Optical Emission Spectroscopy Measurement for Plasma Parameter Identification — from Kinetic Modeling to Data Science

Recent progresses in studies of optical emission spectroscopy (OES) measurement of low-temperature plasmas are presented. Two methods are presented; the first one is line-spectrum measurement assisted with excitation kinetic model like collisional-radiative (CR) model. The second one is continuum-spectrum measurement due to bremsstrahlung of free electrons in low-temperature plasma, which can be applied only to atmospheric-pressure plasmas.

Traditionally, the observed number densities of excited states obtained by line-intensity measurement have been interpreted by excitation kinetic model like modified CR models. Simultaneous equations are solved based on the kinetic models to calculate electron temperature T_e, density N_e, and even the energy distribution parameter x, with which the electron energy distribution function (EEDF) is described as f(ε) ∝ exp[–C(ε/kT_e)^x]. With this scheme, however, troublesome mathematical procedure is inevitable to extract dominant processes. Recently, another strategy is presented to minimize the summation of the square-root-deviation between the excited-state densities observed by the OES and those calculated with the CR model. Such levels must be selected by a trust region method, BOBYQA, which is a root-finding algorithm for bound constrained optimization without using derivatives of the objective function. Examples will be presented for argon plasmas under several discharge conditions, such as low-pressure ICP, medium-pressure MWD and atmospheric-pressure DBD.

On the other hand, continuum emission analysis for the atmospheric-pressure cold plasma can be done by theoretical fitting of the observed spectrum to deduce T_e and N_e. By this spectrum, the EEDF can be also examined to observe through the reinforcement learning-based visible bremsstrahlung inversion (VBI) method, where it is found that the EEDF over a limited energy-range can be reconstructed, which will be discussed in detail in the meeting.