52.25.Xz Magnetized plasmas
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- Plasma (2)
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Institute
The main issue of this thesis was the investigation of dusty plasmas in magnetic fields. We made use of spherical paramagnetic as well as non-magnetic plastic particles in the micrometer range, so-called dust particles. The particles were then trapped in the sheath region of the driven lower electrode of an rf discharge. The plasma chamber was surrounded by coils to apply a horizontal magnetic field with field strengths of up to B=50mT at the particles’ position. In this configuration the sheath electric field and the external magnetic field were perpendicular to each other. Only the electrons could be magnetized but this leads to several forces acting on the dust particles. In some aspects the dust clusters with the magnetic particles show a behavior that is in complete contrast to those consisting of the standard non-magnetic plastic particles. Both types of particles have in common that the dust clusters were found to move either towards the positive or negative ExB-direction as a reaction to the magnetic field. Whether the positive or negative direction was preferred depended on the experimental conditions. The forces that lead to this transport are plasma-based forces induced by the magnetic field. These investigations were performed on two-dimensional horizontal particle systems. Vertically aligned dust particles due to the ion focus interaction have also been studied to determine the influence of horizontal magnetic fields on the stability of such dust pairs. Under certain conditions the vertical alignment can be broken up by the magnetic field. Some additional experiments on the interaction of non-magnetic dust particles in a plasma with UV irradiation were performed, but a significant decrease of dust charge due to a photoelectric effect was not detected. In summary, even relatively weak horizontal magnetic fields have a strong influence on dust particle systems.
Electromagnetic Drift Waves
(2011)
In the rf-plasma of the linear magnetized VINETA experiment, different types of low-frequency waves are observed. The emphasis in this work is on the interaction mechanism between drift waves on the one and kinetic Alfven waves on the other hand. In the peaked density profile of the plasma column drift waves occur as modulation of the plasma density. As gradient driven instability, they draw their energy from the radial density gradients. Alfven waves as magnetic field fluctuations are stable in the present configuration. They are launched by a magnetic excitation antenna. Parallel conduction currents in the plasma are common to both wave phenoma. A B-dot probe as standard diagnostic tool is used to detect the fluctuating magnetic fields of both wave types. The challenge are the small induced voltages due to the low wave frequency. The probe design with an integrated amplifier close to the probe head takes this into acount. The developed B-dot probe is mounted to different positioning systems to characterize both wave phenomena. For Alfven waves, the dispersion relation is recorded experimentally. It is found to be in good agreement with the prediction of the Hall-MHD theory with included resistive term, accounting for the cold collisional plasma. The fluctuating magnetic field pattern is recorded with azimuthal scans. The current density is obained by Amperes law. It is concentrated in helically twisted current filaments. For the unstable drift waves, similar investigations are done with simultaneously recorded density fluctuations. In the azimuthal plane, the locations of the parallel current filaments and the fluctuating density are found to be in phase, supporting the predicted drive of parallel currents by pressure gradients. A mutual influence of the two wave types is observed in an interaction experiment. Assuming parallel currents as coupling quantity, an interpretation of the experimental findings is given based on the linear theory of drift waves.
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.
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.
Turbulenz ist allgegenwärtig in der Natur. Ein wichtiges Charakteristikum sind Fluktuationen auf einer Vielzahl von räumlichen und zeitlichen Skalen, die sowohl in neutralen Fluiden und gasförmigen Systemen, als auch in Plasmen beobachtet werden. Obwohl der elektromagnetische Charakter von Plasmen eine erhöhte Komplexität von Plasmaturbulenz bedingt, sind die grundlegenden Eigenschaften universell. In magnetisch eingeschlossenen Plasmen führen fluktuierende Plasmaparameter zu turbulentem Transport von Plasmateilchen und Energie, der die Einschlusszeit verringert und wichtige Aspekte zukünftiger Fusionskraftwerke beeinflusst. Der intermittente Charakter dieses konvektiven Teilchenflusses ist verbunden mit turbulenten Strukturen mit großen Amplituden, auch "blobs" genannt, die radial durch das Magnetfeld propagieren. Intermittente Fluktuationen im Randplasma von Experimenten mit linearer Magnetfeldgeometrie werden ebenfalls propagierenden turbulenten Strukturen zugeschrieben. Dabei ist der Mechanismus der radialen Propagation kaum verstanden. In dieser Arbeit wird die Bildung und Propagation von turbulenten Strukturen im linear magnetisierten Helikonexperiment Vineta untersucht. Durch Messungen der Fluktuationen in der azimuthalen Ebene mit multi-dimensionalen Sonden wird gezeigt, dass turbulente Strukturen in Driftwellenturbulenz im Gebiet des maximalen Dichtegradienten entstehen. Die turbulenten Strukturen propagieren hauptsächlich azimuthal in Richtung der Hintergrund ExB-Drift, aber sie besitzen auch eine starke radiale Geschwindigkeitskomponente. Die radiale Propagation wird durch das selbstkonsistente Potential der turbulenten Struktur verursacht, dass zu einem fluktuations-induzierten radialen Transport führt. Im Plasmarand werden die turbulenten Strukturen als intermittente Dichteeruptionen mit großen Amplituden beobachtet. Ein Vergleich der experimentellen Ergebnisse mit numerischen dreidimensionalen Fluid-Simulationen mit abgestimmten Geometrie- und Randbedingungen zeigt Übereinstimmung. Die Bildung der turbulenten Strukturen ist kausal mit einer quasi-kohärenten Driftmode verbunden und ihre radiale Propagation wird durch das selbstkonsistente elektrische Feld verursacht, dass aus der dreidimensionalen Dynamik resultiert. Zum Vergleich wird die Propagation von turbulenten Strukturen im Randplasma vom National Spherical Torus Experiment (NSTX) untersucht und mit theoretischen Propagationsmodellen verglichen.