Isotope and density profile effects on pedestal neoclassical transport

Cross-field neoclassical transport of heat, particles and momentum is studied in sharp density pedestals, with a focus on isotope and profile effects, using a radially global approach. Global effects -- which tend to reduce the peak ion heat flux, and shift it outward -- increase with isotope mass for fixed profiles. The heat flux reduction exhibits a saturation with a favorable isotopic trend. A significant part of the heat flux can be convective even in pure plasmas, unlike in the plasma core, and it is sensitive to how momentum sources are distributed between the various species. In particular, if only ion momentum sources are allowed, in global simulations of pure plasmas the ion particle flux remains close to its local value, while this may not be the case for simulations with isotope mixtures or electron momentum sources. The radial angular momentum transport that is a finite orbit width effect, is found to be strongly correlated with heat sources.Comment: 17 pages, 15 figure

Neoclassical flows in deuterium-helium plasma density pedestals

In tokamak transport barriers, the radial scale of profile variations can be comparable to a typical ion orbit width, which makes the coupling of the distribution function across flux surfaces important in the collisional dynamics. We use the radially global steady-state neoclassical {\delta}f code Perfect to calculate poloidal and toroidal flows, and radial fluxes, in the pedestal. In particular, we have studied the changes in these quantities as the plasma composition is changed from a deuterium bulk species with a helium impurity to a helium bulk with a deuterium impurity, under specific profile similarity assumptions. The poloidally resolved radial fluxes are not divergence-free in isolation in the presence of sharp radial profile variations, which leads to the appearance of poloidal return-flows. These flows exhibit a complex radial-poloidal structure that extends several orbit widths into the core and is sensitive to abrupt radial changes in the ion temperature gradient. We find that a sizable neoclassical toroidal angular momentum transport can arise in the radially global theory, in contrast to the local.Comment: 14 pages, 19 figure

Turbulent and neoclassical transport in tokamak plasmas

One of the greatest challenges of thermonuclear fusion is to understand, predict and to some extent control particle and energy transport in fusion plasmas. In the present thesis we consider theoretical and experimental aspects of collisional and turbulent transport in tokamak plasmas.First the collisionality dependence of quasilinear particle flux due to ion temperature gradient (ITG) and trapped electron modes is investigated. A semi-analytical gyrokinetic model of electrostatic microinstabilities is developed and used to study various parametric dependences of ITG stability thresholds and quasilinear particle and energy fluxes, focusing on the effect of collisions.Then corrections to the neoclassical plateau regime transport in transport barriers are calculated. It is found that the ion temperature gradient drive of the bootstrap current can be enhanced significantly, and the ion heat diffusivity and the poloidal flow of trace impurities are also modified in the presence of strong radial electric fields.Furthermore, we investigate the characteristics of microinstabilities in electron cyclotron heated and ohmic discharges in the T10 tokamak using linear gyrokinetic simulations, aiming to find insights into the effect of auxiliary heating on the transport, with special emphasis on impurity peaking.The effect of primary ion species of differing charge and mass on instabilities and transport is studied through linear and nonlinear gyrokinetic simulations. We perform transport analysis of three balanced neutral beam injection discharges from the DIII-D tokamak which have different main ion species (deuterium, hydrogen and helium).Finally the magnitude and characteristics of the error in alkali beam emission spectroscopy density profile measurements due to finite beam width are analyzed and a deconvolution based correction algorithm is introduced

Turbulent and neoclassical transport in tokamak plasmas

One of the greatest challenges of thermonuclear fusion is to understand, predict and to some extent control particle and energy transport in fusion plasmas. In the present thesis we consider theoretical and experimental aspects of collisional and turbulent transport in tokamak plasmas.First the collisionality dependence of quasilinear particle flux due to ion temperature gradient (ITG) and trapped electron modes is investigated. A semi-analytical gyrokinetic model of electrostatic microinstabilities is developed and used to study various parametric dependences of ITG stability thresholds and quasilinear particle and energy fluxes, focusing on the effect of collisions.Then corrections to the neoclassical plateau regime transport in transport barriers are calculated. It is found that the ion temperature gradient drive of the bootstrap current can be enhanced significantly, and the ion heat diffusivity and the poloidal flow of trace impurities are also modified in the presence of strong radial electric fields.Furthermore, we investigate the characteristics of microinstabilities in electron cyclotron heated and ohmic discharges in the T10 tokamak using linear gyrokinetic simulations, aiming to find insights into the effect of auxiliary heating on the transport, with special emphasis on impurity peaking.The effect of primary ion species of differing charge and mass on instabilities and transport is studied through linear and nonlinear gyrokinetic simulations. We perform transport analysis of three balanced neutral beam injection discharges from the DIII-D tokamak which have different main ion species (deuterium, hydrogen and helium).Finally the magnitude and characteristics of the error in alkali beam emission spectroscopy density profile measurements due to finite beam width are analyzed and a deconvolution based correction algorithm is introduced

Runaway dynamics in tokamak disruptions with current relaxation

The safe operation of tokamak reactors requires a reliable modelling capability of disruptions, and in particular the spatio-temporal dynamics of associated runaway electron currents. In a disruption, instabilities can break up magnetic surfaces into chaotic field line regions, causing current profile relaxation, as well as a rapid radial transport of heat and particles. Using a mean-field helicity transport model implemented in the disruption runaway modelling framework Dream, we calculate the dynamics of runaway electrons in the presence of current relaxation events. In scenarios where flux surfaces remain intact in parts of the plasma, a skin current is induced at the boundary of the intact magnetic field region. This skin current region becomes an important centre concerning the subsequent dynamics: it may turn into a hot ohmic current channel, or a sizeable radially localized runaway beam, depending on the heat transport. If the intact region is in the plasma edge, runaway generation in the countercurrent direction can occur, which may develop into a sizeable reverse runaway beam. Even when the current relaxation extends to the entire plasma, the final runaway current density profile can be significantly affected, as the induced electric field is reduced in the core and increased in the edge, thereby shifting the centre of runaway generation towards the edge

Causes and Circumstances of Red Mud Reservoir Dam Failure In 2010 at MAL Zrt Factory Site in Ajka, Hungary

The red mud reservoir failure in Ajka, Hungary has claimed 10 lives and cost millions of dollars in damages. Immediately after emergency measures investigations started to shed light on causes and circumstances. The authors performed an extensive desktop study of about 20 thousand pages reaching back to the 1970s, when the facility has been designed. Beside this study the dam and its area have gone through series of site investigations, from drilling to CPTu testing and laboratory testing. The information so collected resulted in the following conclusion. Contributing factors to this substantial dam failure included poor siting of the facility, partly on top of a diverted creek bed and marshy area; design faults when calculating safety reserves, as well as basic stability at designed maximum reservoir load. Construction technology has not been controlled; its foundation was built unprofessionally. The negligence of regulators at licensing, at commissioning, as well as at the periodic safety reviews. There was no geotechnical monitoring plan, it was considered to be unnecessary. External negative factors further converged with these deficiencies. The frequency of smaller earthquakes was significantly higher in the accident-preceding year than during the previous ten years. Precipitation was unusual, its level during the accident-preceding half year reached an incidence frequency of 3000 years! Heightened groundwater level saturated the clay base surface of the reservoir already weakened by cat-ion exchanges due to high alkalinity of red mud – on sloping surface. The slurry walls built around the reservoir, from environmental protection purpose, have intensified this process. Strong wind gusts shifting direction pressured the dam walls during the day of accident

First principles of modelling the stabilization of microturbulence by fast ions

The observation that fast ions stabilize ion-temperature-gradient-driven microturbulence has profound implications for future fusion reactors. It is also important in optimizing the performance of present-day devices. In this work, we examine in detail the phenomenology of fast ion stabilization and present a reduced model which describes this effect. This model is derived from the high-energy limit of the gyrokinetic equation and extends the existing "dilution" model to account for nontrivial fast ion kinetics. Our model provides a physically-transparent explanation for the observed stabilization and makes several key qualitative predictions. Firstly, that different classes of fast ions, depending on their radial density or temperature variation, have different stabilizing properties. Secondly, that zonal flows are an important ingredient in this effect precisely because the fast ion zonal response is negligible. Finally, that in the limit of highly-energetic fast ions, their response approaches that of the "dilution" model; in particular, alpha particles are expected to have little, if any, stabilizing effect on plasma turbulence. We support these conclusions through detailed linear and nonlinear gyrokinetic simulations.Comment: 29 pages, 10 figures, 3 table

Optimization of flux-surface density variation in stellarator plasmas with respect to the transport of collisional impurities

Avoiding impurity accumulation is a requirement for steady-state stellarator operation. The accumulation of impurities can be heavily affected by variations in their density on the flux-surface. Using recently derived semi-analytic expressions for the transport of a collisional impurity species with high-$Z$ and flux-surface density-variation in the presence of a low-collisionality bulk ion species, we numerically optimize the impurity density-variation on the flux-surface to minimize the radial peaking factor of the impurities. These optimized density-variations can reduce the core impurity density by $0.75^Z$ (with $Z$ the impurity charge number) in the Large Helical Device case considered here, and by $0.89^Z$ in a Wendelstein 7-X standard configuration case. On the other hand, when the same procedure is used to find density-variations that maximize the peaking factor, it is notably increased compared to the case with no density-variation. This highlights the potential importance of measuring and controlling these variations in experiments.Comment: 19 figures, 17 pages. Accepted into Nuclear Fusio