Steep plasma gradients studied with spatially resolved Thomson scattering measurements

Abstract

Abstract. Plasmas created by the microwave torch Torcheà 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.

Effect of remote field electromagnetic boundary conditions on microwave-induced plasma torches

“…It was found that chemical features of the environment indeed have an impact on the plasma. As explained in [13], when the plasma is created in a vessel filled with Ar, we get higher n e -and lower T e -values than when it is operated in air . Typical changes are an increase of n e from 6 · 10 20 m −3 in air to 1.2 · 10 21 m −3 in argon and a change in T e from 19000 K in air to 16000 K in argon.…”

“…Applying absolute intensity measurements [11,12] of atomic lines and the continuum it was possible to determine the electron density n e and temperature T e ; these values have been compared with the results of the Stark intersection method and Thomson scattering [13][14][15][16][17]. Typical reported values for the TIA are n e = 3 · 10 21 m −3 and T e = 22000 K. For the MPT the typical values are n e = 3 · 10 20 m −3 and T e = 18000 K (see [18]).…”

The influence of molecular gases and analytes on excitation mechanisms in atmospheric microwave sustained argon plasmas

“…During steady-state conditions the electron density and temperature at these heights are already much lower than just above the nozzle [7]. Therefore it is very unlikely that in the recombination zone the analyte is still dominantly excited by electron ruled processes.…”

“…Table 2). Due to the high electron temperature in argon plasmas produced by the TIA [7,8], next to dissociative recombination of N 2 + (X) an efficient excitation channel towards the B-state by electron impact is provided as well:…”

“…Other excitation balances which can be distinguished are excitation transfer (ET) (6) or charge transfer (CT) (7) In these equations A and X denote heavy particles whereas p, q, r and s denote principal quantum numbers. The charge transfer balance is not electron temperature dependent and therefore levels populated by CT show no response to electron cooling.…”