Volker Schulz-von der Gathen

Absolute atomic oxygen density profiles in the discharge core of a microscale atmospheric pressure plasma jet

The micro atmospheric pressure plasma jet is an rf driven (13.56 MHz, ∼20 W) capacitively coupled discharge producing a homogeneous plasma at ambient pressure when fed with a gas flow of helium (1.4 slm) containing small admixtures of oxygen (∼0.5%). The design provides excellent optical access to the plasma core. Ground state atomic oxygen densities up to 3×1016 cm−3 are measured spatially resolved in the discharge core by absolutely calibrated two-photon absorption laser-induced fluorescence spectroscopy. The atomic oxygen density builds up over the first 8 mm of the discharge channel before saturating at a maximum level. The absolute value increases linearly with applied power.

Gas flow dependence of ground state atomic oxygen in plasma needle discharge at atmospheric pressure

We present clear evidence that ground state atomic oxygen shows two patterns near a surface in the helium plasma needle discharge. Two-photon absorption laser-induced fluorescence spectroscopy, combined with gas flow simulation, was employed to obtain spatially-resolved ground state atomic oxygen densities. When the feed gas flow rate is low, the radial density peaks along the axis of the needle. At high flow rate, a ring-shaped density distribution appears. The peak density is on the order of 1021 m−3 in both cases. The results are consistent with a previous report of the flow-dependent bacterial killing pattern observed under similar conditions.

Investigation of Surface Etching of Poly(Ether Ether Ketone) by Atmospheric-Pressure Plasmas

An atmospheric-pressure argon plasma jet with varying admixtures of molecular oxygen was used to study the etching mechanism of poly(ether ether ketone) (PEEK). Furthermore, a correlation between plasma-based etching processes on PEEK with the generation of chemically reactive plasma species is proposed. The surface analysis was performed by X-ray photoelectron spectroscopy, atomic force microscopy, and surface profilometry which showed a dramatic increase in the content of oxygen functionalities and surface roughness after long-time Ar/O2-plasma treatment. For the plasma diagnostics, two-photon absorption laser-induced fluorescence spectroscopy was applied. The obtained etching mass as well as the surface roughness for different molecular oxygen admixtures revealed a strong dependence on the atomic-oxygen density. Furthermore, the radial surface profile, affected by plasma etching, might be attributed to the distribution of plasma-generated oxygen species in the plasma jet effluent.

Atomic oxygen dynamics in an air dielectric barrier discharge: a combined diagnostic and modeling approach

Cold atmospheric pressure plasmas are a promising alternative therapy for treatment of chronic wounds, as they have already shown in clinical trials. In this study an air dielectric barrier discharge (DBD) developed for therapeutic use in dermatology is characterized with respect to the plasma produced reactive oxygen species, namely atomic oxygen and ozone, which are known to be of great importance to wound healing. To understand the plasma chemistry of the applied DBD, xenon-calibrated two-photon laser-induced fluorescence spectroscopy and optical absorption spectroscopy are applied. The measured spatial distributions are shown and compared to each other. A model of the afterglow chemistry based on optical emission spectroscopy is developed to cross-check the measurement results and obtain insight into the dynamics of the considered reactive oxygen species. The atomic oxygen density is found to be located mostly between the electrodes with a maximum density of cm. Time resolved measurements reveal a constant atomic oxygen density between two high voltage pulses. The ozone is measured up to 3?mm outside the active plasma volume, reaching a maximum value of cm between the electrodes.

Diagnostic-based modeling on a micro-scale atmospheric-pressure plasma jet

Diagnostic-based modeling (DBM) actively combines complementary advantages of numerical plasma simulations and relatively simple optical emission spectroscopy (OES). DBM is applied to determine spatial absolute atomic oxygen ground-state density profiles in a micro atmospheric-pressure plasma jet operated in He–O2. A 1D fluid model with semi-kinetic treatment of the electrons yields detailed information on the electron dynamics and the corresponding spatio-temporal electron energy distribution function. Benchmarking this time- and space-resolved simulation with phase-resolved OES (PROES) allows subsequent derivation of effective excitation rates as the basis for DBM. The population dynamics of the upper O(3p3P) oxygen state (λ = 844 nm) 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) state (λ = 750.4 nm). The resulting spatial profile for the absolute atomic oxygen density shows an excellent quantitative agreement to a density profile obtained by two-photon absorption laser-induced fluorescence spectroscopy.

Description of HiPIMS plasma regimes in terms of composition, spoke formation and deposition rate

The behaviour of Cu and Cr HiPIMS (high power impulse magnetron sputtering) discharges was investigated by a combination of optical emission spectroscopy, energy-resolved mass spectrometry and optical imaging, for the complete current–voltage characteristic range achievable within our experimental conditions. Inflection points typical of HiPIMS current–voltage characteristics separate plasma regimes perfectly differentiated in terms of flux composition of species towards the substrate, deposition rate, and the nature of plasma self-organization. The reorganization of the HiPIMS plasma into spokes (areas of high ionization over the target) is associated to one regime of high plasma conductivity, where also deposition rate is limited. This spoke-dominated regime can be substituted by a homogeneous regime at higher powers, where there is an increase of deposition rate, which is driven mostly by an increase in the flux of metal neutrals and metal double-charged ions. The relevance of secondary electron emission mechanisms for the support of the spoke-dominated regime in reactive and non-reactive sputtering conditions is discussed.

Pressure broadening of atomic oxygen two-photon absorption laser induced fluorescence

Atomic oxygen, considered to be a determining reactant in plasma applications at ambient pressure, is routinely detected by two-photon absorption laser induced fluorescence (TALIF). Here, pressure broadening of the (2p 4 3 P 2 → 3p 3 P J=0,1,2) two-photon transition in oxygen atoms was investigated using a high-resolution TALIF technique in normal and Doppler-free configurations. The pressure broadening coefficients determined were = 0.40 ± 0.08 cm−1/bar for oxygen molecules and = 0.46 ± 0.03 cm−1/bar for helium atoms. These correspond to pressure broadening rate constants = 9 10–9 cm3 s−1 and = 4 10−9 cm3 s−1, respectively. The well-known quenching rate constants of O(3p 3 P J ) by O2 and He are at least one order of magnitude smaller, which signifies that non-quenching collisions constitute the main line-broadening mechanism. In addition to providing new insights into collisional processes of oxygen atoms in electronically excited 3p 3 P J state, reported pressure broadening parameters are important for quantification of oxygen TALIF line profiles when both collisional and Doppler broadening mechanisms are important. Thus, the Doppler component (and hence the temperature of oxygen atoms) can be accurately determined from high resolution TALIF measurements in a broad range of conditions.

Helium metastable density evolution in a self-pulsing μ-APPJ

Space- and time-resolved helium metastable densities (3S1) have been measured in a radio-frequency driven self-pulsing atmospheric pressure microplasma jet (SP μ-APPJ) using tuneable diode laser absorption spectroscopy. Density maps of metastable atoms have been deduced for different times during a self-pulsing cycle with a time resolution of 20 µs, revealing the metastable dynamics in the discharge. The plasma exhibits a bright propagating constricted discharge during every pulse, co-existing with a homogeneous glow-mode (α-mode). The profiles show significantly increased metastable densities over the whole electrode gap in the region of the constricted discharge. In the sheath region, densities reach the order of 1013 cm−3. These densities are three orders of magnitude higher than the densities that were measured in the homogeneous glow-mode. Time-resolved measurements show that the increased metastable density propagates with the constricted discharge from the gas inlet to the nozzle. Between two pulses the metastable density drops down to the level of the glow-mode. Decay times of the metastables in the order of 100 µs have been measured. The propagation velocity of the constricted discharge has been determined from the movement of the metastable maximum.

Enhanced oxygen dissociation in a propagating constricted discharge formed in a self-pulsing atmospheric pressure microplasma jet

We report on the propagation of a constricted discharge feature in a repetitively self-pulsing microplasma jet operated in helium with a 0.075?vol% molecular oxygen admixture in ambient air environment. The constricted discharge is about 1?mm in width and repetitively ignites at the point of smallest electrode distance in a wedge-shaped electrode configuration, propagates through the discharge channel towards the nozzle, extinguishes, and re-ignites at the inlet at frequencies in the kHz range. It co-exists with a homogeneous, volume-dominated low temperature (T???300?K) ?-mode glow. Time-resolved measurements of nitrogen molecule C-state and nitrogen molecule ion B-state emission bands reveal an increase of the rotational temperature within the constricted discharge to about 600?K within 50??s. Its propagation velocity was determined by phase-resolved diagnostics to be similar to the gas velocity, in the order of 40?m?s?1. Two-photon absorption laser-induced fluorescence spectroscopy synchronized to the self-pulsing reveals spatial regions of increased oxygen atom densities co-propagating with the constricted discharge feature. The generated oxygen pulse density is about ten times higher than in the co-existing homogeneous ?-mode. Densities reach about 1.5???1016?cm?3 at average temperatures of 450?K at the nozzle. This enhanced dissociation of about 80% is attributed to the continuous interaction of the constricted discharge to the co-propagating gas volume.

Ionization wave propagation on a micro cavity plasma array

The simulation was performed using the computer modeling platform nonPDPSIM, described in detail in Refs. 10–12 and briefly discussed here. Poisson’s equation for the electrostatic potential is self-consistently coupled with driftdiffusion equations for the transport of charged species and the surface charge balance equation. The set of equations is simultaneously integrated in time using an implicit Newton iteration technique. This integration step is followed by an implicit update of the electron temperature by solving the electron energy equation. To capture the non-Maxwellian behavior of the electrons, the electron transport coefficients and rate coefficients are obtained by solving the zerodimensional Boltzmann’s equation for the electron energy distribution. A Monte Carlo simulation is used to track the trajectories of sheath accelerated secondary electrons. The transport of photons is treated by means of a Green’s function propagator. The discharge is sustained in argon at atmospheric pressure. The species in the model are electrons, Ar(3s), Ar(4s), Ar(4p), Ar þ ,A r �, and Ar þ . The photon transport we tracked in the model is dimer radiation from Ar � .I n