Steep plasma gradients studied with spatially resolved Thomson scattering measurements

Plasmas created by the microwave torch Torche à Injection Axiale (TIA), which are around 2 mm in diameter and 15 mm long, are investigated. In these plasmas large gradients are present so that the edge is supposed to play an important role. Using global Thomson scattering measurements, in which global refers to the fact that the size of the laser beam is approximately equal to the diameter of the plasma, the electron densities and temperatures were determined. However, these results lead to discrepancies in the particle balance: the production of free electrons is much larger than the classical losses due to recombination, convection and diffusion. Radially resolved Thomson scattering measurements show the plasma has a hollow structure. Although this enhances the losses due to diffusion, still a large discrepancy remains between production and destruction of free electrons in the argon plasmas. Probably some molecular processes are significant as well. A good candidate is the charge transfer between argon ions and nitrogen molecules, since mixing with the surrounding air has a large impact on the plasma.