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Comprehensive study of the discharge mode transition in inductively coupled radio frequency plasmas
(2016)
In this contribution, the mode transition of an inductively coupled radio frequency plasma at low pressure is investigated. Therefore, a comprehensive set of plasma diagnostics were applied to determine plasma and processing parameters. Therewith, the plasma kinetics and especially the important elementary processes were studied. Hence, the reason for the mode transition was identified.
This thesis highlights the impact of surface charges and negative ions on the pre-ionization, breakdown mechanism, and lateral structure of dielectric barrier discharges operated in binary mixtures of helium with nitrogen or electronegative oxygen. Sophisticated diagnostic methods, e.g., non-invasive optical emission spectroscopy and the electro-optic Pockels effect as well as invasive laser photodetachment and laser photodesorption, were applied at one plane-parallel discharge configuration to investigate both relevant volume and surface processes. Moreover, the experimental findings were supported by numerical fluid simulations of the discharge. For the first time, the memory effect of the measured surface charge distribution was quantified and its impact on the local self-stabilization of discharge filaments was pointed out. As well, it turned out that a few additional seed electrons, either desorbed from the charged dielectric surface or detached from negative ions in the volume, significantly contribute to the pre-ionization resulting in a reduced voltage necessary for discharge breakdown. Finally, effective secondary electron emission coefficients of different dielectrics were estimated from the measured breakdown voltage using an analytical model.
The thesis describes experimental results based on optical diagnostics of low- pressure discharges. The models, which are necessary for the interpretation of the experimental data, are developed and simulations are done. The contents can be categorized into the following topics: 1) the time-resolved tunable diode laser absorption spectroscopy of excited states of argon in pulsed magnetron discharge and modeling the plasma afterglow; 2) optical emission- and laser absorption spectroscopy of excited states of argon in radio-frequency (rf) discharge and calculation of the escape factor for self-absorption; 3) fast video recording of the oscillatory motion of a dust particle in rf discharge and analysis of the data.
Magnetic reconnection is a ubiquitous phenomenon observed in a wide range of magnetized plasmas from magnetic confinement fusion devices to space plasmas in the magnetotail. The process enables the release of accumulated magnetic energy by rapid changes in magnetic topology, heating the plasma in the vicinity of the reconnection site, generating fast particles and allowing a wealth of instabilities to grow. This thesis reports on the results from a newly constructed linear, cylindrical and modular guide field reconnection experiment with highly reproducible events, VINETA.II. A detailed analysis of the reconnecting current sheet properties on a macroscopic and microscopic scale in time and space is presented. In the experiment, four parallel axial wires create a figure-eight in-plane magnetic field with an X-line along the central axis, as well as an axial inductive field that drives magnetic reconnection. Particle-in-cell simulations show that the axial current is limited by sheaths at the boundaries and that electrostatic fields along the device axis always set up in response to the induced electric field. Current sheet formation requires an additional electron current source, realized as a plasma gun, which discharges into a homogeneous background plasma created by a rf antenna. The evolution of the plasma current is found to be dominantly set by its electrical circuit. The current response to the applied electric field is mainly inductive, which in turn strongly influences the reconnection rate. The three-dimensional distribution of the current sheet is determined by the magnetic mapping of the plasma gun along the sheared magnetic field lines, as well as by radial cross-field expansion. This expansion is due to a lack of equilibrium in the in-plane force balance. Resistive diffusion of the magnetic field by E=η j is found to be by far insufficient to account for the high reconnection rate E=-dΨ/dt at the X-line, indicating the presence of large electrostatic fields which do not contribute to dissipative reconnection. High-frequency magnetic fluctuations are observed throughout the current sheet which are compared to qualitatively similar observations in the Magnetic Reconnection Experiment (MRX, Princeton). The turbulent fluctuation spectra in both experiments display a spectral kink near the lower hybrid frequency, indicating the presence of lower hybrid type instabilities. In contrast to the expected perpendicular propagation of mainly electrostatic waves, an electromagnetic wave is found in VINETA.II that propagates along the guide field and matches the whistler wave dispersion. Good correlation is observed between the local axial current density and the fluctuation amplitude across the azimuthal plane. Instabilities driven by parallel drifts can be excluded due to the large required drift velocities or low resulting phase velocities that are not observed. It is instead suggested that a perpendicular, electrostatic lower hybrid mode indeed exists that resonantly excites a parallel, electromagnetic whistler wave through linear mode conversion. The resulting fluctuations are found to be intrinsic to the localized current sheet and are independent of the slower reconnection dynamics. Their amplitude is small compared to the in-plane fields, and have a negligible contribution to anomalous resistivity through momentum transport in the present parameter regime.
The present thesis deals with dynamic structures that form during the expansion of plasma into an environment of much lower plasma density. The electron expansion, driven by their pressure, occurs on a much faster time scale than the ion expansion, owed to their mobility. The high inertia of the ions causes the generation of an ambipolar electric field that decelerates the escaping electrons while accelerating the ions. The ambipolar boundary propagates outwards and forms a plasma density front. For a small density differences, the propagation of the front can be described with the linear ansatz for ion acoustic waves. For a large density differences, experiments have shown that the propagation velocity of such a front is still related to the ion sound velocity. However, the reported proportionality factors are scattered over a wide range of values, depending on the considered initial and boundary conditions. In this thesis, the dynamics during plasma expansion are studied with the use of experiments and a versatile particle-in-cell simulation. The experimental investigations are performed in the linear helicon device Piglet. The experiment features a fast valve, which is used to shape the neutral gas density profile. During the pulsed rf-discharges, plasma is generated in the source region and expands collisionless into the expansion chamber. The computer simulation is tailored very close to the experiment and provides a deeper insight in the particle kinetics. The experimental results show the existence of a propagating ion front. Its velocity is typically supersonic and depends on the density ratio of the two plasmas. The ion front features a strong electric field. The front can have similar properties to a double layer is not necessarily a double layer by definition. The computer simulation reveals that the propagating electric field repels the downstream ambient ions. These ions form a stream with velocities up to twice as high as the front velocity. The observed ion density peak is due to the accumulation of the repelled ions and is located at their turning point. The ion front formation depends strongly on the initial ion density profile and is part of a wave-breaking phenomenon. The observed front is followed by a plateau of little plasma density variation. This could be confirmed for the expansion experiment by a comparison with virtual diagnostics in the computer simulation. The plateau has a plasma density determined by the ratio between the high and low plasma density. It consists of streaming ions that have been accelerated in the edge of the main plasma. The presented results confirm and extend findings obtained by independent numerical models and simulations.
This thesis describes how the data of the Langmuir probes in the Wendelstein 7-X (W7X) Test Divertor Unit (TDU) were evaluated, checked for consistency with other diagnostics and used to analyse plasma detachment.
Langmuir probes are an electronic diagnostic, and were among the first to be used in plasma physics to determine particle fluxes, potentials, temperatures and densities.
W7X is a large, advanced stellarator, magnetic confinement fusion experiment, operated at the Max-Planck-Institut for Plasma Physics(IPP) in Greifswald, Germany.
Its TDU is an uncooled graphite component, shaped and positioned to intercept the convective heat load of the plasma.
Detachment describes a desirable operation state of strongly reduced loads on this component.
The evaluation of Langmuir probe data relies heavily on models of the sheath, formed at the interface between plasma and a solid surface, to infer plasma parameters from the directly measured quantities.
Multiple such models are analysed, generalised, and adapted to our use case.
A detailed comparison is made to determine the most suitable model, as this choice strongly affects the predicted parameters.
Special attention is paid to uncertainties on the parameters, which are determined using a Bayesian framework.
From the inferred parameters, heat and particle fluxes are calculated.
These are also indirectly measured by two other, camera-based diagnostic systems.
Observations are compared to test the validity of assumptions and calculations in the evaluation of all three diagnostics by checking their results for consistency.
The first comparison, with the infrared emission camera system, shows good agreement with theoretical predictions and reported measurements of the sheath transmission factor, for which we derive and measure a value in W7X.
Parameter dependencies in the quality of this agreement hint at remaining issues.
The second comparison, with the Hydrogen alpha photon flux camera system, shows significant discrepancy with expectations.
These are argued to originate from systematic differences in the measurement locations, which are quantified and related to the magnetic topology.
Langmuir probe observations of individual discharges are analysed to discuss conditions under which detachment occurs, transition into that state and fluctuations observed prior to and during it.
A spatial parametrisation of the data is developed and used to facilitate this.
These observations contribute to the larger aim of understanding particle balance control and fusion plasma edge processes.
The active screen plasma nitrocarburizing (ASPNC) technology is a state-of-the-art plasma-assisted heat treatment for improving surface hardness and wear resistance of metallic workpieces based on thermochemical diffusion. In comparison to conventional plasma nitrocarburizing, the use of an active screen (AS) improves thermal homogeinity at the workload and reduces soot formation. Further it can serve as a chemical source for the plasma processes, e.g. by use of an AS made of carbon-fibre reinforced carbon. This compilation of studies investigates the plasma-chemical composition of industrial- and laboratory-scale ASPNC plasmas, predominantly using in-situ laser absorption spectroscopy with lead-salt tuneable diode lasers, external-cavity quantum cascade lasers, and a frequency comb. In this way the temperatures and concentrations of the dominant stable molecular species HCN, NH3, CH4, C2H2, and CO, as well as of less prevelant species, were recorded as functions of e.g. the pressure, the applied plasma power, the total feed gas flow and its composition. Additionally, the diagnostics were applied to a chemically similar plasma-assisted process for diamond deposition.
Resulting from this thesis are new insights into the practical application of an AS made of CFC, the plasma-chemistry involving hydrogen, nitrogen, and carbon, and the particular role of CO as an indicator for reactor contamination. The effect of the feed gas composition on the resulting nitrogen- and carbon-expanded austenite layers was proven by combination of in-situ laser absorption spectroscopy with post-treatment surface diagnostics. Furthermore this work marks the first use of frequency comb spectroscopy with sub-nominally resolved Michelson interferometry for investigation of a low-pressure molecular discharge. This way the rotational bands of multiple species were simultaneously measured, resulting in temperature information at a precision hitherto not reached in the field of nitrocarburizing plasmas.
The absolute density of the metastable N2(A,v=0) molecule was extensively studied in nitrogen barrier discharges at 500 mbar. For the detection of the metastables laser-induced fluorescence spectroscopy (LIF) was used, at which for the calibration of the absoute metastables density a comparison with Rayleigh scattering was performed. To get the ratio of the LIF signal to the Rayleigh signal it is shown that the LIF signal is the convolution of the Rayleigh signal with an exponential decay. Besides, the different cross sections are calculated and the ratio of the detection sensitivities at the laser and fluorescence wavelength is determined. As a first step on the way to atmospheric pressure barrier discharges, the laser-induced fluorescence spectroscopy was implemented in low pressure capacitively coupled radio-frequency discharges. The determined metastables density in the capacitively coupled radio-frequency discharge is somewhat below 10^12 cm^(-3) at 40 Pa and somewhat below 10^13 cm^(-3) at 1000 Pa. The axial density profiles show a nearly symmetric shape due to the long lifetime of the metastable state. At a pressure of 500 mbar the two discharge modes of the barrier discharge, the filamentary and the diffuse mode, were analysed. The filamentary mode was mainly investigated in an asymmetric discharge configuration. Typical densities in the detection volume are in the range of 10^13 cm^(-3), resulting in maximal densities of up to 10^15 cm^(-3) in the microdischarge channel. Such large densities are in agreement with the fast decay by the pooling reaction after the maximum of the metastables density in the afterglow of the discharge pulse. The time dependent measurements in the afterglow of single microdischarges offer a delay of the metastables production with respect to the discharge current. This delay indicates that the metastables production takes place mostly by cascades from higher triplet states, which are in turn excited by electron impact. The axial density profiles show a maximum in metastables density in front of the anode in agreement with optical emission spectroscopy, but which cannot be clearly identified because of the asymmetric discharge configuration. The measurements for the diffuse discharge mode were performed in a symmetric discharge configuration. The metastables density is in the range of 10^13 cm^(-3). It increases during the current pulse of the discharge and decays afterwards. The maximum of the metastables density is delayed with respect to the maximum of the discharge current. The depletion of metastables in the early discharge afterglow is dominated by the pooling reaction, afterwards quenching by nitrogen atoms becomes important assuming a nitrogen atom density in the order of 10^14 cm^(-3). As for the filamentary mode, the losses by diffusion are negligible for the measurement positions. The measured axial density profiles show an accumulation of metastables in front of the anode, whereas the density in front of the cathode is below the detection limit. To calculate the metastables current density to the dielectrics after the discharge pulse a simulation is developed including the dominant volume processes for the depletion of metastables and the axial diffusion. Starting point for the simulation is the axial metastables density distribution at the end of the discharge pulse. The calculated metastables current density at the dielectrics is in the range of 10^14 cm^(-2)s^(-1). With the use of recently calculated secondary electron emission coefficients a comparison of the secondary electron emission by metastables with the discharge current is done. It is figured out that the secondary electron emission current is large enough to be important during the discharge ignition. To expand the simulation to the whole voltage cycle, the excitation of metastables is assumed to be proportional to the discharge current and electron density. Using this model, the measured time dependences of the metastables density are well reproduced for the investigated parameter variations. This is not the case for the axial profiles, where a metastables loss process is missed to explain the formation of a density plateau in front of the anode during the discharge pulse. The intended calculation of the metastables current density shows that the delay of the metastables production with respect to the discharge current might be responsible for the ignition of microdischarges at the beginning of the discharge pulse.
This thesis is devoted to experiments on three-dimensional dust clouds which are confined in low temperature plasmas. Such ensembles of highly electrically charged micrometer-sized particles reveal fascinating physics, such as self-excited density waves and vortices. At the same time, these systems are challenging for experimental approaches due to their three-dimensional character. In this thesis, new optical diagnostics for dusty plasmas have been developed and, in combination with existing techniques, have been used to study these 3D dusty plasmas on different size and time scales.
In the last decade a new domain has developed in plasma physics: plasma medicine. Despite the successes that have already been achieved in this exciting new field, the interaction of plasmas with “biological materials” is not yet fully understood. Further investigations in particular with respect to the properties of the applied plasmas sources are therefore essential in order to decode this complex interaction process. Currently, a great variety of different discharge types are used in plasma medical investigation which are generally are operated in noble gases like helium and argon or with dry air. In the present work, the main focuses is on the diagnostics of reactive oxygen and nitrogen species (RONS) resulting from the plasma chemistry of an argon radio-frequency (RF) atmospheric pressure plasma jet (APPJ) and its interaction with the ambient atmosphere. To conduct this study, a commercially available plasma device, so-called kinpen is used due to its technical development maturity and its accessibility on the market. As a method of choice, diagnostic techniques are based on optical spectroscopy known to be a reliable tool to investigate plasmas. Consequently, three complementary optical laser diagnostics, namely quantum cascade laser absorption spectroscopy (QCLAS), laser induced fluorescence (LIF) and planar single shot LIF (PLIF), have been successfully applied to the plasma jet itself or its effluent. All of these diagnostics offer a high species selectivity and an excellent spatial and temporal resolution. They are used in this work for i) the characterization of the plasma chemical dynamics with respect to the generation of biological active RONS – in particular for the case of N2 and O2 admixtures. ii) the measurement of the NO density profile in the plasma effluent iii) the investigation of the flow characteristics of the neutral gas component (laminar vs. turbulent) and its influence on the plasma chemistry. Numerical analysis have been carried out in collaboration with PLASMANT (University of Antwerp) via kinetic simulations of the entire plasma chemistry. Expectingly, atomic oxygen (O) and nitric oxide (NO) turn out to be precursors of ozone (O3) and nitric dioxide (NO2). However, it was intriguing to unveil that atomic oxygen and nitrogen metastable (N2(A)) play together a key part --as intermediate species-- in the generation of more stable RONS, e.g. NO. The absolute density of NO space resolved was measured by LIF and absolutely calibrated molecular beam mass spectrometer. LIF was used to determine relative density of OH radical in the plasma plume. 2D-LIF was used to investigate the gas flow pattern with OH as a flow tracer. The results are discussed in details and show different operating mode of the jet, e.g. laminar or turbulent and that the plasma influences these regimes. The first detection and relative measurement by LIF of nitrogen metastable (N2(A)) produced by an argon APPJ is also shortly reported in this work. The outcome of this thesis will bring new insights in the field of argon APPJs chemistry and its interaction with the ambient atmosphere which can be valuable to support plasma modelling and to consider for the applications in plasma medicine.