Edgar Felizardo

Microwave air plasma source at atmospheric pressure: Experiment and theory

An experimental and theoretical investigation of the axial structure of a surface wave (2.45 GHz) driven atmospheric plasma source in air with a small admixture (1%) of water vapor has been performed. Measurements of the gas temperature and of the intensities of the O(777.4 nm), O(844.6 nm), and O(630 nm) atomic lines and the NO(γ) molecular band versus input power and axial position were carried out. Amplitude and phase sensitive measurements have also been performed to derive the surface wave dispersion characteristics. The experimental results are analyzed in terms of a one-dimensional theoretical model based on a self-consistent treatment of particle kinetics, gas dynamics, and wave electrodynamics. The predicted gas temperature and emission line intensities variations with power and axial position are shown to compare well with experiment. “Hot” excited O atoms (with kinetic energy ∼2 eV) have been detected.

Microwave plasma source operating with atmospheric pressure air-water mixtures

The overall performance of a surface wave driven air-water plasma source operating at atmospheric pressure and 2.45 GHz has been analyzed. A 1D model previously developed has been improved in order to describe in detail the creation and loss processes of active species of interest. This model provides a complete characterization of the axial structure of the source, including the discharge and the afterglow zones. The main electron creation channel was found to be the associative ionization process N + O → NO+ + e. The NO(X) relative density in the afterglow plasma jet ranges from 1.2% to 1.6% depending on power and water percentage, according to the model predictions and the measurements. Other types of species such as NO2 and nitrous acid HNO2 have also been detected by mass and Fourier Transform Infrared spectroscopy. The relative population density of O(3P) ground state atoms increases from 8% to 10% in the discharge zone when the input microwave power increases from 200 to 400 W and the water percent...

Spectroscopic investigation of wave driven microwave plasmas

The study of the emission of “mixed gas” plasmas reveals many surprising results, especially when hydrogen is one of the gases. Anomalous, even extreme, hydrogen line broadening was found in a number of mixed discharge plasmas excited via direct current dc or radio frequency rf electric fields. 1–5 The Balmer line spectra emitted by these discharges have typical multimode behavior, with widely broadened “wings” “fast” hydrogen and a sharp top “slow” hydrogen. The results have usually been explained in terms of Doppler shift and broadening due to the acceleration of charges such as H + ,H 2 , and H3 ions in the high dc electric fields present in the sheath regions of these discharges. The acceleration of hydrogen ions in these dc fields is followed by neutralization and generation of fast excited H atoms. This is the origin of the “wings” in the spectra. Strikingly excessive Balmer- line broadening has been observed in He– H210% microwave discharge, 6 but this has not yet been confirmed by other research groups. On the contrary, measurements of H line profiles emitted by microwave discharges under similar conditions have not revealed excessive broadening. 7–10 Nevertheless, selective hydrogen line broadening has been detected in microwave discharges and their afterglows when there is no significant broadening of noble gas lines or hydrogen molecular lines. 8,9,11 A possible explanation for such selective heating of H atoms may be connected with the main creation processes of excited H atoms, namely, ion conversion and electron impact dissociation. Furthermore, hyperthermal hydrogen atoms have surprisingly been detected at atmospheric pressure Ar– H2 microplasma jets, where the Hn =3 temperatures were found to range from 12,000 to 19,600 K. 12 It is now clear that hydrogen line broadening causes controversy so that more experimental observations are currently needed in order to try to elucidate the mechanisms and processes behind this phenomenon in different types of discharges. The aim of this experimental work is to address some of these problems. This article presents spectroscopic measurements in He– H2 and Ar– H2 low-pressure plasmas generated by a surface wave of frequency / 2 = 2.45 GHz. Results on the line shape and the emission intensities of excited hydrogen and helium atoms, and the emission intensities of the Q-branch of the Fulcher- band d 3 uv =1 →a 3 g v =1 are presented and discussed. Different temperatures are determined from the measured hydrogen and helium emission line shapes and the rotational distribution of hydrogen molecular lines. Furthermore, the population distribution of excited H atoms is determined from measurements of the relative intensities of transitions within the Balmer series.

Hot and super-hot hydrogen atoms in microwave plasma

“Super-hot” (kinetic energy ∼4–8 eV) and “hot” (kinetic energy ∼0.3 eV) H atoms were detected in a surface wave (500 MHz) generated H2 plasma column, at pressure p=0.01 mbar, from the analysis of the Hβ, Hγ, Hδ, and He emission line profiles. These profiles were found to evolve from single Gaussian to bi-Gaussian toward the plasma column end. Population inversion between the levels 5→4 and 6→4 was detected. At pressure p=0.2 mbar, super-hot atoms were not detected and the temperature of the hot atoms was found to increase with the upper level principal quantum number.

Vacuum ultraviolet emission from microwave Ar-H2 plasmas

Vacuum ultraviolet emission from Ar-H2 wave driven microwave (2.45 GHz) plasmas operating at low pressures (0.1–1 mbar) has been investigated. The emitted spectra show the presence of the Ar resonance lines at 104.8 and 106.7 nm and of the Lyman-α,β atomic lines at 121.6 nm and 102.6 nm, respectively. The increase of the hydrogen amount in the mixture results in an abrupt increase of the Werner and Lyman molecular bands intensity. The Lyman-β intensity shows little changes in the range of 5%–30% of hydrogen in the mixture while the Lyman-α intensity tends to decrease as the percentage of hydrogen increases.

Extreme ultraviolet radiation emitted by helium microwave driven plasmas

The extreme ultraviolet radiation emitted by helium microwave-driven (2.45 GHz) plasmas operating at low-pressure conditions was investigated. Novel data regarding emitted spectral lines of excited helium atoms and ions in the 20–33 nm wavelength range and their intensity behavior with variation of discharge operational conditions are presented. The intensity of all the spectral emissions was found to strongly increase with the microwave power delivered to the plasma. Furthermore, the intensity of the ionic spectral emissions decreases by nearly one order of magnitude as the pressure was raised from 0.2 to 0.5 mbar.

Energetic hydrogen atoms in wave driven discharges

Doppler broadened Hγ emission was detected in high frequency (350 and 500 MHz) hydrogen surface wave sustained discharges, revealing the presence of fast excited H atoms with kinetic energies in the range 4–9 eV. Spatially resolved measurements of the Doppler-broadened emission indicate that these fast atoms are predominantly formed near the wall, which suggests that their generation may result from acceleration of H+ ions in the radial dc space charge field followed by recombination at the wall and the return of the neutral atom to the gas phase.

Vacuum ultraviolet emission from hydrogen microwave plasmas driven by surface waves

The vacuum ultraviolet (VUV) radiation emitted by hydrogen surface-wave-driven plasmas operating at microwave frequency (2.45 GHz) and low-pressure conditions (0.1–2 mbar) was investigated, in particular the influence of microwave power and gas pressure on the intensity of the emissions. The strong emission of Lyman H2 and Werner H2 molecular bands in the 80–125 nm spectral range was detected, while the most intense atomic emissions observed correspond to Lyman-α and Lyman-β lines at 121.6 nm and 102.6 nm respectively. An increase of the atomic lines and molecular bands intensities with increasing microwave power at pressure 0.1 mbar was observed. At 2 mbar the VUV spectra are entirely dominated by molecular bands. Theoretical predictions, as obtained from a collisional-radiative model, were validated by the experimental results.

Energetic Hydrogen Atoms in High Frequency Plasmas

Generation of energetic hydrogen atoms, with energy in the range 4-8 eV, was detected throughout the volume of a surface wave generated (500 MHz) plasma column in H2 at pressure p = 0.01 mbar. The Hβ, Hγ, Hδ, and H, line profiles were found to be bi-Gaussian towards the plasma column end. The kinetic temperatures corresponding to the Doppler broadening of the Hβ, Hγ, Hδ, lines are higher than the rotational temperature of the hydrogen molecular Fulcher-α band and the wall temperature. At pressure p = 0.2 mbar, the kinetic temperature of excited H (n = 4-7) atoms, as determined from the fitting of the spectral lines with a single-Gaussian profile, increases with upper level principal quantum number. The experimental results have been analyzed in the framework of a global self-consistent kinetic model describing this surface wave sustained plasma column.