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Magnetic reconnection is a fundamental plasma process where a change in field line connectivity occurs in a current sheet at the boundary between regions of opposing magnetic fields. In this process, energy stored in the magnetic field is converted into kinetic and thermal energy, which provides a source of plasma heating and energetic particles. Magnetic reconnection plays a key role in many space and laboratory plasma phenomena, e.g. solar flares, Earth’s magnetopause dynamics and instabilities in tokamaks. A new linear device (VINETAII) has been designed for the study of the fundamental physical processes involved in magnetic reconnection. The plasma parameters are such that magnetic reconnection occurs in a collision-dominated regime. A plasma gun creates a localized current sheet, and magnetic reconnection is driven by modulating the plasma current and the magnetic field structure. The plasma current is shown to flow in response to a combination of an externally induced electric field and electrostatic fields in the plasma, and is highly affected by axial sheath boundary conditions. Further, the current is changed by an additional axial magnetic field (guide field), and the current sheet geometry was demonstrated to be set by a combination of magnetic mapping and cross-field plasma diffusion. With increasing distance from the plasma gun, magnetic mapping results in an increase of the current sheet length and a decrease of the width. The control parameter is the ratio of the guide field to the reconnection magnetic field strength. Cross-field plasma diffusion leads to a radial expansion of the current sheet at low guide fields. Plasma currents are also observed in the azimuthal plane and were found to originate from a combination of the field-aligned current component and the diamagnetic current generated by steep in-plane pressure gradients in combination with the guide field. The reconnection rate, defined via the inductive electric field, is shown to be directly linked to the time-derivative of the plasma current. The reconnection rate decreases with increasing ratio of the guide field to the reconnection magnetic field strength, which is attributed to the plasma current dependency on axial boundary conditions and the plasma gun discharge. The above outlined results offer insights into the complex interaction between magnetic fields, electric fields, and the localized current flows during reconnection.
Turbulence is a state of a physical system characterized by a high degree of spatiotemporal disorder. Turbulent processes are driven by instabilities exhibiting complex nonlinear dynamics, which span over several spatial as well as temporal scales. Apart from fluids and gases, turbulence is observed in plasmas. While turbulent mixing of a system is sometimes a desired effect, often turbulence is an undesired state. In hot, magnetically confined plasmas, envisaged for energy generation by thermonuclear fusion, plasma turbulence is clearly a problem, since the magnetic confinement time is drastically deteriorated by turbulent transport. Hence, a control mechanism to influence and to suppress turbulence is of significance for future fusion power devices. An important area of plasma turbulence is drift wave turbulence. Drift waves are characterized by currents parallel to the ambient magnetic field, that are tightly coupled to a coherent mode structure rotating in the perpendicular plane. In the present work, the control of drift waves and drift wave turbulence is experimentally investigated in the linear magnetized helicon experiment VINETA. Two different open-loop control systems - electrostatic and electromagnetic - are used to drive dynamically parallel currents. It is observed that the dynamics of the drift waves can be significantly influenced by both control schemes. If the imposed mode number as well as the rotation direction match those of the drift waves, classical synchronization effects like, e.g., frequency locking, frequency pulling, and Arnold tongues are observed. These confirm the nonlinear interaction between the control signal and the drift wave dynamics. Finally, the broadband drift wave turbulence, and thereby turbulent transport, is considerably reduced if the applied control signal is sufficiently large in amplitude.
Impurity ions pose a potentially serious threat to fusion plasma performance by affecting the confinement in various, usually deleterious, ways. Due to the creation of helium ash during fusion reactions and the interaction of the plasma with the wall components, which makes it possible for heavy ions to penetrate into the core plasma, impurities can intrinsically not be avoided. Therefore, it is essential to study their behaviour in the fusion plasma in detail. Within the framework of this thesis, different problems arising in connection with impurities have been investigated. 1. Collisional damping of zonal flows in tokamkas: The effect of impurities on the collisional damping of zonal flows is investigated. Since the Coulomb collision frequency increases with increasing ion charge, heavy, highly charged impurities play an important role in this process. The effect of such impurities on the linear response of the plasma to an external potential perturbation, as caused by zonal flows, is calculated with analytical methods and compared with numerical simulations, resulting in good agreement. 2. Impurity transport driven by microturbulence in tokamaks: Fine scale turbulence driven by microinstabilities is a source of particle and heat transport in a fusion reactor. A semi-analytical model is presented describing the resulting impurity fluxes and the stability boundary of the underlying mode. The results are compared with numerical simulations. Both the impurity flux and the stability boundary are found to depend strongly on the plasma parameters such as the impurity density and the temperature gradient. 3. Pfirsch-Schlüter transport in stellarators: Due to geometry effects, collisional transport plays a much more prominent role in stellarators than in tokamaks. Analytical expressions for the particle and heat fluxes in an impure, collisional plasma are derived from first principles. Contrary to the tokamak case, where collisional transport is exclusively caused directly by friction, in stellarators an additional source of transport exists, namely pressure anisotropy. Since this term is, contrary to the contribution from friction, non-ambipolar, it plays an important role regarding the ambipolar electric field. Furthermore, the behaviour of heavy impurities in the presence of strong radial temperature and density gradients is studied, which lead to a redistribution of the impurities on the flux surfaces. As a consequence, the radial impurity flux is decreased considerably compared with a plasma in which the impurities are evenly distributed on the flux surfaces.
With this thesis, studies which form the bedrock for the long term goal of first wall heat load control and optimization for the advanced stellarator Wendelstein 7-X are developed, described and put into context. It is laid out how reconstruction of features of the edge magnetic field from plasma facing component heat loads is an important first step and can successfully be achieved by artificial neural networks. A detailed study of plasma facing component heat load distribution, potential overloads and overload mitigation possibilities is made in first order approximation of the impact of the main plasma dynamic effects.
Achieving commercial production of electricity by magnetic confinement fusion requires improvements in energy and particle confinement. In order to better understand and optimise confinement, numerical simulations of plasma phenomena are useful. One particularly challenging regime is that in which long wavelength MHD phenomena interact with kinetic phenomena. In such a regime, global electromagnetic gyrokinetic simulations are necessary. In this regime, computational requirements have been excessive for Eulerian methods, while Particle-in-Cell (PIC) methods have been particularly badly affected by the "cancellation problem", a numerical problem resulting from the structure of the electromagnetic gyrokinetic equations. A number of researchers have been working on mitigating this problem with some significant successes. Another alternative to mitigating the problem is to move to a hybrid system of fluid and gyrokinetic equations. At the expense of reducing the physical content of the numerical model, particularly electron kinetic physics, it is possible in this way to perform global electromagnetic PIC simulations retaining ion gyrokinetic effects but eliminating the cancellation problem. The focus of this work has been the implementation of two such hybrid models into the gyrokinetic code EUTERPE. The two models treat electrons and the entire bulk plasma respectively as a fluid. Both models are additionally capable of considering the self-consistent interaction of an energetic ion species, described gyrokinetically, with the perturbed fields. These two models have been successfully benchmarked in linear growth rate and frequency against other codes for a Toroidal Alfvén Eigenmode (TAE) case. The m=1 internal kink mode, which is particularly challenging in terms of the fully gyrokinetic cancellation problem, has also been successfully benchmarked using the hybrid models with the MHD eigenvalue code CKA. Non-linear simulations in this TAE case have been performed confirming the analytical prediction of a quadratic relationship between the linear growth rate of the TAE and the saturated amplitude of the TAE for a range of moderate values of the linear growth rate. At higher linear growth rate, a slower scaling of saturated amplitude with linear growth rate is observed. This analysis has been extended to include the non-linear wave-wave coupling between multiple TAE modes. It has been shown that wave-wave coupling results in a significant reduction in the saturated amplitude. It has been demonstrated that both plasma elongation and ion kinetic effects can exert a stabilising influence on the internal kink mode. A population of energetic particles can also exert a stabilising influence at low normalised pressure. At high normalised fast particle pressure the stabilised kink mode has been shown to give way to the m=1 EPM, which has been simulated both linearly and non-linearly (the "fishbone" mode). The first self-consistent simulations of global modes in the magnetic geometry of the optimised stellarator Wendelstein 7-X have been performed both linearly and non-linearly. Limitations have been encountered in performing simulations in 3D geometry. A hypothesis for the cause of these problems is outlined and ideas for mitigation are briefly described. In addition to the hybrid model simulations, some of the first utilisations of a new scheme for mitigating the cancellation problem in the fully gyrokinetic regime have been carried out in the framework of this thesis. This scheme, which was developed separately, is concisely described in this work. The new scheme has been benchmarked with existing gyrokinetic and hybrid results. The linear Wendelstein 7-X simulations and linear and single mode non-linear TAE simulations have been repeated with the new model. It is shown that bulk plasma kinetics can suppress the growth rate of global modes in Wendelstein 7-X. The results of fully gyrokinetic TAE simulations, the first to have been performed to our knowledge, are shown to be in close agreement with those results obtained using hybrid models. In the TAE case, the hybrid models are an order of magnitude less computationally demanding than the new gyrokinetic scheme, which is in turn at least an order of magnitude less computationally demanding than the previous gyrokinetic scheme.
The development of innovative coatings with multifunctional properties is an ambitious task in modification of material surfaces. A novel approach is a hybrid method combining the non-thermal plasma processing with nanotechnology for the development of multifunctional surface coatings. The conception of the hybrid coating process is based on three steps: the preparation of a suspension consisting of an organic liquid and functional nanoparticles, the deposition of the suspension as a thin liquid film on the material surface, and the plasma modification of the liquid organic film to achieve a thin solid composite film with embedded nanoparticles demonstrating multifunctional properties and good adherence on the substrate material. In this work the liquid polydimethylsiloxane (PDMS) was applied as a model system, and the experimental investigations were focused on the PDMS plasma modification. In particular, the specific role of the different plasma components and the influence of the plasma and processing parameters on the PDMS modification were studied. The applied capacitively coupled radio frequency (CCRF) plasma was analyzed by electric probe measurements and optical emission spectroscopy, whereas the molecular changes in PDMS due to plasma-induced chemical reactions were studied by the Fourier transform infrared reflection absorption spectroscopy. Additionally, the photocatalytic activity of thin composite films consisting of plasma cross-linked PDMS with embedded TiO2 nanoparticles was demonstrated. During the investigation it was found that the CCRF discharge modifies efficiently thin liquid PDMS films to solid coatings. The samples were positioned in the plasma bulk at floating potential. The penetration depth of particles like neutrals, ions, electrons and radicals in the film is strongly limited. The heating of samples in the CCRF discharge is weak to modify PDMS by itself and only the plasma radiation is able to transform the liquid bulk to solid one. It is known that the absorption onset of PDMS lies in the VUV region (below 200 nm). The energetic VUV radiation penetrates into the PDMS film on a thickness from several hundred nanometers to few micrometers and initiates photochemical reactions there. Thus, different gases like Ar, Xe, O2, H2O, air and H2 were tested to provide the strongest VUV emission intensity of the CCRF discharge. Discharge pressure and power were varied for all these gases and it was found that at all conditions the H2 plasma demonstrates drastically stronger emission. Thus, H2 gas was selected for the plasma treatment of liquid PDMS films. The IRRAS analysis revealed the transformation process of PDMS with the degradation of CH3 groups, the formation of new groups like SiOH, CH2 and SiH, the formation of the SiOx material and crosslinking. It was found that the modification effect is not uniform across the film thickness. The top region with an initial thickness up to 100 nm loses all CH3 groups, in the underlying region the CH3 concentration increases gradually from zero to the value for PDMS, if the film was thick enough. The methyl-free SiOx top layer contains also SiOH and SiH groups. Furthermore, the SiH groups are concentrated only in a very thin layer with a thickness below 10 nm. The presence of the unscreened polar SiOSi and SiOH groups on the surface causes the adsorption of H2O from the atmosphere, which was also observed by IRRAS. By means of the spectroscopic ellipsometry it was found out that all above described regions experience a shrinking. The reason is the crosslinking and loss of material. The most shrunken layer is the top SiOx layer with the shrinking ratio (final thickness/initial thickness) of 0.55 - 0.60. Further, this ratio gradually rise up to the value of 0.95 in the deeper region, which has the concentration of CH3 groups of about that for PDMS. After the analysis of all results the depth of effective modification was estimated at 300 400 nm for the most optimal conditions. The optimization of the plasma VUV intensity was realized by variation of discharge pressure and power. The strongest plasma emission at studied conditions provided the irradiance of the sample of ca. 13 mW/cm2. However, such strong radiation causes very strong production rate of the gases. These products leave the modifying film slower as they are produced, what causes their accumulation in there. Their pressure grows up leading to formation of bubbles, which later explode. Finally, the film becomes heavily damaged. To avoid this effect the pressure and the RF power were changed to reduce the irradiance to 6 - 7 mW/cm2. This resulted in the absence of any damages.
Ion thrusters are Electric Propulsion systems used for satellites and space missions. Within
this work, the High Efficient Multistage Plasma Thruster (HEMP-T), patented by the
THALES group, is investigated. It relies on plasma production by magnetised electrons.
Since the confined plasma in the thruster channel is non-Maxwellian, the near-field plume
plasma is as well. Therefore, the Particle-In-Cell method combined with a Monte-Carlo
Collision model (PIC-MCC) is used to model both regions. In order to increase the sim-
ulated near-field plume region, a non-equidistant grid is utilised, motivated by the lower
plasma density in the plume. To minimise artificial self-forces at grid points bordered by
cells of different size a modified method for the electric field calculation was developed in
this thesis. In order to investigate the outer plume region, where electric field and collisions
are negligible, a ray-tracing Monte-Carlo model is used. With these simulation methods,
two main questions are addressed in this work.
What are the basic mechanisms for plasma confinement, plasma-wall-interaction
and thrust generation?
For the HEMP-T the plasma is confined by magnetic fields in the thruster channel, generated
by cylindrical permanent magnets with opposite polarity. Due to different Hall parameters,
electrons are magnetised, while ions are not. Therefore, the dominating electron transport
is parallel to the magnetic field lines. In the narrow cusp regions, the magnetic mirror effect
reduces the electron flux towards the wall and confines the electrons like in a magnetic
bottle. At the anode, propellant gas streams into the thruster channel, which gets ionised
by the electrons creating the plasma. As a result of the electron oscillation between the two
cusp regions, ionisation of the propellant gas is efficient.
The magnetic field configuration of the HEMP-T also influences the plasma potential inside
the thruster channel. Close to the symmetry axis, the mainly axial magnetic field results in
a flat potential. At the inner wall, the field configuration reduces the plasma wall interaction
to only the narrow cusp regions. Here, the floating potential of the dielectric channel wall
and its plasma sheath result in a rather low radial potential drop compared to the applied
anode potential. As a result, the electric potential is rather flat and impinging ions at the
thruster channel wall have energies below the sputter threshold energy of the wall material.
Therefore, no sputtering appears at the dielectric wall. At the thruster exit the confinement
by the magnetic field is weakened and the potential drops with nearly the full anode voltage.
The resulting electric field accelerates the generated ions into the plume and generate the
thrust, but they are also able to sputter surfaces. During terrestrial testing, sputteringat vacuum vessel walls leads to the production of impurities. The amount of back-flux
towards the channel exit is determined by the sputter yield of the vacuum chamber wall. A
large distance between thruster exit and vessel wall reduces the back-flux and smooths the
pattern of deposition inside the thruster channel. Dependent on their material, the evolving
deposited layers can get conductive, modify by this the potential distribution and reduce
the thrust.
For the HEMP-T, ions are mainly generated at high potential close to the applied anode
potential. Therefore, the accelerated ions producing the thrust gain the maximum energy
as observed in experiment. Ions emitted from the thruster into different angles in the
plume contribute mainly to the ion current at angles between 30 ◦ and 90 ◦ . They mainly
originate from ionisation at the thruster exit. The resulting angular distribution of the
ejected ion current is close to the one of the experiment, slightly shifted by about ten
degrees to higher emission angles. In front of the thruster exit, electrons are trapped by
electrostatics forces. This enhanced density allows ionisation and an additional electron
density structure establishes.
What are possible physics based ideas for optimisation of an ion thruster?
An optimised thruster should have a high ionisation rate inside the thruster channel, low
erosion and an ion angular distribution with small contributions at high angles for min-
imised thruster satellite interactions. In experiments, the HEMP-T satisfies already quite
nicely these requests. In the simulations, low erosion inside the thruster channel and angular
ion distributions close to the experimental data are demonstrated. However, the ionisation
efficiency is lower and radial ion losses are larger than in experiment. A possible explanation
of these differences is an underestimated transport perpendicular to the magnetic field lines,
well known for magnetised plasmas.
A successful example for an optimisation using numerical simulations is the reduction of
back-flux of sputtered impurities during terrestrial experiments by an improved set-up of
the vacuum vessel. The implementation of baffles reduces the back-flux towards the thruster
exit and therefore deposition inside the channel. These improvements were successfully im-
plemented in the experiment and showed a reduction of artefacts during long time measure-
ments. This leads to a stable performance, as it is expected in space.
There is a wide variety of Alfvén waves in tokamak and stellarator plasmas. While most of them are damped, some of the global eigenmodes can be driven unstable when they interact with energetic particles. By coupling the MHD code CKA with the gyrokinetic code EUTERPE, a hybrid kinetic-MHD model is created to describe this wave–particle interaction in stellarator geometry. In this thesis, the CKA-EUTERPE code package is presented. This numerical tool can be used for linear perturbative stability analysis of Alfvén waves in the presence of energetic particles. The equations for the hybrid model are based on the gyrokinetic equations. The fast particles are described with linearized gyrokinetic equations. The reduced MHD equations are derived by taking velocity moments of the gyrokinetic equations. An equation for describing the Alfvén waves is derived by combining the reduced MHD equations. The Alfvén wave equation can retain kinetic corrections. Considering the energy transfer between the particles and the waves, the stability of the waves can be calculated. Numerically, the Alfvén waves are calculated using the CKA code. The equations are solved as an eigenvalue problem to determine the frequency spectrum and the mode structure of the waves. The results of the MHD model are in good agreement with other sophisticated MHD codes. CKA results are shown for a JET and a W7-AS example. The linear version of the EUTERPE code is used to study the motion of energetic particles in the wavefield with fixed spatial structure, and harmonic oscillations in time. In EUTERPE, the gyrokinetic equations are discretized with a PIC scheme using the delta-f method, and both full orbit width and finite Larmor radius effects are included. The code is modified to be able to use the wavefield calculated externally by CKA. Different slowing-down distribution functions are also implemented. The work done by the electric field on the particles is measured to calculate the energy transfer between the particles and the wave and from that the growth rate is determined. The advantage of this approach is that the full magnetic geometry is retained without any limiting assumptions on guiding center orbits. Extensive benchmarks have been performed to test the new CKA-EUTERPE code. Three tokamak benchmarks are presented, where the stability of TAE modes are studied as a function of fast particle energy, or in one case as a function of the fast particle charge. The benchmarks show good agreement with other codes. Stellarator calculations were performed for Wendelstein 7-AS and the results demonstrate that the finite orbit width effects tend to be strongly stabilizing.
Within the scope of this work, a versatile large linear magnetised plasma experiment was designed, constructed, and subsequently put into operation. The magnetised plasma was used to investigate the dispersion of whistler waves (circular polarised electromagnetic waves) with regard to the influence of the plasma boundaries. After a brief review over electromagnetic plasma waves and the three discharge modes of a helicon source, the experimental device and the diagnostic tools are explained in detail. Great attention is devoted to the identification of a reliable, calibrated magnetic fluctuation probe design. To the understanding of dynamical phenomena in ionospheric plasmas, whistler wave measurements in laboratory experiments may contribute significantly because of the ability to vary plasma parameters and to do measurements with high spatial and temporal resolution. However, the boundaries of laboratory experiments change the dispersion behaviour of whistler waves significantly if compared to the unbounded ionospheric situation. The influence of the plasma boundary is studied in the present work on three different levels of increasing complexity. First, a high density, small wavelength regime is established to make the effect of the boundary negligible. Measurements are in full agreement with whistler wave theory for unbounded plasma geometry. Measurements below the ion cyclotron frequency reveal the strong influence of the ion dynamics on whistler wave propagation, but are not straightforward to interpret in terms of dispersion theory. Second, the other limit case is examined: bounded plasma helicon modes. These waves are, mathematically speaking, eigenfunctions of the plasma-boundary system and are of great practical importance for high density plasma discharges, the helicon source. Careful measurements of the equilibrium plasma parameters as well as the magnetic fluctuation profiles of the helicon source are done in all three modes of operation, the capacitive, inductive, and helicon wave sustained mode. The first two modes are fairly well understood and the measurements are consistent with existing models. The high density helicon mode, however, is still a scientific case. The measurements partially confirm existing assumptions. It is demonstrated that the plasma production is detached from the antenna edge region. Moreover, it is shown that the plasma parameters are self-consistently determined by the antenna geometry and the discharge parameters according to basic helicon wave theory. Finally, it is ruled out that the plasma density is the control parameter determining the transition point into the high density helicon mode. The measurements rather suggest that the rf power density is the important value. As a third aspect, whistler waves in an intermediate wavelength regime are studied and the transition from unbounded to bounded plasma wave dispersion is systematically investigated. It is shown both experimentally and numerically that the wave dispersion in a plasma filled metal waveguide cannot be determined solely from wave vector measurements parallel to the magnetic field. For a correct description, the perpendicular mode profile has to be correctly taken into account. In contrast to simple helicon wave theory, it is demonstrated that the perpendicular mode profile is not only determined by the conducting vessel boundaries alone but the entire plasma-boundary system has to be considered as a unity. To summarise, this work has contributed to a better understanding of the physics of the propagation of whistler waves, where the particular role of metal boundaries acting as wave guides was highlighted. This basic science approach to the waves' dynamics is believed to be of significance in the course of the scientific debate on the physics principles of helicon discharges.
Die vorliegende Arbeit liefert Beiträge zur optischen und elektrischen Charakterisierung des dynamischen Verhaltens von Plasmaspezies in Atmosphärendruck-Plasmen insbesondere mit Hinsicht auf den Einsatz in der Plasmamedizin. Dabei wurde ein breites Spektrum verschiedener Diagnostiken angewandt, um die Zugänglichkeit zur Bestimmung weiterer Plasmaparameter an Atmosphärendruck zu prüfen. Diese Arbeit stellt eine neue Methode zur Bestimmung der Ionendichte bei Atmosphärendruck- Bedingungen vor, bei der elektrische Oszillationen ausgewertet werden, deren Ursprung ionenakustische Wellen im Plasma sind. Weiterhin wurden neben relativen optischen Messungen wie der phasenaufgelösten optischen Fotografie (PROI) und der Kreuz- Korrelations-Spektroskopie (CCS) auch absolute optische Messungen mit der interferometrischen Hakenmethode und dem Pockels-Effekt durchgeführt. Anhand von elektrischen Messungen wurde ferner gezeigt, dass mit einer Strom- und Spannungs-Charakteristik der Einfluss von Aufbauparametern, wie der Kapillarposition oder dem Gasfluss, auf das Plasma untersucht werden kann. Gegenstand der Untersuchungen waren verschiedene Plasmaquellen, die für eine Nutzung in der Plasmamedizin entwickelt wurden. Sowohl die elektrischen Messungen des Parametereinflusses als auch die Bestimmung der Ionendichte erfolgten an der selbstpulsenden transienten Funkenentladung in Argon an offener Atmosphäre. Der geringe Filamentdurchmesser und der dennoch hohe Entladungsstrom ermöglichen die Detektion der ionenakustischen Instabilität. Darüber hinaus wurde diese erratisch zündende Entladung räumlich und zeitlich aufgelöst mit der CCS spektroskopisch untersucht. Dabei wird insbesondere die Selbst-Triggerung der CCS ausgenutzt, um einen Zeitbezug trotz des großen Entladungsjitter zu erhalten. Für die PROI wurden die räumlich und zeitlich stabilen Entladungsanordnungen der Nadel-Platte-Geometrie und des Kapillarjets in Helium gewählt. Die Anordnungen wurden mit einer periodischen Sinusspannung betrieben und wiesen Entladungsspalte von d = 5 - 15 mm auf. Eine besondere Anforderung der Messung mit dem Pockels-Effekt ist zu der räumlichen und zeitlichen Stabilität eine dielektrische Gegenelektrode, welche bei der Anordnung des Kapillarjets möglich war. Bei der Anwendung der interferometrischen Hakenmethode kam neben einem Erdgas-Sauerstoff-Mischgasbrenner sowohl eine Mikrowellen-Entladung (Plexc) als auch ein MHz-Plasmajet (kINPen) zur Anwendung. Die Bedeutung der elektrischen Messungen, besonders der Strom- und Spannungscharakteristik einer Entladung, wurde an dem Parametereinfluss der Kapillarposition einer erratisch zündenden transienten Funkenentladung vorgestellt. Es konnte gezeigt werden, dass der Zeitunterschied zwischen dem Stromsignal eines Vorstreamers und der Hauptentladung durch das Einbringen einer Kapillare in den Entladungsspalt deutlich verringert wird. Insbesondere der Beitrag der lokalen elektrischen Feldstärkeerhöhung an der Kapillarkante und der Diffusionsanteil der Umgebungsluft wurden als Ursachen, durch Vergleich einer Feldsimulation mit der Beobachtung der Vorphase an der Kapillarkante in den CCS-Messungen, diskutiert. Anschließend konnte gezeigt werden, dass der Leistungseintrag in die Vorphase durch die Platzierung der Kapillare deutlich reduziert werden kann. Ein wesentliches Ergebnis dieser Arbeit ist die Beobachtung von ionenakustischen Wellen als Oszillationen im Abklingen des Stromsignals einer erratisch zündenden transienten Funkenentladung. Hierzu war es nötig, elektrische Störungen zu erkennen und zu eliminieren. Es konnte ein Erdschleifen-freier Aufbau realisiert werden. In diesem Aufbau zeigt sich, dass die Signale der ionenakustischen Welle ausschließlich in einem bestimmten Gasflussbereich beobachtet werden. Die gemessene Frequenz der Oszillationen wurde als Ionenplasmafrequenz f_{pl ,i} identifiziert und enthält daher Angaben zu den Ionendichten im Bereich von n_{Ar_2^+} = 3•10^{14} cm^{-3} bis 1•10^{12} cm^{-3}. Nach einer Abschätzung der zu erwartenden Elektronendichte, die der gemessenen Ionendichte sehr nahe kommt, wurde die Dispersionsrelation für die vorhandenen Entladungsbedingungen aufgestellt und gelöst. Dabei zeigt sich eine starke zeitliche Dämpfung über die Ionen-Neutralteilchenstöße sowie eine räumliche Verstärkung für die Ionenplasmafrequenz. Aus der Dämpfung der Oszillationsamplituden konnte die Ionen- Neutralteilchen-Stoßfrequenz nu_i = 3•10^7 Hz ermittelt werden. Weiterhin ergibt sich aus der Lösung der Dispersionsrelation ein Existenzbereich für die ionenakustischen Wellen in Abhängigkeit von der Ionendichte und der elektrischen Feldstärke.