A langmuir multi-probe system for the characterization of atmospheric pressure arc plasmas

It is well known that the use of Langmuir probes in `high' pressure plasmas (atmospheric and above) is extremely difficult due to the high degree of collisionality. In addition, electric arcs used in industry are highly non-homogeneous. In particular the degree of ionization varies from zero at the arc edges to values close to full ionization near the axis. Existing probe theories are therefore inappropriate. The `traditional' use of the characteristic curve for arc temperature and particle density mapping is time consuming and also leads to errors due to the collisional nature of the discharge. Existing probe theories are examined with respect to the application at high pressure and in this, the first of a series of papers, an interpretation based on different assumptions is presented. A simple apparatus for the study of tungsten inert gas (TIG) welding arcs is described permitting data collection over short time periods (~0.1 s) for operating currents in the range 50-150 A. Difficulties encountered with probe applications are discussed highlighting the need for further theoretical developments. The axial electric field is given by the derivative of the measured potential with respect to the axial position and shown to be in agreement with previous estimations. Results are discussed with reference to the sparse previous publications concerned with high pressure arc plasmas.