Development of diagnostics of electron density and temperature for atmospheric-pressure helium plasma based on optical emission spectroscopy analysis and a collisional-radiative model

Abstract

An algorithm for diagnosing the electron density and temperature of helium plasma at atmospheric pressure has been developed based on a revised helium collisional-radiative (CR) model. Atomic collision processes are included and part of the atomic data of electron collision processes in the conventional CR model has been updated to expand its valid pressure. The algorithm uses eight emission lines in the visible-wavelength range as inputs to determine the electron density, electron temperature, and number density of the two metastable states by fitting the number density of the states corresponding to the emission lines. The algorithm has a considerably small theoretical error. In the microwave-discharged low-pressure helium plasma experiment, the results obtained with the algorithm agreed well with the results obtained with the probe method. The electron density and temperature of the atmospheric-pressure helium plasma obtained with the algorithm agreed well with the results of the continuum spectrum analysis.

Quantitative analysis of optical emission spectroscopy for plasma process monitoring

“…Hence, in order to confirm the potential of the method of OES measurement of continuum emission and the specific precautions to be taken when implementing it, the author's group also launched experimental research and confirmed the effectiveness of this method through various measurements and cross-checks. 57,[60][61][62] The author's group have been studying with validity examination whether the EEDF should be Maxwellian, Druyvesteynian, or something else. The author is also proposing mathematical methods to presume the arbitrary EEDF from the observed continuum emission as remarkable issues with originality.…”

“…The strategy for spectral analysis is to obtain the continuum spectrum of electron-atomic bremsstrahlung obtained as shown in Fig. 14, using T e and N e in the theoretical equation as best fitting parameters, 57,60) which will be described in this chapter in detail.…”

“…On the other hand, as will be explained later, concerning OES measurements of non-equilibrium atmospheric-pressure plasmas for medical/dental applications, etc., similar to lowpressure plasma, it is also possible to interpret the line spectrum intensity based on atomic and molecular processes to find the electron temperature and density as macroscopic parameters of the plasma. 48,[52][53][54][55][56][57] However, it is now becoming clear that its application is difficult due to the uncertainty in the cross-sections of excitation and de-excitation caused by atomic collisions, or the rate coefficient of quenching by collisions with heavy particles. Currently, no standard measurement method of atmospheric-pressure nonequilibrium plasma has been established for electron temperature or density.…”

“…58,59) Therefore, the author's laboratory has also been intensively pursuing this possibility over the past few years. 57,[60][61][62] Nowadays, the OES of the continuum spectrum is confirmed as a reliable measurement method of not only the electron temperature and density, but also the electron energy distribution function (EEDF), using simple and practical experimental equipment. [63][64][65] In the case of atmospheric-pressure nonequilibrium plasma, there are many neutral atoms to collide with, so bremsstrahlung as a continuous spectrum can be measured with considerable intensity.…”