Reinterpretation of Laser Scattering Measurements of Welding Arc Temperature and the Existence of Local Thermodynamic Equilibrium in Arcs(Physics, Processes, Instruments & Measurements, INTERNATIONAL SYMPOSIUM OF JWRI 30TH ANNIVERSARY)

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Thomson scattering measurements of electron temperature in atmospheric-pressure free-burning arcs and plasma jets have indicated that the electron temperature in these thermal plasmas is up to 7000K greater than the ion temperature and the excitation temperature measured by spectroscopy. In the Thomson scattering measurements, the electrons were strongly heated by the laser beam. This heating was taken into account by measuring the electron temperature as a function of the laser pulse energy, and linearly extrapolating the results to zero pulse energy to obtain an unperturbed electron temperature. It is shown here that the absorption of laser energy by the electrons, and the collisional and radiative cooling of the electrons, are strongly dependent on the electron temperature, and therefore on the pulse energy. A one-dimensional fluid dynamic model is developed that takes into account these processes. The use of this model to extrapolate the measurements gives unperturbed electron temperatures in much closer agreement with ion and excitation temperatures than the electron temperatures obtained using linear extrapolation. The laser heating of the electrons means that, over the duration of the laser pulse, scattering occurs from electrons at a range of temperatures. The difficulty in deriving an electron temperature from the resulting spectrum of scattered radiation is pointed out. The accuracy of Thomson scattering measurements that use an expanded laser beam to minimise the electron heating is also examined. The defocusing of the laser beam, combined with the large temperature gradients that occur in thermal plasmas, again leads to scattering from electrons at a range of temperatures. Simulations indicate that the resulting distortion of the spectrum of scattered radiation can lead to the derivation of electron temperatures that are significantly greater than those existing anywhere in the scattering volume. It is concluded that Thomson scattering measurements significantly overestimate electron temperatures, and do not provide reliable evidence of deviations from local thermodynamic equilibrium, in free-burning arcs and plasma jets at atmospheric pressure.
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Reinterpretation of Laser Scattering Measurements of Welding Arc Temperature and the Existence of Local Thermodynamic Equilibrium in Arcs(Physics, Processes, Instruments & Measurements, INTERNATIONAL SYMPOSIUM OF JWRI 30TH ANNIVERSARY)

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