Aufbau und Erprobung eines repetierenden Laser-Blow-Off-Systems zur Untersuchung elektrostatischer Elektronendichtefluktuationen in der Rand- und Abschaelschicht von TEXTOR und Wendelstein 7-AS

High anomalous transport is one of the key problems on the way to a burning plasma. Today it is generally assumed that this anomalous transport is caused by correlated fluctuations of the electric and magnetic fields, as well as of the particle densities and temperatures. The exact mechanisms are not yet, however, completely understood. Hence, the investigation of fluctuations currently has a high priority in fusion research. Therefore, the aim of this work was to determine to which extent Li-beams that are produced by means of laser blow-off (LBO) can be used for the investigation of fluctuations of the electron density in the edge and scrape-off layer of fusion devices and to carry out fluctuation measurements. For this purpose the LBO-system installed on the TEXTOR tokamak has been upgraded for repetitive operation and used for fluctuation measurements on TEXTOR and on the Wendelstein 7-AS stellarator. The principle of this diagnostic is based on the excitation of Li atoms by collisions with electrons of the plasma and the following relaxation into the ground state. This transition into the ground state is accompanied by photon emission. For the total beam, the line emission is proportional the density of both, the Li atoms and the electrons. The penetration depth of the beam is limited by the increasing ionisation of the injected atoms caused by the rise of the electron density towards the plasma center. The spatial resolution in the direction of propagation of atomic beam diagnostics is determined by the velocity of the atoms and the relaxation time of the atomic processes involved. In the case of Li-LBO the spatial resolution is about 2 mm, which is ideal for the investigation of the spatial structure of electron density fluctuations, as these have a characteristic length of 5-15 mm in radial as well as in poloidal direction. Problems arise from the short length of the LBO pulses (ca. 100 #mu#s) and the strong temporal variation of the Li flux, that has to be taken into account during the data evaluation. (orig.)SIGLEAvailable from TIB Hannover: RA 831(3993) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Zur Anwendung der driftkinetischen Theorie in Monte-Carlo-Studien zum Verunreinigungstransport in Tokamak-Plasmen

A drift kinetic model for the transport of non-Maxwellian impurities is derived from first principles. This model fits in a consistent way in a hybrid concept, i.e. a combination of a fluid model for the background particles and a kinetic test particle description for the impurities. Impurities in a stationary toroidal magnetic field configuration are considered taking into account the interaction of the impurities with the background particles via Coulomb collisions. Also relevant processes like ionization, recombination and interactions with boundary materials in the tokamak are considered in the model. A Monte-Carlo-algorithm is presented for the numerical solution of the resulting model equations in realistic tokamak configurations. Results of simulations performed with the code DORIS for carbon and neon impurities in the tokamak TEXTOR are presented and discussed. (orig.)67 refs.Available from TIB Hannover: RA 831(3508) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Untersuchungen zur Energiedeposition bei Plasmadisruptionen

In the development of a reactor based on controlled nuclear fusion, the tokamak is currently the most advanced concept. The twisting of the magnetic field lines ensures that in normal operation the hot plasma from the inner zone is confined on magnetic flux surfaces and only diffuses very slowly to the walls. A very disturbing exception to this normal case is disruptions where the magnetic confinement of the hot plasma suddenly collapses. The collapse of the plasma flux induces fluxes in the mechanical components and the associated forces may lead to destruction at the tokamak. Impingement of the particles previously enclosed by the magnetic field on the wall materials leads, moreover, to high heat fluxes so that particularly exposed wall materials (limiter, divertor target plates) may melt or partially vaporize. Within the framework of this PhD thesis, the disruptive heat fluxes at the TEXTOR tokamak were investigated. The surface temperature on the ALT-II limiter was measured by an infrared camera. A major technical innovation in this work was an improved time correlation of the IR video image with the other TEXTOR data. The absolute time correlation is a few milliseconds and the relative correlation (i.e. for short periods of time) about 0.1 ms. The local heat flux was determined from the temperature rise after disruptive heat pulses. The heat pulses last less than 100 ms: during this period the local power flux increases to more than 1000-fold that of the mean flux in the nondisruptive phase. The distribution of the heat flux deposition is probably not homogeneous but it is consistent to assume a peaking factor of about 3. (orig.)88 refs.Available from TIB Hannover: RA 831(3664) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Energieabfuhr in Fusionsplasmen durch Linienstrahlung von Verunreinigungen

The following topics were covered: The problem of energy losses in tokamaks, line radiation in plasma edges, radial radiation profiles, influence of magnetic fields, plasma parameters, plasma impurities, TEXTOR tokamak, stationary caseSIGLEAvailable from TIB Hannover: RA 831(3651) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Untersuchung des plasmainduzierten Aufbaus grosser Molekuele in einer Niederdruck-Hohlkathodenanordnung

DC-Glow-Discharges in organic gases are widely used for the carbonization of surfaces. It is known, that these plasmas are suitable to constitute agglomers, which can be applied systematically for the sythesis of nanoparticles, but can be problematic with respect to the plasma induced thin film deposition. Since the beginning of the 90th the mechanisms leading to the generation of these large particles are intensively investigated, but in many cases not well understood. This experimental work is dealing with an examination of responsible processes, which enhance the generation of large molecules in a hollow-cathode-discharge. Discharges in rare gases, methane, ethane, ethene, ethine and mixtures of them are employed in a pressure regime between 0.1 Pa and 5 Pa. The experimental apparatus is equipped with a plasma process monitor (Balzers PPM 421 Spec.) as the central diagnostic for the energy-dispersive plasma-ion-mass-spectrometry (PIMS). This diagnostic has been especially designed for the the demands of this work. A retarding field analyzer is used to calibrate the plasma monitor. Therefore quantitative data from the measured ion-fluxes can be obtained. In addition, the experiment is fitted with a conventional quadrupole-mass-spectrometer (Balzers QME 112) for the rest gas analysis and with electrical probes for the characterization of elementary plasma parameters. Special efforts have been spended to find negative ions. This has lead to the development of a modulation technic, which is suitable to extract negatively charged particles from the plasma bulk. Due to the time-correlated switching of the discharge and the detector sensitivity, the influence of the active discharge periode to the formation of negative ions can be investigated. (orig.)SIGLEAvailable from TIB Hannover: RA 831(3562) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Control of plasma-surface-interactions by thin films

Changing composition and structure of the near surface region of wall materials in fusion devices can modify substantially the plasma-surface interaction and hence influence the performance of fusion plasmas. Main aspects are the control of the plasma impurity production and of hydrogen recycling. Important mechanisms are reviewed and examples from major fusion devices are discussed. (orig.)72 refs.Available from TIB Hannover: RA 831(3124) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Ladungsaustausch-Spektroskopie an der Balmer-#alpha#-Linie an TEXTOR

The quantitative measurement of ion temperatures is of great relevance for the interpretation of experiments related to controlled fusion research. The diagnostics, which have been developed for this purpose, are often indirect and have to be checked for consistency. For spectrocopic diagnostics this concerns in particular the comparison between measurements on impurities with those on the Hydrogen isotopes. Charge exchange recombination spectroscopy (CXRS) is a diagnostic, which allows the determination of radial resolved ion temperatures. The analysis of the H- or D-spectra are problematic because of several effects. For this reason, normally spectra of light impurities are used for CXRS. In this report the charge exchange spectrum of H and D is analysed and the results are compared to other independent measurements. The two effects, which are the most important for complicating the analysis, are the dependence of the effective emission coefficient on the collision energy and the halo effect. Both have been simulated numerically. In the case of the #sigma#_C_X(E_r_e_l)-dependence it is shown, that for the parameter range of TEXTOR the resulting spectrum can be approximated with a Gaussian. For the calculation of ion temperatures and toroidal velocities correction values have been introduced with the species distribution of the neutral beam particles taken into account. (orig.)43 refs.Available from TIB Hannover: RA 831(3375) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Numerical study of plasma-wall transition in a magnetic field

The present report describes numerical results obtained with the Vlasov code. The agreement of our results with those obtained by Chodura is good, and this gives us confidence about the accuracy of our code and its reliability to correctly describe plasma sheaths. As a further step, we have started to include one more spatial dimension. This extension will enable us to describe plasma flows for arbitrary angles, down to one degree. These flows have been found to be widely present in the edge plasma of TEXTOR and are also expected to be important near a divertor. (orig.)7 refs.Available from TIB Hannover: RA 831(3378) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Eine Monte-Carlo-Methode zur Untersuchung von Verunreinigungsionen in Tokamak-Plasmen

The following topics were dealt with: A kinetic equation for the transport of charged test particles in the Tokamak plasma, particle transport and diffusion processes (Chapman-Kolmogorov equation for Markov processes, Fokker-Planck equation and Monte Carlo solution), models with particle creation and annihilation, Monte Carlo code for numerical applications for TEXTOR plasmas15 refs.Available from TIB Hannover: RA 831(3173) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Modellierung der Plasma-Wand-Wechselwirkung und des lokalen Teilchentransportes in der Plasmarandschicht von Tokamaks

In fusion research the interaction of the plasma with the walls is one of the critical issues. On the one hand the plasma induced erosion limits the lifetime of wall components. On the other hand eroded particles can be transported into the core plasma where they dilute the fusion plasma and lead to energy losses due to radiation. To achieve ignition conditions the dilution as well as the cooling has to be reduced to a minimum. Potential wall candidates under discussion are low-Z materials with low atomic numbers like carbon and high-Z elements with high atomic numbers like tungsten. High-Z materials exhibit in comparison to low-Z ones the advantage of small physical sputtering yields. On the other hand one has to consider the high radiation potential due to the high atomic number. Especially carbon-like materials are suitable for high heat flux conditions. Due to their thermodynamical properties they do not suffer even under extreme power loads. However, low-Z materials exhibit high sputtering rates and in the case of carbon-like materials there exists the additional erosion channel of chemical erosion. Particularly at low temperatures, where the physical erosion yield decreases, the chemical erosion determines the erosion behaviour of these materials. (orig.)SIGLEAvailable from TIB Hannover: RA 831(3741) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman