van Afh Bram Gessel

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.

Laser scattering on an atmospheric pressure plasma jet: disentangling Rayleigh, Raman and Thomson scattering

Laser scattering provides a very direct method for measuring the local densities and temperatures inside a plasma. We present new experimental results of laser scattering on an argon atmospheric pressure microwave plasma jet operating in an air environment. The plasma is very small so a high spatial resolution is required to study the effect of the penetration of air molecules into the plasma. The scattering signal has three overlapping contributions: Rayleigh scattering from heavy particles, Thomson scattering from free electrons and Raman scattering from molecules. The Rayleigh scattering signal is filtered out optically with a triple grating spectrometer. The disentanglement of the Thomson and Raman signals is done with a newly designed fitting method. With a single measurement we determine profiles of the electron temperature, electron density, gas temperature, partial air pressure and the N2/O2 ratio, with a spatial resolution of 50 µm, and including absolute calibration. (Some figures may appear in colour only in the online journal)

NO production in an RF plasma jet at atmospheric pressure

A time modulated RF atmospheric pressure plasma jet, operated in ambient air with a flow of argon with a few per cent of air, N2 or O2, was characterized by measuring the gas temperature with Rayleigh scattering, the absolute NO density with laser-induced fluorescence, and the emission of NO A and N2 C with time resolved optical emission spectroscopy. The gas temperature, NO density and the emission measurements are carried out both time and spatially resolved. The atmospheric pressure plasma jet has the advantage that the plasma dissipated power can be measured, and it was found that the gas temperature depends on the power, rather than the gas mixture. The NO density increases with increasing plasma power, and was found to have a maximum around 1.5 × 1021 m−3 at an air admixture of 2%. The N2 C emission is modulated by the 13.9 MHz RF frequency, while the NO A emission front increases with much slower velocity during the 20 kHz duty cycle, which gives an insight into the excitation mechanisms in the plasma. Through the addition of either N2 or O2 to the plasma it was experimentally confirmed that the production of atomic N radicals are of key importance for the NO production in this atmospheric pressure plasma jet.

Nitric oxide density distributions in the effluent of an RF argon APPJ: effect of gas flow rate and substrate

The effluent of an RF argon atmospheric pressure plasma jet, the so-called kinpen, is investigated with focus on the nitric-oxide (NO) distribution for laminar and turbulent flow regimes. An additional dry air gas curtain is applied around the plasma effluent to prevent interaction with the ambient humid air. By means of laser-induced fluorescence (LIF) the absolute spatially resolved NO density is measured as well as the rotational temperature and the air concentration. While in the laminar case, the transport of NO is attributed to thermal diffusion; in the turbulent case, turbulent mixing is responsible for air diffusion. Additionally, measurements with a molecular beam mass-spectrometer (MBMS) absolutely calibrated for NO are performed and compared with the LIF measurements. Discrepancies are explained by the contribution of the NO2 and N O 2 to the MBMS NO signal. Finally, the effect of a conductive substrate in front of the plasma jet on the spatial distribution of NO and air diffusion is also investigated.

Atomic oxygen TALIF measurements in an atmospheric-pressure microwave plasma jet with in situ xenon calibration

Two-photon absorption laser-induced fluorescence (TALIF) is used to measure the absolute density of atomic oxygen (O) in a coaxial microwave jet in ambient air at atmospheric pressure, operated with a mixture of He and a few per cent of air. The TALIF signal is calibrated using a gas mixture containing Xe. A novel method to perform calibration in situ, at atmospheric pressure, is introduced. The branching ratios of several Xe mixtures are reported, to enable us to perform the Xe calibration without the need for a vacuum vessel. The O densities are measured spatially resolved, and as a function of admixed air to the He, and microwave power. The electron density and temperatures are measured using Thomson scattering, and the N2 and O2 densities are measured using Raman scattering. O densities are found to have a maximum of (4?6)???1022?m?3, which indicate that O2 is close to fully dissociated in the plasma. This is confirmed by the Raman scattering measurements. O is found to recombine mainly into species other than O2 in the afterglow, which is suggested to consist of O3 and oxidized components of NO.

The effect of collisional quenching of the O 3p 3 P J state on the determination of the spatial distribution of the atomic oxygen density in an APPJ operating in ambient air by TALIF

The spatial profile of the absolute atomic oxygen density is obtained by two-photon absorption laser-induced fluorescence (TALIF) in an Ar+2% air cold atmospheric pressure plasma jet (APPJ) operating in ambient air. The varying air concentration in the jet effluent which contributes to the collisional quenching of the O 3p 3PJ state, pumped by the laser, strongly influences the recorded TALIF signal under the present experimental conditions. The spatially resolved air densities obtained from Raman scattering measurements have been reported in our previous work (van Gessel et al 2013 Appl. Phys. Lett. 103 064103). These densities allow us to calculate the spatially dependent collisional quenching rate for the O 3p 3PJ state and reconstruct the spatial O density profile from the recorded TALIF signal. Significant differences between the TALIF intensity profile and the actual O density profile for the investigated experimental conditions are found.

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.

Temperature fitting of partially resolved rotational spectra

In this paper we present a method to automatically fit the temperature of a rotational spectrum. It is shown that this fitting method yields similar results as the traditional Boltzmann plot, but is applicable in situations where lines of the spectrum overlap. The method is demonstrated on rotational spectra of nitric oxide from an atmospheric pressure microwave plasma jet operated with a flow of helium and air, obtained with two different methods: laser induced fluorescence and optical emission spectroscopy. Axial profiles of the rotational temperatures are presented for the ground NO X state and the excited NO A state.

Simultaneous Thomson and Raman Scattering on an Atmospheric-Pressure Plasma Jet

In this paper, laser scattering is applied to a cold atmospheric-pressure microwave plasma argon jet in direct contact with air. Spatially resolved measurements clearly show the air entrainment in the plasma jet. Consequently, the contributions from Thomson scattering and Raman scattering (N2 and O2) overlap. With a specially designed fitting method, we are able to obtain ne and Te, in spite of the significant Raman contribution.

Thermalization of rotational states of NO A 2Σ +(v = 0) in an atmospheric pressure plasma

Laser induced fluorescence (LIF) measurements of nitric oxide (NO) are performed in an atmospheric pressure microwave plasma jet, operated with a mixture of He and 3% air. The fluorescence signal of NO A(2)Σ(+)(v = 0) is measured time and fluorescence wavelength resolved. Based on the evolution of the rotational spectrum at different positions in the plasma, we determined the thermalization time of the rotational distribution of NO A after pumping a single transition, at temperatures in the range 300-1500 K. Also, a LIF-RET (rotational energy transfer) model is developed to simulate the RET and to calculate the thermalization time. The RET rate coefficients are calculated using the energy corrected sudden-exponential power scaling law. It was found that it is necessary to take the fine structure of the rotational states into account. At room temperature the results of the measurement and the simulation are consistent, and the thermalization occurs during the laser pulse (11 ± 1 ns). At elevated temperatures the measurements show a large increase in thermalization time, up to 35 ± 4 ns at 1474 K. This time is much longer than the laser pulse, and of the order of the NO A lifetime. This means that for spectroscopy measurements of the rotational states of NO A, the RET has to be taken into account to derive gas temperatures from the rotational distribution of NO A.