V Schulz-von der Gathen

Absolute atomic oxygen density measurements by two-photon absorption laser-induced fluorescence spectroscopy in an RF-excited atmospheric pressure plasma jet

The atmospheric pressure plasma jet is a capacitively coupled radio frequency discharge (13.56 MHz) running with a high helium flux (2 m3 h−1) between concentric electrodes. Small amounts (0.5%) of admixed molecular oxygen do not disturb the homogeneous plasma discharge. The jet effluent leaving the discharge through the ring-shaped nozzle contains high concentrations of radicals at a low gas temperature—the key property for a variety of applications aiming at treatment of thermally sensitive surfaces. We report on absolute atomic oxygen density measurements by two-photon absorption laser-induced fluorescence (TALIF) spectroscopy in the jet effluent. Calibration is performed with the aid of a comparative TALIF measurement with xenon. An excitation scheme (different from the one earlier published) providing spectral matching of both the two-photon resonances and the fluorescence transitions is applied.

Absolute calibration of atomic density measurements by laser-induced fluorescence spectroscopy with two-photon excitation

The two-photon resonances of atomic hydrogen (λ = 2×205.1 nm), atomic nitrogen (λ = 2×206.6 nm) and atomic oxygen (λ = 2×225.6 nm) are investigated together with two selected transitions in krypton (λ = 2×204.2 nm) and xenon (λ = 2×225.5 nm). The natural lifetimes of the excited states, quenching coefficients for the most important collisions partners, and the relevant ratios of the two-photon excitation cross sections are measured. These data can be applied to provide a calibration for two-photon laser-induced fluorescence measurements based on comparisons with spectrally neighbouring noble gas resonances.

Atomic oxygen formation in a radio-frequency driven micro-atmospheric pressure plasma jet

Atomic oxygen formation in a radio-frequency driven micro-atmospheric pressure plasma jet is investigated using both advanced optical diagnostics and numerical simulations of the dynamic plasma chemistry. Laser spectroscopic measurements of absolute densities of ground state atomic oxygen reveal steep gradients at the interface between the plasma core and the effluent region. Spatial profiles resolving the interelectrode gap within the core plasma indicate that volume processes dominate over surface reactions. Details of the production and destruction processes are investigated in numerical simulations benchmarked by phase-resolved optical emission spectroscopy. The main production mechanisms are electron induced and hence most efficient in the vicinity of the plasma boundary sheath, where electrons are energized. The destruction is driven through chemical heavy particle reactions. The resulting spatial profile of atomic oxygen is relatively flat. The power dependence of the atomic oxygen density obtained by the numerical simulation is in very good agreement with the laser spectroscopic measurements.

Characterization of the effluent of a He/O2 microscale atmospheric pressure plasma jet by quantitative molecular beam mass spectrometry

The effluent of a microscale atmospheric pressure plasma jet (μ-APPJ) operated in helium with a small admixture of molecular oxygen (<1.6%) has been analyzed by means of two independent diagnostics, quantitative molecular beam mass spectrometry (MBMS) and two-photon absorption laser-induced fluorescence spectroscopy (TALIF). The atomic oxygen density, the ozone density and the depletion of molecular oxygen have been measured by MBMS and the atomic oxygen density has been validated by TALIF. Absolute atomic oxygen densities in the effluent up to 4.7×1015 cm-3 could be measured with a very good agreement between both diagnostics. In addition, ozone densities in the effluent up to 1.4×1015 cm-3 and an O2 depletion up to 10% could be measured by MBMS. The atomic oxygen density shows a maximum value at an O2 admixture of 0.6%, whereas the ozone density continues to increase toward higher O2 admixtures. With increasing distance from the jet, the atomic oxygen density decreases but is still detectable at a distance of 30 mm. The ozone density increases with distance, saturating at a distance of 40 mm. By applying higher powers to the μ-APPJ, the atomic oxygen density increases linearly whereas the ozone density exhibits a maximum.

Discharge comparison of nonequilibrium atmospheric pressure Ar/O2 and He/O2 plasma jets

A plasma jet has been developed that operates using rf power and produces a stable homogeneous discharge at atmospheric pressure. Discharge characteristics for the gas mixture of He/O2 and Ar/O2 were studied. The temperature distribution at the exit of the nozzle of Ar/O2 discharge is about 100 K higher than that for the He/O2 discharge—identified by an increased temperature—extends much farther downstream for Ar/O2 discharge in contrast to the He/O2 discharge—250 mm as compared to 75 mm. Optical emission spectra for both discharges in the jet effluent shows only fractions of the peak intensities of Ar, He, and O atomic lines compared with spectra taken looking obliquely inside the jet volume. The spatial profile of excited atomic oxygen at λ=777 nm (4S0 3p–3s) taken side on to the flow direction outside the nozzle shows good correlation to the temperature distribution for both discharges. These results indicate that the Ar/O2 discharge has better energy transfer efficiency than the He/O2 discharge. Study...

Absolute atomic oxygen density distributions in the effluent of a microscale atmospheric pressure plasma jet

The coplanar microscale atmospheric pressure plasma jet (μ-APPJ) is a capacitively coupled radio frequency discharge (13.56 MHz, ~15 W rf power) designed for optimized optical diagnostic access. It is operated in a homogeneous glow mode with a noble gas flow (1.4 slm He) containing a small admixture of molecular oxygen (~0.5%). Ground state atomic oxygen densities in the effluent up to 2 × 1014 cm−3 are measured by two-photon absorption laser-induced fluorescence spectroscopy (TALIF) providing space resolved density maps. The quantitative calibration of the TALIF setup is performed by comparative measurements with xenon. A maximum of the atomic oxygen density is observed for 0.6% molecular oxygen admixture. Furthermore, an increase in the rf power up to about 15 W (depending on gas flow and mixture) leads to an increase in the effluents atomic oxygen density, then reaching a constant level for higher powers.

Atomic oxygen in a cold argon plasma jet: TALIF spectroscopy in ambient air with modelling and measurements of ambient species diffusion

By investigating the atomic oxygen density in its effluent, two-photon absorption laser-induced fluorescence (TALIF) spectroscopy measurements are for the first time performed in a cold argon/oxygen atmospheric pressure plasma jet. The measurements are carried out in ambient air and quenching by inflowing air species is considered. We propose a novel absorption technique in the VUV spectral range, where emission originating from within the discharge is used as light source to determine the inflow of atmospheric oxygen into the effluent. Furthermore, we propose a modelling solution for the on-axis density of inflowing ambient air based on the stationary convection?diffusion equation.

Spatially resolved diagnostics on a microscale atmospheric pressure plasma jet

Despite enormous potential for technological applications, fundamentals of stable non-equilibrium micro-plasmas at ambient pressure are still only partly understood. Micro-plasma jets are one sub-group of these plasma sources. For an understanding it is particularly important to analyse transport phenomena of energy and particles within and between the core and effluent of the discharge. The complexity of the problem requires the combination and correlation of various highly sophisticated diagnostics yielding different information with an extremely high temporal and spatial resolution. A specially designed rf microscale atmospheric pressure plasma jet (μ-APPJ) provides excellent access for optical diagnostics to the discharge volume and the effluent region. This allows detailed investigations of the discharge dynamics and energy transport mechanisms from the discharge to the effluent. Here we present examples for diagnostics applicable to different regions and combine the results. The diagnostics applied are optical emission spectroscopy (OES) in the visible and ultraviolet and two-photon absorption laser-induced fluorescence spectroscopy. By the latter spatially resolved absolutely calibrated density maps of atomic oxygen have been determined for the effluent. OES yields an insight into energy transport mechanisms from the core into the effluent. The first results of spatially and phase-resolved OES measurements of the discharge dynamics of the core are presented.

High power impulse magnetron sputtering discharges: Instabilities and plasma self-organization

We report on instabilities in high power impulse magnetron sputtering plasmas which are likely to be of the generalized drift wave type. They are characterized by well defined regions of high and low plasma emissivity along the racetrack of the magnetron and cause periodic shifts in floating potential. The azimuthal mode number m depends on plasma current, plasma density, and gas pressure. The structures rotate in E→×B→ direction at velocities of ∼10 km s−1 and frequencies up to 200 kHz. Collisions with residual gas atoms slow down the rotating wave, whereas increasing ionization degree of the gas and plasma conductivity speeds it up.

Diagnostic based modelling of radio-frequency driven atmospheric pressure plasmas

Diagnostic based modelling (DBM) actively combines complementary advantages of numerical plasma simulations and relatively simple optical emission spectroscopy (OES). DBM is employed to determine absolute atomic oxygen ground state densities in a helium?oxygen radio-frequency driven atmospheric pressure plasma jet. A comparatively simple one-dimensional simulation yields detailed information on electron properties governing the population dynamics of excited states. Important characteristics of the electron dynamics are found to be largely insensitive to details of the chemical composition and to be in very good agreement with space and phase-resolved OES. Benchmarking the time and space resolved simulation allows us to subsequently derive effective excitation rates as the basis for DBM with simple space and time integrated OES. The population dynamics of the upper O 3p?3P (? = 844?nm) atomic oxygen state is governed by direct electron impact excitation, dissociative excitation, radiation losses and collisional induced quenching. Absolute values for atomic oxygen densities are obtained through tracer comparison with the upper Ar 2p1 (? = 750.4?nm) state. The presented results for the atomic oxygen density show excellent quantitative agreement with independent two-photon laser-induced fluorescence measurements.