Eddie M. van Veldhuizen

Spatially resolved ozone densities and gas temperatures in a time modulated RF driven atmospheric pressure plasma jet : an analysis of the production and destruction mechanisms

In this work, a time modulated RF driven DBD-like atmospheric pressure plasma jet in Ar?+?2%O2, operating at a time averaged power of 6.5?W is investigated. Spatially resolved ozone densities and gas temperatures are obtained by UV absorption and Rayleigh scattering, respectively. Significant gas heating in the core of the plasma up to 700?K is found and at the position of this increased gas temperature a depletion of the ozone density is found. The production and destruction reactions of O3 in the jet effluent as a function of the distance from the nozzle are obtained from a zero-dimensional chemical kinetics model in plug flow mode which considers relevant air chemistry due to air entrainment in the jet fluent. A comparison of the measurements and the models show that the depletion of O3 in the core of the plasma is mainly caused by an enhanced destruction of O3 due to a large atomic oxygen density.

A spectroscopic method to determine the electron temperature of an argon surface wave sustained plasmas using a collision radiative model

A method is presented to determine the electron temperature in a low pressure argon plasma using emission spectroscopic measurements and a collisional radiative (CR) model. Absolute line intensity measurements are made in order to construct the atomic state distribution function. In addition to the excited states, the ground state density is also taken into account. Because of this, the excitation temperature can be determined with high precision. A CR-model has been used to determine the degree of equilibrium departure and to obtain the relationship between the excitation temperature and the electron temperature. This method is applied to a microwave plasma which has been generated inside a quartz tube using a surfatron device. The densities of argon levels close to the continuum are used to get an estimated value of the electron density. These values are used as input data for the CR-model. For an argon pressure of 6 mbar, the 4p level densities vary between 8 × 1014 and 6 × 1015 m−3. Using the estimated values for the electron density, between 2 × 1019 and 3 × 1019 m−3, the electron temperature was found to range between 1.15 and 1.20 eV. An extensive error analysis showed that the relative error in the electron temperature is less than 6%.

Spatio-temporal dynamics of a pulsed microwave argon plasma: ignition and afterglow

In this paper, a detailed investigation of the spatio-temporal dynamics of a pulsed microwave plasma is presented. The plasma is ignited inside a dielectric tube in a repetitively pulsed regime at pressures ranging from 1 up to 100 mbar with pulse repetition frequencies from 200 Hz up to 500 kHz. Various diagnostic techniques are employed to obtain the main plasma parameters both spatially and with high temporal resolution. Thomson scattering is used to obtain the electron density and mean electron energy at fixed positions in the dielectric tube. The temporal evolution of the two resonant and two metastable argon 4s states are measured by laser diode absorption spectroscopy. Nanosecond time-resolved imaging of the discharge allows us to follow the spatio-temporal evolution of the discharge with high temporal and spatial resolution. Finally, the temporal evolution of argon 4p and higher states is measured by optical emission spectroscopy. The combination of these various diagnostics techniques gives deeper insight on the plasma dynamics during pulsed microwave plasma operation from low to high pressure regimes. The effects of the pulse repetition frequency on the plasma ignition dynamics are discussed and the plasma-off time is found to be the relevant parameter for the observed ignition modes. Depending on the delay between two plasma pulses, the dynamics of the ionization front are found to be changing dramatically. This is also reflected in the dynamics of the electron density and temperature and argon line emission from the plasma. On the other hand, the (quasi) steady state properties of the plasma are found to depend only weakly on the pulse repetition frequency and the afterglow kinetics present an uniform spatio-temporal behavior. However, compared to continuous operation, the time-averaged metastable and resonant state 4s densities are found to be significantly larger around a few kHz pulsing frequency.

Laser-induced fluorescence spectroscopy for phenol and intermediate products in aqueous solutions degraded by pulsed corona discharges above water

Laser-induced fluorescence (LIF) spectroscopy is introduced as an in situ diagnostic for phenol and intermediate products in an aqueous solution degraded by corona discharges. The complications that are inherent in applying LIF as a diagnostic for aqueous solutions are experimentally examined. The LIF intensities of phenol and the intermediate products are measured as a function of time. The absolute phenol concentration is determined. We confirm the applicability of LIF spectroscopy for monitoring phenol concentration during degradation.

Electron impact transfer rates between metastable and resonant states of argon investigated by laser pump-probe technique

The laser pump-probe technique is used to study the electron impact transfer between the 1s5 and 1s4 states of argon (in Paschens notation) belonging to the 2P3/2 ion core for electron temperatures in the range of 1-2 eV. A rate coefficient of m3s-1 is determined for the transfer from 1s5 to 1s4 state. Different pumping schemes between the 1s and 2p states but also 3p states are used to verify the obtained value but also to probe the transfers with ion-core change toward the 2P1/2 ion-core. Our results show the presence of an important transfer channel between 1s2 and 1s4 states, and a rate coefficient of m3 s-1 is estimated for this transfer. The present results confirm that recent quantum mechanical calculations by Zatsarinny et al [1] underestimate significantly the cross sections for electron impact transfers between 1s states of argon.

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.

Characteristics of a novel nanosecond DBD microplasma reactor for flow applications

We present a novel microplasma flow reactor using a dielectric barrier discharge (DBD) driven by repetitive nanosecond high-voltage pulses. Our DBD-based geometry can generate a non-thermal plasma discharge at atmospheric pressure and below in a regular pattern of micro-channels. This reactor can work continuously up to about 100 min in air, depending on the pulse repetition rate and operating pressure. We here present the geometry and main characteristics of the reactor. Pulse energies of 1.46 and 1.3 μJ per channel at atmospheric pressure and 50 mbar, respectively, have been determined by time-resolved measurements of current and voltage. Time-resolved optical emission spectroscopy measurements have been performed to calculate the relative species concentrations and temperatures (vibrational and rotational) of the discharge. The effects of the operating pressure and flow velocity on the discharge intensity have been investigated. In addition, the effective reduced electric field strength has been obtained from the intensity ratio of vibronic emission bands of molecular nitrogen at different operating pressures and different locations. The derived increases gradually from about 550 to 4600 Td when decreasing the pressure from 1 bar to 100 mbar. Below 100 mbar, further pressure reduction results in a significant increase in up to about 10000 Td at 50 mbar.

Time and spatially resolved OH dynamics in a nanosecond pulsed filamentary discharge in atmospheric pressure he-h2o

Summary form only given. Atmospheric pressure plasmas are investigated more and more in view of environmental en biomedical applications. One of the active species that are important for applications such as air cleaning is the OH radical1. This radical is ubiquitous in the water containing air plasmas and is very efficient due to its high oxidation potential.