Detlef Loffhagen

Particles as probes for complex plasmas in front of biased surfaces

An interesting aspect in the research of complex (dusty) plasmas is the experimental study of the interaction of micro-particles with the surrounding plasma for diagnostic purposes. Local electric fields can be determined from the behaviour of particles in the plasma, e.g. particles may serve as electrostatic probes. Since in many cases of applications in plasma technology it is of great interest to describe the electric field conditions in front of floating or biased surfaces, the confinement and behaviour of test particles is studied in front of floating walls inserted into a plasma as well as in front of additionally biased surfaces. For the latter case, the behaviour of particles in front of an adaptive electrode, which allows for an efficient confinement and manipulation of the grains, has been experimentally studied in terms of the dependence on the discharge parameters and on different bias conditions of the electrode. The effect of the partially biased surface (dc and rf) on the charged micro-particles has been investigated by particle falling experiments. In addition to the experiments, we also investigate the particle behaviour numerically by molecular dynamics, in combination with a fluid and particle-in-cell description of the plasma.

Diagnostic studies of H2?Ar?N2 microwave plasmas containing methane or methanol using tunable infrared diode laser absorption spectroscopy

Tunable infrared diode laser absorption spectroscopy has been used to detect the methyl radical and nine stable molecules, CH4, CH3OH, C2H2, C2H4, C2H6, NH3, HCN, CH2O and C2N2, in H2–Ar–N2 microwave plasmas containing up to 7% of methane or methanol, under both flowing and static conditions. The degree of dissociation of the hydrocarbon precursor molecules varied between 20% and 97%. The methyl radical concentration was found to be in the range 1012–1013 molecules cm−3. By analysing the temporal development of the molecular concentrations under static conditions it was found that HCN and NH3 are the final products of plasma chemical conversion. The fragmentation rates of methane and methanol (RF(CH4) = (2–7) × 1015 molecules J−1, RF(CH3OH) = (6–9) × 1015 molecules J−1) and the respective conversion rates to methane, hydrogen cyanide and ammonia (RCmax(CH4) = 1.2 × 1015 molecules J−1, RCmax(HCN) = 1.3 × 1015 molecules J−1, RCmax(NH3) = 1 × 1014 molecules J−1) have been determined for different hydrogen to nitrogen concentration ratios. An extensive model of the chemical reactions involved in the H2–N2–Ar–CH4 plasma has been developed. Model calculations were performed by including 22 species, 145 chemical reactions and appropriate electron impact dissociation rate coefficients. The results of the model calculations showed satisfactory agreement between calculated and measured concentrations. The most likely main chemical pathways involved in these plasmas are discussed and an appropriate reaction scheme is proposed.

Low-pressure mercury-free plasma light sources: experimental and theoretical perspectives

The replacement of mercury in conventional fluorescent lamps by other components is highly desirable for environmental reasons. This paper gives a short review of new types of mercury-free plasma light sources operating at low pressure. In particular, the features of cylindrical glow discharges in rare-gas mixtures including xenon are discussed, focusing on the generation of the 147 nm resonance radiation of xenon and its transition into visible light by new phosphors with sufficient efficiency. Laser absorption and vacuum ultraviolet emission spectroscopy are applied for several rare-gas mixtures to reveal the contributions of the different gas components and their excited states to the power balance and radiation output. The experimental research is assisted by theoretical studies applying self-consistent hybrid models of the cylindrical column plasma. The good agreement between experimental and theoretical results obtained at selected discharge conditions makes it possible to predict optimum discharge parameters by means of extensive model calculations. It is demonstrated that about half of the efficacy of a mercury fluorescent lamp can be reached if the rare-gas discharge is operated at pressures below 100 Pa.

Fluid modelling of CO2 dissociation in a dielectric barrier discharge

The dissociation of CO2 in a geometrically symmetric dielectric barrier discharge has been analysed by means of numerical modelling. A time- and space-dependent fluid model has been used, taking into account the spatial variation of the plasma between the plane-parallel dielectrics covering the electrodes. The main features of the model, including an extensive reaction kinetics for the vibrational states of CO2, are given. The modelling studies have been performed for different applied voltages, discharge frequencies, pressures, gas temperatures, and relative permittivities of the dielectrics. The model calculations show that the discharges in the positive and negative half-cycles are different for the considered standard condition, leading to a spatially asymmetric distribution of the stable neutrals like CO molecules and O atoms. The generation of CO mainly takes place during the discharge pulses, and it is dominated by electron impact dissociation. The specific energy input obtained for the broad range of parameters considered and determined for residence times reported in the literature agrees well with the corresponding experimental values. In accordance with these experiments, the calculated degree of CO2 conversion has been found to increase almost linearly with the specific energy input. Remaining discrepancies between the measured and calculated energy efficiencies are discussed.

Comparison of kinetic calculation techniques for the analysis of electron swarm transport at low to moderate E/N values

We present a comparison between results for the electron velocity distribution function (evdf), and transport and rate coefficients of an electron swarm obtained under different assumptions for the space and angular dependence of the evdf. Several solution techniques for the Boltzmann equation as well as Monte Carlo simulations have been tested. The comparison is made in neon at a constant and homogeneous reduced electric field in the range 10 Td ≤ E/N ≤ 500 Td taking into account the production of electrons in ionizing collisions. The results show that to obtain an accurate description of the electron swarm we need to take into account the variation in space of the electron density in the representation of the evdf. In what regards the angular dependence on velocity we discuss criteria to estimate the importance of the anisotropy of the evdf for any gas. Depending on the solution technique and on the E/N value, we find good to excellent agreement between the Boltzmann results obtained with a half-range method, a multi-term Legendre expansion, an elliptic approximation and the Monte Carlo results. The accuracy of the transport and rate coefficients obtained with each approach is evaluated and it is found that although the two-term velocity expansion is not sufficiently accurate to be used for cross section fitting, the corresponding rate and transport coefficients can generally be used in discharge modelling.

Breakdown characteristics in pulsed-driven dielectric barrier discharges: influence of the pre-breakdown phase due to volume memory effects

The pre-phase of the breakdown of pulsed-driven dielectric barrier discharges (DBDs) was investigated by fast optical and electrical measurements on double-sided DBDs with a 1 mm gap in a gas mixture of 0.1 vol% O2 in N2 at atmospheric pressure. Depending on the pulse width (the pause time between subsequent DBDs), four different breakdown regimes of the following discharge were observed. By systematically reducing the pulse width, the breakdown characteristics could be changed from a single cathode-directed propagation (positive streamer) to simultaneous cathode- and anode-directed propagations (positive and negative streamer) and no propagation at all for sub-μs pulse times. For all cases, different spatio-temporal emission structures in the pre-phase were observed. The experimental results were compared with time-dependent, spatially one-dimensional fluid model calculations. The modelling results confirmed that different pre-ionisation conditions, i.e. considerably high space charges in the volume created by the residual electrons and ions from the previous discharge, are the reason for the observed phenomena.

Study of the electron kinetics in the anode region of a glow discharge by a multiterm approach and Monte Carlo simulations

A recently developed method for the solution of the space-dependent electron Boltzmann equation in higher-order accuracy has been adopted to study the behaviour of the electrons in the anode region of a dc glow discharge. This method is based upon a multiterm approximation of the Legendre polynomial expansion of the electron velocity distribution function. Generalizing the boundary conditions, in particular for the partially absorbing anode, the impact of the anode fall and the influence of the electron absorption at the anode on the spatial behaviour of the electron kinetic properties have been investigated in various approximation orders. The analysis has shown that the simplified treatment of the kinetic equation using only the first two terms of the velocity distribution expansion can lead to considerable falsifications of the convergent behaviour. In general, the convergent solution of the significant components of the electron velocity distribution and all important macroscopic quantities is obtained by a multiterm approximation including six to eight terms of that expansion. The discrepancies between the two-term and convergent results are found to depend sensitively on the parameters of the anode fall. In addition, the multiterm results are compared with corresponding ones obtained by accurate Monte Carlo simulations. Very good agreement between the convergent eight-term Boltzmann and Monte Carlo simulations is found.

Breakdown characteristics of high pressure xenon lamps

An investigation of the breakdown of high intensity discharge (HID) lamps filled with xenon at pressures from 0.1 to 5 bar is presented. Three power supplies were used in order to provide voltage rates of increase covering about four orders of magnitude from 5 mV ns−1 to 100 V ns−1, the latter being typical for electronic ballasts driving commercial HID lamps. Customized lamps ensure a volume breakdown between the tungsten tip electrodes of the lamp. Voltage and current waveforms were measured by means of electrical probes and the transient optical radiation was captured by a fast camera system. The breakdown voltage increases with growing pressure and voltage rate up to several 10 kV. Additional UV illumination decreases the breakdown voltage and reduces its mean variation. The experimental results were reproduced with good agreement by a fluid model taking into account the electron energy balance. The model shows an ionization front propagating towards the cathode. The front moves due to continuous field compression and relies on electron avalanches initiated by secondary electrons at the cathode.

The electron kinetics in the cathode region of H2/Ar/N2 discharges

Theoretical and experimental investigations of the cathode region of dc discharges in H2/Ar/N2 mixtures are presented as a first step to a deeper understanding of the TiN deposition in such mixture plasmas containing in addition TiCl4. A recently developed multiterm technique for the solution of the space-dependent Boltzmann equation of the electrons has been generalized and adapted to analyse the behaviour of the velocity distribution function and significant macroscopic quantities of the electrons. The basics of this theoretical treatment are presented. On the basis of the calculated results, the main features of the kinetics of the electrons in the cathode region are illustrated and comprehensively discussed. In particular, the analysis of the results shows extreme alterations of the properties of the electron gas in the transition from the cathode fall to the negative glow, caused by intensive collisional dissipation mainly by the H2 component. Probe diagnostics have been used to measure the electron velocity distribution function and related macroscopic quantities in the negative glow and the Faraday dark space of the discharges. The experimental results are discussed and compared with corresponding results of model calculations. The agreement between the measured and calculated results is satisfactory for the velocity distribution function and good for the mean electron energy. The remaining discrepancies between the results are critically evaluated with regard to various possible reasons.

A stabilized finite element method for modeling of gas discharges

Abstract An efficient stabilized finite element method for modeling of gas discharge plasmas is represented which provides wiggle-free solutions without introducing much artificial diffusion. The stabilization is achieved by modifying the standard Galerkin test functions by means of a weighted quadratic term that results in a consistent Petrov–Galerkin formulation of the charge carriers in the plasma. Using the example of a glow discharge plasma in argon, it is shown that this efficient method provides more accurate results on the same spatial grid than the widely used finite difference approach proposed by Scharfetter–Gummel if the weighting factor is determined in dependence on the local Peclet number and the modified test functions are consistently applied to all terms of the governing equations.