Open Access

In this work, spatial distributions for reactive stable and transient species that are involved in the reaction cycle of H2O2, a key species for biomedical applications, were determined directly in the effluent of a kINPen-sci plasma jet. The small diameter of cold atmospheric pressure plasma jets and their operation at atmospheric pressure that causes strong quenching reactions make diagnostics challenging. Here, various diagnostic techniques have been employed and adapted for the use in the effluent of a cold atmospheric pressure plasma jet, which were laser atomic absorption spectroscopy (LAAS) at 811.5 nm for the detection of Ar(3P2), picosecond two-photon absorption laser-induced fluorescence spectroscopy (ps-TALIF) at 225 nm and 205 nm for the detection of O and H atoms, respectively, and continuous wave cavity ring-down spectroscopy (cw-CRDS) at 1.506 µm for the detection of HO2, and cw-CRDS at 8000 µm for the detection of H2O2. All these methods provide absolute number densities. In this work, spatial distributions within the small diameter of the effluent of a CAPJ were obtained, which have not been reported so far literature. In order to overcome the line-of-sight limitations of CRDS, radial scans were performed and transformed into a spatial distribution by using Abel inversion. Based on the determined spatial density distributions for H atoms, O atoms, HO2 radicals, and H2O2 molecules, together with the investigated impact of humidity in the feed gas on the excitation dynamics and the production of Ar(3P2), and finally on a comparison of the experimental results to a plasma chemical and reacting flow model, three different zones with varying reaction kinetics were identified. The densities close to the nozzle of the kINPen-sci plasma jet were dominated by reactions within the plasma zone including the dissociation of H2O added to the Ar feed gas and O2 that was presumably transferred into the plasma zone by counter-propagating ionisation waves. Notably, also the larger molecules, such as HO2 and H2O2 were mainly formed within the plasma zone of the plasma jet. Between 1.5 mm and 5 mm below the nozzle, the atomic species and molecular radicals generated in the plasma zone were consumed by chemical reactions with the surrounding gas, whose composition was controlled by applying a gas curtain. At further distances from the nozzle, where typically biological samples are positioned, only H2O2 and HO2 were observed. With this work, it is successfully demonstrated that even for the small diameters of cold atmospheric pressure plasma jets the determination of spatial profiles for reactive transient and stable species is possible within the effluent. By combining the experimental results, important insights into the formation and consumption of H2O2 and its precursors were gained, which are essential for the understanding of use of plasmas in biomedical applications.

In this thesis, charge transport at the plasma-wall interface is investigated theoretically, on a semiclassical, microscopic level. Based on the Boltzmann and Poisson equations a set of equations is derived and numerically solved to model charge carriers both within a semiconducting wall and a gaseous plasma in front of it. While the plasma is considered collision-free, within the solid, phonon collisions, as well as recombination processes between conduction band electrons and valence band holes are considered. This results, for the first time, in a self-consistent modeling of both the gaseous electron-ion plasma and the electron-hole plasma in the solid on the same footing. Utilizing specific approximations for different physical scenarios, numerical solutions are presented both for the floating and the electronically contacted (biased) interface. In the latter case, the current voltage characteristic is calculated and shown to heavily depend on the charge kinetics within the wall. Furthermore, we present optical methods to measure the wall charge noninvasively. These utilize the influence of the deposited surplus charges on the optical reflection coefficient of the surface. By calculating the optical response of these charges, we show that the magnitude of the surface charge can be inferred from the change in the reflectivity of the surface caused by the presence of the plasma. While nonlocal effects are considered, it is shown analytically and numerically that these can be neglected at the scales of the considered physical systems.

The layer-by-layer method is a robust way of surface functionalization using a wide range of materials, e.g. synthetic and natural polyelectrolytes (PEs), proteins and nanoparticles. Thus, this method yields films with applications in diverse areas including biology and medicine. Sequential adsorption of different oppositely charged macromolecules can be used to prepare tailored films with controlled molecular organization. In biomedical research, electrically conductive coatings are of interest. In manuscript 1, we investigated films sequentially assembled from the polycation poly (diallyldimethyl-ammonium) (PDADMA) and modified carbon nanotubes (CNTs), with CNTs serving as the electrically conductive material. We assume that charge transport occurs through CNT contacts. We showed that with more than four CNT/PDADMA bilayers, the electrical conductivity is constant and independent of the number of CNT/PDADMA bilayers. A conductivity up to 4∙10^3 S/m was found. It is possible to control the conductivity with the CNT concentration of the CNT deposition suspension. A higher CNT concentration resulted in thicker CNT/PDADMA bilayers, but in a lower conductivity per bilayer. We suspect that an increased CNT concentration leads to a rapid CNT adsorption without the possibility to rearrange themselves. If PDADMA then adsorbs on the disordered CNTs in the next deposition step, the average thickness of the polymer layer is thicker than on the more ordered CNT layer from the dilute solution. This leads to an increased PE monomer/CNT ratio and lower conductivity. More polycations between the CNT layers leads to less CNT contacts. Thus, the controlled composition of films can be used to fulfill specific requirements. For many applications of polyelectrolyte multilayers (PEMs), cheap PEs with a broad distribution of molecular weights are used. It was unknown whether the distribution of molecular weights of the PE in the adsorption solution is maintained during the adsorption process and hence in the film. To investigate this, the PSS adsorption solution in article 2 consisted of a binary mixture of short and long poly (styrene sulfonate) (PSS). A good model system to study layered films in terms of composition are PDADMA/PSS multilayers. Neutron reflectivity and in-situ ellipsometry measurements were carried out to determine the PSS composition in the film and the growth regimes. At a mole fraction of long PSS of 5 % or more in solution, the exponential growth (which is characteristic of short PSS) is totally suppressed, and only long PSS is deposited in the resulting multilayer. Variation of adsorption time of PSS showed that short PSS first adsorbs to the surface but is displaced by long PSS. Between 0 and 5 % of long PSS in the adsorption solution exponential growth occurs. The fraction of short PSS in the film continuously decreases with the increase of long PSS in the adsorption solution. In the assembly of films prepared from binary PSS mixtures, the short PSS leaves the film through adsorption/desorption steps both during PSS adsorption and during PDADMA adsorption (as PDADMA/PSS complexes). Both techniques show that the composition of the film does not correspond to that of the deposition solution. The composition and thus the properties of the resulting multilayer are influenced by the choice of adsorption time. Moreover, we conclude that a multilayer grown from a polydisperse polyelectrolyte contains fewer mobile low molecular weight polymers than the deposition solution. In manuscript 1 and article 2, the composition of multilayers was studied. In manuscript 1 adsorption kinetics were important for the arrangement of CNTs on the surface. In article 2, the adsorption kinetics, i.e. the diffusion of the polyelectrolytes to the surface, was also investigated. In article 3, we investigated the influence of the composition of the film as well as the preparation condition on the mobility of PEs in the film. The molecular weight of the polycation PDADMA and the NaCl concentration of the deposition solution were varied. The vertical PSS diffusion constant D_PSS within the PDADMA/PSS multilayers was measured using neutron reflectivity. The salt concentration of the preparation solution defines the polymer conformation during deposition. The molecular weight of the polycation determines the degree of intertwining. Together, both parameters determine the polyanion-polycation coupling and thus the PSS mobility within the network. Log−log display of D_PSS vs the molecular weight of PDADMA and fits to two power laws (D_PSS ∝ X_n(PDADMA)^(-m) ∝ M_w(PDADMA)^(-m)) reveals for films built from 10 or 200 mM NaCl a kink. Below and above the kink, the dependence of D_PSS on M_w(PDADMA) can be described by different power laws. For Χ_n(PDADMA) < X_n,kink(PDADMA) ≈ 288, the exponents are consistent with the predictions of the sticky reptation model. X_n(PDADMA) ≈ 288 is the entanglement limit. For Χ_n(PDADMA) > X_n,kink(PDADMA) ≈ 288, the decrease of D_PSS with M_w(PDADMA) is larger than below the entanglement limit, which is indicative of sticky reptation and entanglement. The PSS diffusion constant of films built from 100 mM NaCl drops three orders of magnitude when increasing the molecular weight of PDADMA from 45 kDa to 72 kDa. To figure out if an immobile PSS fraction exists in the film built from 72 kDa PDADMA (beyond the entanglement limit), the film was annealed at different conditions in article 4: both temperature and salt concentration were varied. For data analysis, the simplest model with two PSS fractions with different diffusion constants was used. These diffusion constants increase as the temperature of the surrounding solution is increased. As assumed in article 3, an immobile PSS fraction exists when annealing at room temperature. At higher annealing temperatures, at least two diffusion processes must be distinguished: the diffusion of the highly mobile PSS fraction through the entire film and a slow PSS fraction, mowing in a limited way. The choice of preparation conditions determines whether a polyelectrolyte multilayer can intermix completely. It is not clear if complete intermixing will ever occur for films built with PDADMA beyond the entanglement limit. It is possible that the diffusion is more complex. Long-term measurements will clarify this question. Calculating scattering length density profiles with subdiffusive behavior would be interesting and is a challenge for the future. Furthermore, immobile fractions are only visible with long annealing times. We hypothesize that an immobile or nearly immobile fraction is present whenever the dependence of D_PSS on the molecular weight of PDADMA cannot be described by the sticky reptation. To verify this hypothesis, further studies are necessary. All results presented and discussed in the manuscript and articles show that by varying the preparation conditions, tailored films can be built. The composition of the film is also determined by the adsorption kinetics. In addition, the mobility of the PEs within the multilayers can be controlled by varying the conformation, mingling and entanglement of the chains within the film. The influence of the salt concentration in the preparation solution on the growth regimes during film formation is part of our future research. It is planned to investigate films built of different PDADMA molecular weights under varied annealing conditions to better understand the mobile and immobile fractions.

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.

Modern space missions depend more and more on electric propulsion devices for in-space flights. The superior efficiency by ionizing the feedgas and propelling them using electric fields with regard to conventional chemical thrusters makes them a great alternative. To find optimized thruster designs is of high importance for industrial applications. Building new prototypes is very expensive and takes a lot of time. A cheaper alternative is to rely on computer simulations to get a deeper understanding of the underlying physics. In order to gain a realistic simulation the whole system has to be taken into account including the channel and the plume region. Because numerical models have to resolve the smallest time and spatial scales, simulations take up an unfeasible amount of time. Usually a self-similarity scaling scheme is used to greatly speed up these simulations. Until now the limits of this method have not been thoroughly discussed. Therefore, this thesis investigates the limits and the influence of the self-similarity scheme on simulations of ion thrusters. The aim is to validate the self-similarity scaling and to look for application oriented tools to use for thruster design optimization. As a test system the High-Efficiency-Multistage-Plasma thruster (HEMP-T) is considered. To simulate the HEMP-T a fully kinetic method is necessary. For low-temperature plasmas, as found in the HEMP-T, the Particle-in-Cell (PIC) method has proven to be the best choice. Unfortunately, PIC requires high spatial and temporal resolution and is hence computationally costly. This limits the size of the devices PIC is able to simulate as well as limiting the exploration of a wider design space of different thrusters. The whole system is physically described using the Boltzmann and Maxwell equations. Using these system of equations invariants can be derived. In the past, these invariants were used to derive a self-similarity scaling law, maintaining the exact solution for the plasma volume, which is applicable to ion thrusters and other plasmas. With the aid of the self-similarity scaling scheme the computation cost can be reduced drastically. The drawback of the geometrical scaling of the system is, that the plasma density and therefore the Debye length does not scale. This expands the length at which charge separation occurs in respect to the system size. In this thesis the limits of this scaling are investigated and the influence of the scaling at higher scaling factors is studied. The specific HEMP-T design chosen for these studies is the DP1. Because the application of scaling laws is limited by the increasing influence of charge separation with increased scaling, PIC simulations still are computationally costly. Another approach to explore a wider design space is given using Multi-Objective-Design-Optimization (MDO). MDO uses different tools to generate optimized thruster designs in a comparatively short amount of time. This new approach is validated using the PIC method. During this validation the drawback of the MDO surfaces. The MDO calculations are not self-consistent and are based on empirical values of old thruster designs as input parameters, which not necessarily match the new optimized thruster design. By simulating the optimized thruster design with PIC and recalculate the former input parameters, a more realistic thruster design is achieved. This process can be repeated iteratively. The combination of self-consistent PIC simulations with the performance of MDO is a great way to generate optimized thruster designs in a comparatively short amount of time. The proof of concept of such a combination is the pinnacle of this thesis.