The radial contraction of argon microwave plasmas studied by Thomson scattering

Radial electron density n(e)(r) and temperature T-e(r) profiles of a microwave argon plasma at intermediate pressure were investigated by Thomson scattering. This method allows one to get n(e)(r) and T-e(r) spatially resolved without any a priori assumption on the shape of the profile. Data were acquired in the pressure range 5-88 mbar where a transition from wall-stabilized to a radially contracted plasma mode was observed. It was found that the fitting of the radial profile can be done with a Bessel function for which the boundary radius R defined by n(e)(R) = 0 is a free parameter. For pressures above 20 mbar the electron density profile undergoes radial contraction, so R goes down from 3 mm at 5 mbar (wall position) to 2.09 mm at 88 mbar. The electron temperature T-e(r) on the other hand is flat in the centre and rises towards the wall. For low pressures, this rise is moderate but for pressures of 20 mbar and above the increase is more pronounced.