Electron density measurements in an atmospheric pressure air plasma by means of infrared heterodyne interferometry

Abstract

An infrared heterodyne interferometer has been used to measure the spatial distribution of the electron density in direct current, atmospheric pressure discharges in air. Spatial resolution of the electron density in the high-pressure glow discharge with characteristic dimensions on the order of 100 µm required the use of a CO 2 laser at a wavelength of 10.6 µm. For this wavelength and electron densities greater than 10 11 cm −3 the index of refraction of the atmospheric air plasma is mainly determined by heavy particles rather than electrons. The electron contribution to the refractive index was separated from that of the heavy particles by taking the different relaxation times of the two particle species into account. With the discharge operated in a repetitive pulsed mode, the initial rapid change of the refractive index was assumed to be due to the increase in electron density, whereas the following slower rise is due to the decrease in gas density caused by gas heating. By reducing the time between pulses, direct current conditions were approached, and the electron density as well as the gas density, and gas temperature, respectively, were obtained through extrapolation. A computation inversion method was used to determine the radial distribution of the plasma parameters in the cylindrical discharge. For a direct-current filamentary discharge in air, at a current of 10 mA, the electron density was found to be 10 13 cm −3 in the centre, decreasing to half of this value at a radial distance of 0.21 mm. Gaussian temperature profiles with σ = 1.1 mm and maximum values of 1000-2000 K in the centre were also obtained with, however, larger error margins than for electron densities.

Electron density measurement in atmospheric pressure plasma jets: Stark broadening of hydrogenated and non-hydrogenated lines

“…First beam with frequency ω is passed through the discharge region where a phase shift is appeared due to a change in refractive index of the medium. After two beams merging at the interferometer, a phase shift ΔΦ between the two beams can be measured by the heterodyne technique [27,28]. The method is sensitive to change in density of both heavy particles and electrons.…”

“…Schoenbach et al [27] applied heterodyne interferometry to microhollow cathode discharge in air and estimated electron density to be about 10 13 cm −3 . The results of laser heterodyne interferometry are found to be in good agreement with other methods with a very high limit of detection 1 × 10 12 cm −3 [29].…”

Basic Studies on High Pressure Air Plasmas

“…Leipold et al [10] have measured the density in a DC discharge by periodically pulsing the discharge and progressively increasing the duty cycle to tend to a DC regime. It could be possible to modulate the current without turning off the plasma, with a period smaller that the characteristic time of neutral gas temparature variation.…”

Heterodyne Interferometer for Measurement of Electron Density in High-Pressure Plasmas

“…For the separation of two components in the phase shift of the CO 2 laser beam, difference of time constants of the two phenomena is utilized, because the change of electron density is much faster than that of gas temperature in the high-pressure discharge (Leipold et al, 2000;Choi et al, 2009). Applicability of this technique can be confirmed by experimental evidence as explained below.…”

“…Meanwhile, a heterodyne interferometer of CO 2 laser beam, which is a theme in this chapter, can be in a category of the refractive-index measurement methods and have a good spatial resolution at the same time. This interferometer detects the laser beam's phase shift caused by the presence of tested plasma and provides line-integrated information of electron density with a spatial resolution in sub mm order (Leipold et al, 2000;Choi et al, 2009). …”