ABSTRACT
The key result of the present work is the theoretical prediction and observation of the formation of a new type of transport barrier in fusion plasmas, called F-ATB (fast ion-induced anomalous transport barrier). As demonstrated through state-of-the-art global electrostatic and electromagnetic simulations, the F-ATB is characterized by a full suppression of the turbulent transport-caused by strongly sheared, axisymmetric E×B flows-and an increase of the neoclassical counterpart, albeit keeping the overall fluxes at significantly reduced levels. The trigger mechanism is shown to be a mainly electrostatic resonant interaction between suprathermal particles, generated via ion-cyclotron-resonance heating, and plasma microturbulence. These findings are obtained by realistic simulations of the ASDEX Upgrade discharge No. 36637-properly designed to maximized the beneficial role of the wave-particle resonance interaction-which exhibits the expected properties of improved confinement produced by energetic particles.
ABSTRACT
Turbulence is widely expected to limit the confinement and, thus, the overall performance of modern neoclassically optimized stellarators. We employ novel petaflop-scale gyrokinetic simulations to predict the distribution of turbulence fluctuations and the related transport scaling on entire stellarator magnetic surfaces and reveal striking differences to tokamaks. Using a stochastic global-search optimization method, we derive the first turbulence-optimized stellarator configuration stemming from an existing quasiomnigenous design.
ABSTRACT
BACKGROUND: No commonly accepted criteria to evaluate bony incorporation of cages as vertebral body replacement in thoracolumbar fractures exist. The goal of this study was a thorough radiological evaluation of the fusion process in posterior-anterior stabilized fractures. PATIENTS AND METHODS: In this study 35 patients were evaluated radiologically including computed tomography (CT) scanning and bone mineral density measurement inside the cages. Correction loss, cage subsidence and tilting, bone growth in and around the cages as well as bone mineral density were assessed. Fusion grading was assessed with defined criteria (i.e. bridging bone, bone growth through the cage, stability in functional X-rays and no radiolucent lines). RESULTS: After 12 months minor subsidence and tilting of the cages had caused significant correction loss of the basal plate angle of 2.4° on average. Of the patients 20 (57%) fulfilled the criteria for complete or incomplete fusion and 5 (14%) showed no signs of bony fusion. Bone mineral density measurements were unreliable due to metallic artefacts. CONCLUSIONS: The advocated criteria allow accurate assessment of bony incorporation of cages. Bony incorporation can be detected in and around the cages over time; however, only 57% of patients showed signs of bony fusion after 1 year.
Subject(s)
Bone Plates , Prostheses and Implants , Spinal Fractures/surgery , Spinal Fusion/instrumentation , Thoracic Vertebrae/injuries , Thoracic Vertebrae/surgery , Adolescent , Adult , Aged , Female , Humans , Longitudinal Studies , Male , Middle Aged , Prosthesis Design , Radiography , Spinal Fractures/diagnostic imaging , Treatment Outcome , Young AdultABSTRACT
A hitherto unexplained feature of electromagnetic simulations of ion temperature gradient turbulence is the apparent failure of the transport levels to saturate for certain parameters; this effect, termed here nonzonal transition, has been referred to as the high-ß runaway. The resulting large heat fluxes are shown to be a consequence of reduced zonal flow activity, brought on by magnetic field perturbations shorting out flux surfaces.
ABSTRACT
In gyrokinetic theory, the quadratic nonlinearity is known to play an important role in the dynamics by redistributing (in a conservative fashion) the free energy between the various active scales. In the present study, the free energy transfer is analyzed for the case of ion temperature gradient driven turbulence. It is shown that it shares many properties with the energy transfer in fluid turbulence. In particular, one finds a (strongly) local, forward (from large to small scales) cascade of free energy in the plane perpendicular to the background magnetic field. These findings shed light on some fundamental properties of plasma turbulence, and encourage the development of large-eddy-simulation techniques for gyrokinetics.
ABSTRACT
The scaling of turbulence-driven heat transport with system size in magnetically confined plasmas is reexamined using first-principles based numerical simulations. Two very different numerical methods are applied to this problem, in order to resolve a long-standing quantitative disagreement, which may have arisen due to inconsistencies in the geometrical approximation. System size effects are further explored by modifying the width of the strong gradient region at fixed system size. The finite width of the strong gradient region in gyroradius units, rather than the finite overall system size, is found to induce the diffusivity reduction seen in global gyrokinetic simulations.
ABSTRACT
Nonlinear gyrokinetic simulations of microturbulence simultaneously driven by electron temperature gradient modes, trapped electron modes, and ion temperature gradient modes are presented, covering both electron and ion spatiotemporal scales self-consistently. It is found that, for realistic ion heat (and particle) flux levels and in the presence of unstable electron temperature gradient 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 high-wave-number contributions.
ABSTRACT
Ion-temperature-gradient turbulence constitutes a possibly dominant transport mechanism for optimized stellarators, in view of the effective suppression of neoclassical losses characterizing these devices. Nonlinear gyrokinetic simulation results for the Wendelstein 7-X stellarator [G. Grieger, in (IAEA, Vienna, 1991) Vol. 3, p. 525]-assuming an adiabatic electron response-are presented. Several fundamental features are discussed, including the role of zonal flows for turbulence saturation, the resulting flux-gradient relationship, and the coexistence of ion-temperature-gradient modes with trapped ion modes in the saturated state.
