Edge plasma turbulence

One of the most important goals of modern plasma physics is the realisation of the gain of electric energy from the fusion of light nuclei in a hot magnetically confined plasma. However, especially the radial loss of particles from such a plasma still poses great problems. There have been intensive theoretical and experimental efforts to clarify the reasons for the high particle losses, which impair the magnetic confinement. In spite of that, these phenomena are still far from being comprehensively understood. The most important loss mechanism is due to strong fluctuations in the edge region of a magnetically confined plasma torus or cylinder. In toroidal magnetised plasmas at high densities so-called ELMs (edge localized modes), i.e. bursts of fluctuations with high amplitudes, are responsible for the strongest loss events. For a better understanding and for the development of better theoretical models of these phenomena, in particular of ELMs, it is essential to measure these fluctuations in the edge plasma region as exactly as possible with good temporal and spatial resolution. The main purpose of the project is thus to determine the plasma potential, the electric field, the plasma density, the electron and ion temperatures and the magnetic fluctuations in different plasma devices, in particular two with toroidal and one with cylindrical geometry, in order to compare the results with each other and with computer simulations and to develop improved models for edge plasma transport phenomena. This shall be done in collaboration with four European plasma research institutions (in Prague, Czech Republic, Greifswald, Germany, Garching near Munich, Germany, and Risø, Denmark). To measure the electric field, new plasma diagnostic tools shall be used, which have been developed by us. These are various types of electric plasma probes, in particular electron emissive probes. With such types of probes the plasma potential (and thereby the electric field, when several such probes are used) can be determined with a higher accuracy and faster than with the hitherto used cold probes. In addition also more sophisticated tunnel probes and so-called ball-pen probes shall be used, by which also the ion temperature perpendicular and parallel to the magnetic field can be determined. To measure the fluctuations of the magnetic field a set of three Mirnov coils will be used by which the magnetic field can be picked up in all three dimensions of space. After thorough evaluation of the data and comparison with suitable computer simulations we expect new insights into the edge plasma transport phenomena and in particular into ELMs of various types.

Plasmas, as the so-called fourth state of matter, have increasing importance for numerous aspects of fundamental physics, energy physics and technology. Plasmas are already indispensable for the production of nano-scale integrated circuits, surface treatment (mainly coating, etching and modifications of the surface energy), medical application, light sources, waste management and many other industrial applications. Extremely hot magnetically confined plasma will in future fusion reactors be the medium for nuclear fusion reaction to convert the nuclear binding energy into usable energy. Fusion reactors will soon be vital for humanity to secure the future provision with affordable, reliable, clean, CO2-free energy. The title of the project was "Turbulence in the edge region of magnetized plasmas" but during the execution of the project this restricted subject was considerably, effectively and successfully expanded to some related topics. Thus in this project mainly the following four different aspects of modern plasma physics were treated: Turbulence-driven plasma transport across magnetic fields mainly in tokamaks: Numerous investigations of radial plasma losses across the magnetic field have been carried out in small and medium-sized tokamaks in collaboration with several European fusion research centres (Lisbon, Prague, Padua and mainly the Max-Planck-Institute for Plasma Physics [IPP] in Garching near Munich) with probes developed by the Innsbruck Experimental Plasma Physics Group (IEPPG) (see also next topic). In particular in ASDEX Upgrade (IPP) many valuable insights into turbulence-driven transport of particles, Reynolds stress and radial transport of poloidal momentum were obtained which were instrumental for a better understanding especially of strong transport events like Edge-Localized Modes (ELMs) in the H-mode of tokamaks. Further development and investigations of various plasma probes for magnetized and unmagnetized plasmas. Electron-emissive probes, in particular laser-heated emissive probes were investigated and developed further on. Concerning the use of emissive probes as such in small tokamaks and in the development of laser-heated emissive probes the IEPPG has carried out pioneering work. For ASDEX Upgrade a probe head with several cold probes and a triple magnetic pick-up coil was developed and used in many fundamentally important transport measurement campaigns (see above topic 1). Also in further investigations with ball-pen probes in tokamaks (COMPASS, Prague, and ASDEX Upgrade) the IEPPG has taken part. Low frequency instabilities and fluctuations in magnetized plasmas. In various magnetized plasma devices, mainly in the Innsbruck Q-Machine (the only plasma machine of this type in Europe) three typical low frequency instabilities and their interaction with each other were thoroughly investigated. These were the potential relaxation instability, the electrostatic ion-cyclotron instability and the Kelvin-Helmholtz instability. Localized potential structures and their low and high frequency fluctuations in magnetized and unmagnetized plasmas. Interesting investigations of so-called fireballs were undertaken and published, meeting great interest by fellow scientists. Fireballs are nonlinear localized potential structures which can be created by additional small electrodes immersed in thin background plasma. The electrodes are either positively or negatively biased with respect to the background plasma. When small wire cages or strong permanent magnets were used as electrodes, particularly fascinating and aesthetically appealing luminous plasma structures could be produced. These investigations were also of high relevance for a better understanding of plasma probes.