T Tiny Verreycken

Power dissipation, gas temperatures and electron densities of cold atmospheric pressure helium and argon RF plasma jets

A set of diagnostic methods to obtain the plasma parameters including power dissipation, gas temperature and electron density is evaluated for an atmospheric pressure helium or argon radio frequency (RF) plasma needle for biomedical applications operated in open air. The power density of the plasma is more or less constant and equal to 1.3 ? 109?W?m?3. Different methods are investigated and evaluated to obtain the gas temperature. In this paper the gas temperatures obtained by rotational spectra of OH(A?X) and (B?X) are compared with Rayleigh scattering measurements and measurements of the line broadening of hydrogen and helium emission lines. The obtained gas temperature ranges from 300 to 650?K, depending on the gas. The electron densities are estimated from the Stark broadening of the hydrogen ? and ? lines which yield values between 1019 and 1020?m?3. In the case of helium, this is an overestimate as is shown by a power balance from the measured power density in the plasma jet. The obtained plasma parameters enable us to explain the radial contraction of the argon plasma compared with the more diffuse helium plasma. The accuracy of all considered diagnostics is discussed in detail.

Optical emission spectroscopy as a diagnostic for plasmas in liquids: opportunities and pitfalls

In this contribution, optical emission spectroscopy is evaluated and thoroughly analysed as a diagnostic to characterize plasmas in and in contact with liquids. One of the specific properties of plasmas in and in contact with liquids is the strong emission of OH(A?X) and of hydrogen lines. As an example a 600?ns pulsed dc excited discharge in Ar, He and O2 bubbles in water is investigated by time resolved optical emission spectroscopy. It is shown that the production processes of excited species and the plasma kinetics strongly influence the emission spectrum. This complicates the interpretation of the spectra but provides the opportunity to derive production mechanisms from the time resolved emission. The importance of recombination processes compared with direct electron excitation processes in the production of excited states of the water fragments in plasmas with high electron densities is shown. The OH(A?X) emission spectrum illustrates that even in these highly collisional atmospheric pressure discharges the rotational population distribution deviates from equilibrium. A two-temperature fit of the OH rotational population distribution leads to realistic gas temperatures for the temperature parameter corresponding to small rotational numbers. The H? and H? lines are fitted with two component profiles corresponding to two different electron densities. The obtained electron density is in the range 1021?1023?m?3. Possible complications in the interpretation of obtained temperatures and electron densities are discussed.

Time and spatially resolved LIF of OH in a plasma filament in atmospheric pressure He–H2O

The production of OH in a nanosecond pulsed filamentary discharge generated in pin–pin geometry in a He–H2O mixture is studied by time and spatially resolved laser-induced fluorescence. Apart from the OH density the gas temperature and the electron density are also measured. Depending on the applied voltage the discharge is in a different mode. The maximum electron densities in the low- (1.3 kV) and high-density (5 kV) modes are 2 × 1021 m−3 and 7 × 1022 m−3, respectively. The gas temperature in both modes does not exceed 600 K. In the low-density mode the maximum OH density is at the centre of the discharge filament, while in the high-density mode the largest OH density is observed on the edge of the discharge. A chemical model is used to obtain an estimate of the absolute OH density. The chemical model also shows that charge exchange and dissociative recombination can explain the production of OH in the case of the high-density mode.

Spectroscopic study of an atmospheric pressure dc glow discharge with a water electrode in atomic and molecular gases

The positive column of dc excited atmospheric pressure glow discharges in a metal pin–water electrode system is investigated in air, N2, He, Ar, N2O, CO2 and He–N2 mixtures. The electric field and the plasma temperatures in the positive column are measured and the effect of the filling gas on the optical emission is examined. Estimates of the electron temperature and density and the observed emission properties indicate that the formation processes of the excited dissociation fragments of H2O are due to recombination processes rather than direct electron excitation. Deviation from a Boltzmann rotational population distribution of OH(A) is observed in all gases due to the formation process of OH(A). In the case of He the non-Boltzmann behaviour is more pronounced which can be caused by the lower gas temperature as a higher gas temperature leads to more thermalization of the rotational population distribution of OH(A). The results in this paper suggest that vibrational energy transfer contributes to the non-Boltzmann distribution of OH(A). The temperature corresponding to the small rotational numbers of OH(A) can be used as an estimate of the gas temperature.

Absolute OH density measurements in the effluent of a cold atmospheric-pressure Ar–H2O RF plasma jet in air

Absolute OH densities are obtained in a radio-frequency-driven Ar–H2O atmospheric-pressure plasma jet by laser-induced fluorescence (LIF), calibrated by Rayleigh scattering and by UV broadband absorption. The measurements are carried out in ambient air and the effect of air entrainment into the Ar jet is measured by analyzing the time-resolved fluorescence signals. The OH densities are obtained for different water vapor concentrations admixed to the Ar and as a function of the axial distance from the nozzle. A sensitivity analysis to deduce the accuracy of the model-calculated OH density from the LIF measurement is reported. It is found that the UV absorption and the LIF results correspond within experimental accuracy close to the nozzle and deviate in the far effluent. The possible reasons are discussed. The OH densities found in the plasma jet are in the range (0.1–2.5) × 1021 m−3 depending on the water concentration and plasma conditions.

Time-resolved optical emission spectroscopy of nanosecond pulsed discharges in atmospheric pressure N2 and N2/H2O mixtures

In this contribution, nanosecond pulsed discharges in N2 and N2/0.9% H2O at atmospheric pressure (at 300?K) are studied with time-resolved imaging, optical emission spectroscopy and Rayleigh scattering. A 170?ns high-voltage pulse is applied across two pin-shaped electrodes at a frequency of 1?kHz. The discharge consists of three phases: an ignition phase, a spark phase and a recombination phase. During the ignition phase the emission is mainly caused by molecular nitrogen (N2(C?B)). In the spark and recombination phase mainly atomic nitrogen emission is observed. The emission when H2O is added is very similar, except the small contribution of H? and the intensity of the molecular N2(C?B) emission is less.The gas temperature during the ignition phase is about 350 K, during the discharge the gas temperature increases and is 1??s after ignition equal to 750?K. The electron density is obtained by the broadening of the N emission line at 746?nm and, if water is added, the H? line. The electron density reaches densities up to 4???1024?m?3. Addition of water has no significant influence on the gas temperature and electron density.The diagnostics used in this study are described in detail and the validity of different techniques is compared with previously reported results of other groups.

Absolute calibration of OH density in a nanosecond pulsed plasma filament in atmospheric pressure He–H2O: comparison of independent calibration methods

The absolute density of OH radicals generated in a nanosecond pulsed filamentary discharge in atmospheric pressure He +0.84% H2O is measured independently by UV absorption and laser induced fluorescence (LIF) calibrated with Rayleigh scattering. For the calibration of LIF with Rayleigh scattering, two LIF models, with six levels and four levels, are studied to investigate the influence of the rotational and vibrational energy transfers. In addition, a chemical model is used to deduce the OH density in the afterglow from the relative LIF intensity as function of time. The different models show good correspondence and by comparing these different methods, the accuracy and the effect of assumptions on the obtained OH density are discussed in detail. This analysis includes an analysis of the sensitivity to parameters used in the LIF models.

Anode pattern formation in atmospheric pressure air glow discharges with water anode

Pattern formation in the anode layer at a water electrode in atmospheric pressure glow discharges in air is studied. With increasing current a sequence of different anode spot structures occurs from a constricted homogeneous spot in the case of small currents to a pattern consisting of small distinct spots for larger currents. The dependence of the number of spots on the current is (18.5+0.4N) mA with N the number of spots. The spots group together and form stripe patterns. Disappearance of the spots with increasing conductivity of the electrode seems to indicate that mechanisms of electrical nature govern the phenomenon. Similarities and differences with other experiments and models revealing pattern formation in glow discharges are discussed.

Validation of gas temperature measurements by OES in an atmospheric air glow discharge with water electrode using Rayleigh scattering

Rayleigh scattering is used to determine the gas temperature of an atmospheric pressure dc excited glow discharge in air with a water electrode. The obtained temperatures are compared with calculated rotational temperatures measured by optical emission spectroscopy of OH(A–X) and N2(C–B). At a current of 15 mA a deviation is found between Trot(OH) and the gas temperature obtained from Rayleigh scattering of about 1000 K. The gas temperatures obtained from Rayleigh scattering, N2(C) and OH(A) in the positive column are, respectively, 2600 ± 100 K, 2700 ± 150 K and 3600 ± 200 K. It is shown that the rotational temperature of N2(C) is a reliable measurement of the gas temperature while this is not the case for OH(A). The results are explained in the context of quenching processes of the excited states. Spatially resolved gas temperatures in both longitudinal and radial directions are presented. The observed strong temperature gradients near the electrodes are checked to be consistent with the power dissipation and the heat transfer in the discharge. The effect of the polarity of the water electrode and filamentation on the measured temperatures is discussed.

OH density measurements in nanosecond pulsed discharges in atmospheric pressure N2-H2O mixtures

Time and spatially resolved OH densities are determined in a nanosecond pulsed filamentary discharge in N2–H2O for varying water concentration using laser-induced fluorescence (LIF). The OH densities are absolutely calibrated using Rayleigh scattering. A dip in the LIF signal is found at the centre of the filament. In the case of 2.6% H2O a remarkable increase in OH density is found in the far afterglow and is discussed in analogy with the pink splash or pink afterglow. It is believed that the large amount of energy released in the recombination of atomic nitrogen plays an important role in the production of OH in the far afterglow.