Dynamic Procedure for Filtered Gyrokinetic Simulations

Large Eddy Simulations (LES) of gyrokinetic plasma turbulence are investigated as interesting candidates to decrease the computational cost. A dynamic procedure is implemented in the GENE code, allowing for dynamic optimization of the free parameters of the LES models (setting the amplitudes of dissipative terms). Employing such LES methods, one recovers the free energy and heat flux spectra obtained from highly resolved Direct Numerical Simulations (DNS). Systematic comparisons are performed for different values of the temperature gradient and magnetic shear, parameters which are of prime importance in Ion Temperature Gradient (ITG) driven turbulence. Moreover, the degree of anisotropy of the problem, that can vary with parameters, can be adapted dynamically by the method that shows Gyrokinetic Large Eddy Simulation (GyroLES) to be a serious candidate to reduce numerical cost of gyrokinetic solvers.Comment: 10 pages, 10 figures, submitted to Physics of Plasma

Exploring fusion-reactor physics with high-power electron cyclotron resonance heating on ASDEX Upgrade

The electron cyclotron resonance heating (ECRH) system of the ASDEX Upgrade tokomak has been upgraded over the last 15 years from a 2MW, 2 s, 140 GHz system to an 8MW, 10 s, dual frequency system (105/140 GHz). The power exceeds the L/H power threshold by at least a factor of two, even for high densities, and roughly equals the installed ion cyclotron range of frequencies power. The power of both wave heating systems together (>10MW in the plasma) is about half of the available neutral beam injection (NBI) power, allowing significant variations of torque input, of the shape of the heating profile and of Qe/Qi, even at high heating power. For applications at a low magnetic field an X3-heating scheme is routinely in use. Such a scenario is now also forseen for ITER to study the first H-modes at one third of the full field. This versatile system allows one to address important issues fundamental to a fusion reactor: H-mode operation with dominant electron heating, accessing low collisionalities in full metal devices (also related to suppression of edge localized modes with resonant magnetic perturbations), influence of Te/Ti and rotational shear on transport, and dependence of impurity accumulation on heating profiles. Experiments on all these subjects have been carried out over the last few years and will be presented in this contribution. The adjustable localized current drive capability of ECRH allows dedicated variations of the shape of the q-profile and the study of their influence on non-inductive tokamak operation (so far at q$_{95}$>5.3). The ultimate goal of these experiments is to use the experimental findings to refine theoretical models such that they allow a reliable design of operational schemes for reactor size devices. In this respect, recent studies comparing a quasi-linear approach (TGLF) with fully non-linear modeling (GENE) of non-inductive high-beta plasmas will be reported

One year`s radiological results of ventral spondylodesis in fractures of the thoracic and lumbar spine with expandable cage

GesamtdissertationZiel dieser Arbeit war die radiologische Beurteilung einer, nach Fraktur der Brust- und Lendenwirbelsäule mittels expandierbarem Wirbelkörperersatzsystem VBR®, versorgten Patientengruppe. Dazu wurden 35 Patienten durchschnittlich 12,4 Monate postoperativ radiologisch nachuntersucht. Vorausgehend erhielt jeder Patient eine dorsale Stabilisierung mittels Fixateur interne, Typ USS®. 16 Patienten erhielten wegen einer intraoperativ befundeten erheblichen Instabilität zusätzlich eine ventrale Plattenosteosynthese. Durch konventionelle Röntgenbilder, Funktionsaufnahmen in Flexions- und Extensions- stellung, CT-Aufnahmen und 2-D-Rekonstruktionen sowie quantitativer Comoputertomographie wurden Korrekturverlust, das Auftreten einer knöchernen Fusion und die Dichteänderung der intraoperativ in den Cage eingebrachten Spongiosa beurteilt. Zum Zeitpunkt der Nachuntersuchung zeigte sich ein Korrekturverlust von durchschnittlich 2,4° wobei nur 3 Patienten mit jeweils 6°, 7° und 11° Werte von mehr als 5° boten. 20 Patienten (57 %) erfüllten alle Kriterien für eine komplette bzw. inkomplette Fusion und ein Implantatversagen wurde bei keinem Patienten festgestellt. Die quantitative Computertomographie zeigte bis auf eine tendenzielle Dichtezunahme im oberen und unteren Ansatzstück keine statistisch signifikante Dichteänderung im Cage. Bezüglich des Korrekturverlustes konnte durch die Verwendung des expandierbaren Cages VBR® ein mit anderen Studien vergleichbares Ergebnis registriert werden. Im Literatur-vergleich wurde eine geringere knöcherne Fusionsrate ermittelt, wobei in dieser Arbeit durch die Einteilung in vier mögliche Fusionsgrade eine differenziertere Beurteilung dieses dynamischen Prozesses erfolgte.The aim of this study was to evaluate radiologically a group of patients who were treated with the expandable vertebral body replacement system VBR after a fracture of the thoracolumbar spine. We have examined 35 patients for an average of 12.4 month after surgical treatment combined with the implantation of a dorsal fixateur interne, type USS. 16 patients received an additional ventral plate-osteosynthesis because of an intraoperatively stated relevant instability. By means of conventional X-ray, functional roentgenogram in flexion and extension, computed tomography, 2D-reconstruction and quantitative computed tomography we evaluated the loss of correction, the appearance of osseous fusion and the change of density of the spongiosa which was intraoperatively implanted into an expandable cage. On final examination we saw an average loss of correction of 2.4° with only 3 patients presenting with values higher than 5° (6°, 7° and 11°, respectively). 20 patients (57%) fulfilled all criteria for the complete or incomplete osseous fusion and there was no case of implant-failure. Using quantitative computed tomography we could not see any statistically significant change of density in the cage apart from a slightly increased density in the upper and the lower insertion area. The loss of correction after the implantation of the expandable cage VBR was comparable to other studies. In comparison with literature sources of other studies we found a minor rate of osseous fusion. That may be explained by our new classification into four different fusion grades, which allows a more differentiated judgement on this dynamic process

Multiscale effects in plasma microturbulence

Microinstabilities are one of the key physics problems on the way to efficient nuclear fusion power plants. They cause anomalous heat and particle transport which significantly degrades the plasma confinement, thus preventing self-sustaining plasma burning in present-day experiments. Due to their complex dynamics and highly nonlinear character, the underlying equations can only be solved numerically, and corresponding computations would not be feasible if all involved space and time scales were to be considered. However, in this context, multiscale approaches allow for a reduction to the relevant domain of interest and are used in the plasma turbulence code GENE. The latter numerically solves the gyrokinetic Vlasov-Maxwell system of equations and has been significantly extended during this thesis project by relaxing the local approximation in the radial direction. An important application of the code is the investigation of coupled microturbulence on different space and time scales which are not affected by the gyrokinetic approximation. Traditionally, much of the heat transport and thus the confinement degradation in fusion experiments is attributed to microturbulence with wavelengths of the order of the ion gyroradius. However, several recent theoretical and experimental findings indicate significant contributions originating from the considerably smaller electron gyroradius scales and hence motivate simulations covering both ion and electron spatial and temporal scales self-consistently. It is found that for realistic ion heat flux levels and in the presence of unstable electron temperature gradient driven modes, there tends to be a scale separation between electron and ion thermal transport. In contrast to the latter, the former may exhibit substantial or even dominant small-scale contributions. Furthermore, it is investigated in which way this behavior is reflected in several experimentally accessible quantities, including frequency or density spectra

bei der Eröffnung des Promotionsverfahrens:

Microinstabilities are one of the key physics problems on the way to efficient power plants based on nuclear fusion. They cause anomalous heat and particle transport which significantly degrades the plasma confinement quality, thus preventing self-sustaining plasma burning in present-day experiments. However, due to their complex dynamics and highly nonlinear character, it is impossible to solve the underlying equations of turbulent systems analytically—a problem which is also well known in several other physics research fields, e.g. aerodynamics. Theoretical descriptions and predictions are therefore typically based on numerical simulations. Here, the multitude of involved space and time scales may cause problems since the parameters required for a numerical treatment – e.g., the grid resolutions – often turn out to be infeasible for computations. However, if scales are clearly separated – as it is the case in magnetically confined fusion plasmas – multiscale approaches allow for a reduction of the problem under investigation to the relevant domain of interest. In this context, gyrokinetics is well-established as one of the most powerful theoretical descriptions. It serves as a basis for the plasma turbulence code Gene which numerically solves the modified Vlasov-Maxwell system o

Extension of LES approaches to conductive fluids and plasmas

The extension of LES methods to the MHD description of conductive fluids and plasmas as well as the kinetic description of small scales of turbulence in plasmas is presented. The results obtained during the last decade in the framework of MHD equations will highlight and motivate the current try to perform filtered gyrokinetic simulations.SCOPUS: cp.kinfo:eu-repo/semantics/publishe