remote microwave plasma operating at a discharge power of about 250 W. The gas flowed through a quartz-glass tube with inner and outer diameters of 3.8 and 6.0 mm, respectively. The exhaust of the quartz tube widened to cones of various geometries. The spatial distribution of atomic oxygen was determined for cones with an outer diameter of up to 40 mm. The tube with the widest cone (Tube 3) provided the largest O-atom density of 6 × 10 20 m −3 in the upper part of the afterglow chamber away from the main gas stream in the pressure range from 50–200 Pa, while the tube (Tube 1) with the narrowest cone enabled an O-atom density of up to 2 × 10 20 m −3 . The differences in measured oxygen density for three tubes at positions ‘up’ and ‘down’ decreased with increasing pressure and were below the detection limit at pressures above 350 Pa. In the case of the ‘middle’ position, Tube 2 with an outer diameter of 19.3 mm exhibited a sharper decrease in oxygen density compared to Tube 1 and Tube 3. The O-atom density in the middle of the afterglow chamber increased with the increase in the percentage of pump valve opening at the lowest probed constant pressure of 40 Pa, but it stayed constant for the opening of the pump valve above 70%. For constant pressures above 100 Pa the O-atom density decreases with the larger pump valve opening. The pressure is kept constant by the corresponding increase in oxygen gas flow while increasing the percentage of the pump valve opening. The spatial profiles are explained by the effects of gas flow and diffusion.