A departure from local thermodynamic equilibrium within a freely burning arc and asymmetrical Thomson electron features

Spectrally resolved measurements of Thomson scattering were made in a freely burning are in argon al a pressure of 1 atm. The radial and axial profiles in electron temperature and number density exhibited extensive regions of departure from local thermodynamic equilibrium, so confirming the findings of Snyder et al in Phys. Rev. E 48 4124-7, 1993. Electron temperatures at least 4000 K above those of the gas were found. Asymmetry in the Thomson electron feature was found and shown to depend on which segment of the laser-profile scattering data was sourced. This arises from an asymmetry in the are-laser interaction. Also described is a simple technique for accurately defining the position of the cathode tip once an are has been ignited.