The effect of toroidal flows on the stability of ITGs in MAST

The free energy in the large temperature and density gradients in tokamaks can drive microinstabilities, which in turn drive turbulence. This turbulence is responsible for the transport of energy and particles over and above that predicted by neoclassical theory. Sheared toroidal rotation can suppress the turbulence and stabilise the underlying microinstabilities, thereby reducing the transport. This thesis investigates how variation of the equilibrium temperature and density profiles, over the same scales associated with the microinstabilities, affects how the ow shear stabilises the linear modes and suppresses the turbulence. A global gyrokinetic code is employed in this investigation, which retains the profile variation and simulates the full 3D domain of a tokamak plasma. How much ow shear is needed to stabilise the linear ion temperature gradient (ITG) mode is found to be dependent on its poloidal wavenumber, with longer wavelength modes needing more ow shear than the fastest growing mode. This dependence is present whether the ow shear is constant across the radius or if it has the variation typical in an experimental rotation profile. There is an asymmetry with respect to the sign of the ow shear in the effectiveness of the stabilisation, with the maximum linear growth rate occurring at finite negative shearing rates for the plasma studied here. This asymmetry arises from the profile variation, and is found to be significant in simulations of MAST L-mode plasmas, especially when the effects of kinetic trapped electrons are included in the simulations. Flow shear asymmetry is still present in nonlinear simulations, and the suppression of fully-developed turbulence depends on the sign of the shearing rate. With the experimental rotation profile, the heat ux arising from ITG turbulence is reduced by an amount comparable to the reduction in the linear growth rates. When the direction of the rotation profile is reversed, such that the sign of the ow shear is ipped while the magnitude remains the same, the turbulence is almost completely suppressed. A new diagnostic on MAST, beam emission spectroscopy (BES), is used to make a direct comparison between density fluctuations from simulation, and from experiment. Collisionless, electrostatic simulations with rotation are found to disagree significantly with experiment in the level of ITG turbulence activity and the correlation times and lengths of the turbulence. The inclusion of electron-electron and electron-ion collisions into static simulations is enough to bring the level of turbulent density uctuations down to within a factor two of the experimental levels, with the correlation lengths becoming comparable, while the correlation times remain an order of magnitude too large

Nuclear Fusion Programme: Annual Report of the Association Karlsruhe Institute of Technology/EURATOM ; January 2013 - December 2013 (KIT Scientific Reports ; 7671)

The Karlsruhe Institute of Technology (KIT) is working in the framework of the European Fusion Programme on key technologies in the areas of superconducting magnets, microwave heating systems (Electron-Cyclotron-Resonance-Heating, ECRH), the deuterium-tritium fuel cycle, He-cooled breeding blankets, a He-cooled divertor and structural materials, as well as refractory metals for high heat flux applications including a major participation in the preparation of the international IFMIF project

Structural, Thermodynamic, and Electronic Properties of Mixed Ionic/Electronic Conductor Materials

Due to the mainstream CMOS technology facing a rapid approach to the fundamental downscaling limit, beyond CMOS technologies are under active investigation and development with the intention of revolutionizing and sustaining a wide range of applications including sensors, cryptography, neuromorphic and quantum computing, memory, and logic, among others. Resistive switching electronics, for example, are devices that can change their electrical resistance with an applied external field. Despite their simple metal-insulator-metal structure, resistive switching devices exhibit an intricate set of IV characteristics based on the chemical composition of the solid electrolyte that ranges from non-volatile bipolar and non-polar switching to volatile threshold switching (abrupt but reversible change in resistance). This rich variety of electrical responses offer new alternatives to traditional CMOS applications in the areas of RF-signal switching, relaxation oscillators, over-voltage protection, and notably, memory cells and two-terminal non-linear selector devices. With the aim of unraveling the physics behind two of such conduction mechanisms, filamentary and threshold, in electrochemical cells consisting solid mixed ionic-electronic conductor electrolytes, this work focused on using first-principles calculations to characterize the structural, thermodynamic, and electronic properties of copper-doped amorphous silicon dioxide and copper-doped germanium-based glassy chalcogenides. The Cu/a-SiO2 system is a promising candidate for resistive switching memory applications. The conduction mechanism in the low-resistance state is known to be filamentary, that is, a physical metallic filament bridges between the metallic electrodes through the amorphous silica. However, many fundamental materials processes that would aid the design and optimization of this devices, such the shape and size of stable metallic filaments, remain unknown. In the first part of this work, the morphology and diffusion of small copper clusters embedded in amorphous silicon dioxide were characterized by density functional theory calculations. The average formation energy of a single copper ion in the amorphous matrix is found to be 2.4 eV, about 50% lower than in the case of silicon dioxide in its cristobalite or quartz phases. The theoretical predictions show that copper clusters with an equiaxed morphology are always energetically favorable relative to the dissolved copper ions in a-SiO2; hence, stable clusters do not exhibit a critical size. The stochasticity in the atomistic structure of the amorphous silicon dioxide leads to a broad distribution activation energies for diffusion of copper in the matrix, ranging from 0.4 to 1.1 eV. Since ab initio molecular dynamics are prohibitively expensive to simulate the switching process in Cu/a-SiO2 electrochemical metallization cells, a multi-scale simulation approach based on electrochemical dynamics with implicit degrees of freedom and density functional theory was developed to study the electronic evolution during metallic filament formation cells. These simulations suggest that the electronic transport in the low-resistance configuration is attributed to copper derived states belonging to the filament bridging between electrodes. Interestingly, the participation of states derived from intrinsic defects in the amorphous SiO2 around the Fermi energy are negligible and do not contribute to conduction. Unlike the Cu/a-SiO2 system which only exhibits filamentary switching, copper-doped germanium-based glassy chalcogenides show filamentary or threshold type of conduction depending on the chemical composition of the glass and copper concentration. Ab initio molecular dynamics based on density functional theory is used to understand the atomistic origin of the electronic transport in these materials. The theoretical predictions show that glasses containing tellurium tend to favor the formation of copper clusters; hence, these materials exhibit filamentary conduction. Threshold conduction is predicted to be the transport mechanism for glassy sulfides and selenides due to the ability of copper to remain dissolved in the amorphous matrix even at high metal concentration. Furthermore, the charge carrier transport in sulfur and selenium glasses was found to be assisted by defective states derived from chalcogen atoms whose bonds exhibit a polar character. Finally, taking advantage of the van der Waals gap as intercalation sites and crystal order in molybdenum disulfide, a novel mixed ionic-electronic conductor material based on copper and silver intercalation of MoS2 is proposed. The theoretical predictions show that on average, the intercalation energy of copper into MoS2 is 1 eV, while intercalation of silver shows a strong dependence on concentration ranging from 2.2 to 0.75 eV for low and high concentrations, respectively. The activation energy for diffusion of copper and silver intercalated within the van der Waals gap of MoS2 is predicted to be 0.32 and 0.38 eV, respectively, comparable to other superionic conductors. Upon Cu and Ag intercalation, MoS2 undergoes a semiconductor-to-metal transition, where the in-plane and out-of-plane conductances are comparable and exhibit a linear dependence with metal concentration

U(VI) retention by Ca-bentonite and clay minerals at (hyper)alkaline conditions

Clays are considered as potential host rocks and backfill material for deep geological repositories for radioactive waste. Therefore, profound understanding of radionuclide retention processes at clay mineral surfaces is essential for a long-term safety assessment. This understanding has already been generated in the past for simple chemical systems, in which experiments are easy to conduct and interpretation is straightforward. However, there is still a lack of molecular process understanding when considering complex natural systems (low radionuclide concentrations, high ionic strength, high pH values, multi-mineral solid phases, complex solution composition). This thesis aims to close some of these knowledge gaps, focusing on U(VI) and Np(VI) sorption on clays at (hyper)alkaline conditions. pH values between 10 and 13 can prevail in the near-field of a radioactive waste repository as a result of the degradation of concrete, which is part of the geo-engineered barrier. Existing studies on radionuclide sorption on clays do not exceed pH 10. Therefore, within this work, a comprehensive investigation in the pH range 8-13 was conducted. This included the quantification of radionuclide retention in batch sorption experiments as well as spectroscopic investigations to generate understanding about the underlying retention mechanisms on a molecular level. Beside the pH, additional focus was on the influence of dissolved carbonate and calcium on radionuclide sorption at (hyper)alkaline conditions. Next to two small chapters dealing with the stability and surface charge of Ca-bentonite at (hyper)alkaline conditions (chapter 4.1) and the influence of ISA on U(VI) sorption at high pH values (chapter 4.3), the thesis can be subdivided in two major parts. The first part (chapter 4.2) is a detailed investigation of U(VI) sorption on Ca-bentonite at (hyper)alkaline conditions in mixed electrolyte solutions. Batch sorption experiments were conducted, varying a number of experimental parameters (sorption time, S/L ratio, U(VI) concentration, pH value, carbonate concentration) and assessing their effect on U(VI) sorption. In order to be able to explain the observed sorption behavior, next to U(VI) solubility tests, spectroscopic techniques were applied. The aqueous speciation of U(VI) was investigated with TRLFS, while its surface speciation was probed with ATR FT-IR, site-selective TRLFS, EXAFS and CTR/RAXR. Since the results of this chapter indicated a great importance of the presence of calcium (see below), the second major part of the thesis (chapter 4.4) was dedicated to a careful evaluation of the influence of calcium on An(VI) sorption on clay minerals at (hyper)alkaline conditions. This encompasses the sorption of Ca(II) on Ca-bentonite and its effect on the bentonite surface charge. Furthermore, U(VI) batch sorption experiments with Na-montmorillonite, synthetic kaolinite and muscovite were conducted in 0.1 M NaCl as well as in 0.1 M NaCl + 0.02 M CaCl2 at pH 8-13, in order to quantify the influence of calcium on U(VI) sorption on supposedly Ca-free mineral phases. Site-selective TRLFS was applied with the aim to observe U(VI) sorption species involving calcium. Finally, complementary sorption experiments Np(VI) on muscovite were performed in order to check whether its sorption behavior is analogous to U(VI) under the given conditions. Batch sorption experiments demonstrate that U(VI) retention on Ca-bentonite can be very effective at pH > 10, even in the presence of carbonate and despite the prevalence of anionic aqueous species. Above a certain pH, depending on the concentration of carbonate in solution, carbonate does not play a role in the aqueous U(VI) speciation anymore due to the predominance of hydrolysis. TRLFS measurements revealed a clear correlation between sorption behavior and aqueous U(VI) speciation, showing that retention reaches a maximum at pH 10-12, where UO2(OH)3− is the predominant aqueous species. This raised the question whether the strong retention can be achieved by adsorption of an anionic species to the negatively charged mineral surface or rather by precipitation of uranates. By in situ ATR FT-IR and CTR/RAXR experiments the formation of U(VI) precipitates on the mineral surface was observed at U(VI) concentrations of 2×10-5 M and 5×10-5 M, respectively. However, solubility tests at sub-micromolar U(VI) concentrations, which were also applied in the batch sorption experiments, showed that the observed complete U(VI) removal at pH 10-12 cannot be attributed to precipitation of (earth) alkali-uranates from the solution. In order to unambiguously distinguish between surface precipitation and surface complexation, direct spectroscopic investigations of the U(VI) complexes on the Ca-bentonite surface were performed with site-selective TRLFS and EXAFS. The occurrence of luminescence line-narrowing and the frequency of the total symmetric stretch vibration obtained from the site-selective TRLFS emission spectra, indicate the presence of two U(VI) surface complexes. Also EXAFS spectroscopy confirmed the presence of two independent U(VI) sorption species on Ca-bentonite at pH 8-13. With increasing pH, the nature of the retained U(VI) complexes shifts from bidentate inner-sphere surface complexes with an overall equatorial coordination of five adsorbed on aluminol or silanol edge sites to surface complexes with a 4-fold equatorial coordination, resembling the aqueous species UO2(OH)42−. For the first time, a 4-fold coordination in the equatorial plane of U(VI) was univocally proven with the help of a multiple-scattering feature originating from the strong symmetry of the complexes, and without the need for error-prone shell fitting. The lack of scattering paths from the substrate and the comparatively high value for the total symmetric stretch vibration indicate that the high-pH-component is an outer-sphere complex. Concerning the character of the second sorption species at very high pH it was hypothesized that the anionic uranyl hydroxide complexes are mediated to the surface by calcium cations. It was found that calcium sorbs strongly on Ca-bentonite between pH 8 and 13. Also zeta potential measurements showed a partial compensation of the strongly negative surface charge of Ca-bentonite due to adsorption of calcium. U(VI) sorption on kaolinite and muscovite was strongly reduced in the absence of calcium at pH > 10. An increased retention upon addition of calcium proved the sorption enhancing effect of calcium at pH 10-12. Site-selective TRLFS allowed the spectroscopic observation and identification of calcium-induced U(VI) sorption complexes on muscovite. The obtained spectra correspond to the outer-sphere species found on Ca-bentonite. Combining the findings from batch sorption, zeta potential, TRLFS and EXAFS suggests that calcium adsorbs to the mineral surface in the first place, displaying locally positively charged sites which enable an electrostatically driven attachment of anionic uranyl hydroxides. The same effect could also be demonstrated for Np(VI) sorption on muscovite, which was also strongly enhanced in the presence of calcium at pH 9-12. ISA leads to a mobilization of U(VI) at (hyper)alkaline conditions only when present in very high excess of U(VI). A reduction of sorption on Ca-bentonite and the formation of aqueous U(VI)-ISA complexes, detected with TRLFS, occurred at an U:ISA ratio of 1:100,000. Such conditions are not likely to be found in deep geological repository environments. Based on these findings it can be concluded that under certain alkaline repository conditions, where precipitation does not occur (due to very low concentrations or kinetic restraints), U(VI) and Np(VI) are still effectively retained in argillaceous minerals and rocks by adsorption despite the anionic character of prevailing aqueous species. Repulsive forces between the actinide species and the mineral surfaces are overcome by mediating Ca2+. This finding is of great relevance, as also the migration of very small amounts of uranium or neptunium out of waste repositories could lead to a hazardous accumulation in the long term. The achieved knowledge gain concerning radionuclide retention at environmental conditions helps to take the next step towards realistic long-term safety assessment of nuclear waste repositories

Towards AMR Simulations of Galaxy Formation

Numerical simulations present a fundamental building block of our modern theoretical understanding of the Universe. As such the work in this thesis is primarily concerned with understanding fundamental differences that lie between the different hydrodynamic schemes. In chapter 3 I outline the optimisations I make to the FLASH code to enable larger simulations to be run. These include developing and testing a new FFT gravity solver. With these complete, in chapter 4 I present results from a collaborative code comparison project in which we test a series of different hydrodynamics codes against a suite of demanding test problems with astrophysical relevance. As the problems have known solutions, we can quantify their performance and are able to develop a resolution criteria which allows for the two different types to be reliably compared. In chapter 5 we develop an analytic model for ram pressure stripping of the hot gaseous haloes of galaxies in groups and clusters. We test the model against a suite of hydrodynamic simulations in the SPH GADGET-2 code. To ensure that the spurious suppression of hydrodynamic instabilities by SPH codes does not bias our results, I compare our findings to those obtained with the FLASH AMR code and find excellent agreement. Chapter 6 presents work in which we unambiguously determine the origin of the difference between the entropy cores formed in AMR and SPH codes. By running mergers of model clusters we are able to systematically explore the various proposed mechanisms and determine that turbulent mixing generates the higher entropy cores within AMR codes but is suppressed in SPH codes. The startling differences between the two hydrodynamic schemes presented in chapter 6 leads me to investigate their affect upon different sub-grid physical recipes. In chapter 7 I present the implementation of a sub-grid star formation recipe in FLASH and find strong differences in the way the two codes model pressure laws. I extend the work in chapter 8 by implementing a kinetic supernova feedback mechanism in FLASH and contrasting it with the results from the GADGET-2 code. I find that AMR codes dissipate energy much more efficiently than in SPH codes