High-resolution electron density and temperature maps of a microwave plasma torch measured with a 2-D Thomson scattering system

Abstract

Thomson scattering (TS) measurements were performed on plasmas produced by a microwave plasma torch (MPT). This MPT creates plasmas with powers in the range 50–150 W in argon flows of typically 1000 ml/min. For the detection of the TS signal, we used a homemade Triple Grating Spectrograph (TGS), which eliminates the influence of the (false) stray light at the central wavelength. The two-dimensional (one spatial and one wavelength) image constructed by the TGS is projected onto an intensified Charge-Coupled Device so that in short measurement times, both spatial and spectral information are obtained for the plasma chord along the laser beam. This makes it relatively easy to map the properties of the electron gas within half an hour. The detection limits of this set-up (associated with this measurement time) are on the order of 10 18 electrons per m 3. By getting more detailed insight in the electron density n e and electron temperature T e fields, we can now firmly conclude that there are large discrepancies between the results of experiments and models based on the theory of disturbed Bilateral Relations (dBR). The theoretical T e value of 10 kK is substantially lower than the experimental value (17 kK), whereas the theoretical n e value of 1.6×10 22 m −3 is much larger than what is found experimentally (2×10 21 m −3).