Advanced Computational Methods for Oncological Image Analysis.

The Special Issue "Advanced Computational Methods for Oncological Image Analysis", published for the Journal of Imaging, covered original research papers about state-of-the-art and novel algorithms and methodologies, as well as applications of computational methods for oncological image analysis, ranging from radiogenomics to deep learning [...]

Scaling laws for electron kinetic effects in tokamak scrape-off layer plasmas

Tokamak edge (scrape-off layer) plasmas can exhibit non-local transport in the direction parallel to the magnetic field due to steep temperature gradients. This effect along with its consequences has been explored at equilibrium for a range of conditions, from sheath-limited to detached, using the 1D kinetic electron code SOL-KiT, where the electrons are treated kinetically and compared to a self-consistent fluid model. Line-averaged suppression of the kinetic heat flux (compared to Spitzer-Harm) of up to 50% is observed, contrasting with up to 98% enhancement of the sheath heat transmission coefficient, $\gamma_e$. Simple scaling laws in terms of basic SOL parameters for both effects are presented. By implementing these scalings as corrections to the fluid model, we find good agreement with the kinetic model for target electron temperatures. It is found that the strongest kinetic effects in $\gamma_e$ are observed at low-intermediate collisionalities, and tend to increase at increasing upstream densities and temperatures. On the other hand, the heat flux suppression is found to increase monotonically as upstream collisionality decreases. The conditions simulated encompass collisionalities relevant to current and future tokamaks.Comment: 24 pages, 14 figure

ReMKiT1D -- A framework for building reactive multi-fluid models of the tokamak Scrape-Off Layer with coupled electron kinetics in 1D

In this manuscript we present the recently developed flexible framework for building both fluid and electron kinetic models of the tokamak Scrape-Off Layer in 1D - ReMKiT1D (Reactive Multi-fluid and Kinetic Transport in 1D). The framework can handle systems of non-linear ODEs, various 1D PDEs arising in fluid modelling, as well as PDEs arising from the treatment of the electron kinetic equation. As such, the framework allows for flexibility in fluid models of the Scrape-Off Layer while allowing the easy addition of kinetic electron effects. We focus on presenting both the high-level design decisions that allow for model flexibility, as well as the most important implementation aspects. A significant number of verification and performance tests are presented, as well as a step-by-step walkthrough of a simple example for setting up models using the Python interface

The physics of turbulence localised to the tokamak divertor volume

Fusion power plant designs based on magnetic confinement, such as the tokamak design, offer a promising route to sustainable fusion power but require robust exhaust solutions capable of tolerating intense heat and particle fluxes from the plasma at the core of the device. Turbulent plasma transport in the region where the interface between the plasma and the materials of the device is handled - called the divertor volume - is poorly understood, yet impacts several key factors ultimately affecting device performance. In this article a comprehensive study of the underlying physics of turbulence in the divertor volume is conducted using data collected in the final experimental campaign of the Mega Ampere Spherical Tokamak device, compared to high fidelity nonlinear simulations. The physics of the turbulence is shown to be strongly dependant on the geometry of the divertor volume - a potentially important result as the community looks to advanced divertor designs with complex geometry for future fusion power plants. These results lay the foundations of a first-principles physics basis for turbulent transport in the tokamak divertor, providing a critical step towards a predictive understanding of tokamak divertor plasma solutions

Edge turbulence in ISTTOK : a multi-code fluid validation

Fluid models used to study the edge plasma region need to be benchmarked against similar conditions given that models can strongly differ in complexity and therefore the results they produce. Via this validation study undertaken through the framework of EUROfusion Enabling Research, four state-of-the art models - GBS, Hermes/BOUT++, HESEL and TOKAM3X - are compared to experimental plasma turbulence measurements on the ISTTOK tokamak. Statistical comparisons of simulation and experiment data show that fluid models used here can replicate most of the experiment in terms of $I_{sat}$ and $V_{float}$ fluctuations. Furthermore, it is shown that without including more complex information (like core turbulence information and domain geometry details and magnetic topological aspects) in fluid models, the results recovered can fall short from the experimental results. Via the simulations using these codes, it is demonstrated that fluid models continue to be a good cost-effective tool in recovering many global aspects of edge plasma behaviour

Effect of the relative shift between the electron density and temperature pedestal position on the pedestal stability in JET-ILW and comparison with JET-C

The electron temperature and density pedestals tend to vary in their relative radial positions, as observed in DIII-D (Beurskens et al 2011 Phys. Plasmas 18 056120) and ASDEX Upgrade (Dunne et al 2017 Plasma Phys. Control. Fusion 59 14017). This so-called relative shift has an impact on the pedestal magnetohydrodynamic (MHD) stability and hence on the pedestal height (Osborne et al 2015 Nucl. Fusion 55 063018). The present work studies the effect of the relative shift on pedestal stability of JET ITER-like wall (JET-ILW) baseline low triangularity (\u3b4) unseeded plasmas, and similar JET-C discharges. As shown in this paper, the increase of the pedestal relative shift is correlated with the reduction of the normalized pressure gradient, therefore playing a strong role in pedestal stability. Furthermore, JET-ILW tends to have a larger relative shift compared to JET carbon wall (JET-C), suggesting a possible role of the plasma facing materials in affecting the density profile location. Experimental results are then compared with stability analysis performed in terms of the peeling-ballooning model and with pedestal predictive model EUROPED (Saarelma et al 2017 Plasma Phys. Control. Fusion). Stability analysis is consistent with the experimental findings, showing an improvement of the pedestal stability, when the relative shift is reduced. This has been ascribed mainly to the increase of the edge bootstrap current, and to minor effects related to the increase of the pedestal pressure gradient and narrowing of the pedestal pressure width. Pedestal predictive model EUROPED shows a qualitative agreement with experiment, especially for low values of the relative shift

Linear theory and saturation of tearing modes in plasmas

In this work, the linear theory and the saturation of the Tearing Mode in plasmas are investigated. This instability is associated to magnetic reconnection, and has been proved a valid model to explain several plasma processes, such as the solar flares, the behavior of Earth's magnetosphere and the physics of the experimental devices used in nuclear fusion research (Tokamak). Specific attention is devoted to the influence of the asymmetries of the equilibrium current density. It is shown that an asymmetric equilibrium can affect significantly both the linear dispersion relation of the mode and the overall saturation level. A rigorous mathematical procedure, which employs perturbative techniques, is developed to solve the nonlinear saturation problem. This procedure allows reliable predictions of the final width of the magnetic island associated with the instability. Three relevant physical regimes of plasma are investigated, depending on the model for the evolution of the resistivity, which may be affected by the growth of the mode. In the final section of the work, part of the analytical results obtained is employed in the understanding of the Tearing Mode in a physical framework relevant for fusion plasmas. In particular, the mathematical model is extended to include effects related to the so-called Neoclassical description of the Tokamak. The complexity of the nonlinear problem does not allow a straightforward analytical approach, and has to be handled with numerical tools. A systematic numerical investigation of the saturation of the Neoclassical Tearing Mode is presented, the results of which are interpreted with a simplified theoretical modelDans ce travail sont étudiés la théorie linéaire et la saturation du Tearing Mode dans le plasma. Cette instabilité est associée la reconnexion magnétique, et a été avére un modèle valide pour expliquer plusieurs phénomènes de plasma, comme les éruptions chromosphériques, le comportement de la magnétosphère de la terre, et la physique des dispositifs expérimentaux utilisés dans le cadre de la recherche de la fusion contrôlée (Tokamak). Une attention particulière est portée à l'influence des asymétries de la densité de courant d'équilibre. Il est montré qu'un équilibre asymétrique peut affecter de manière signicative la relation de dispersion linéaire du mode et le niveau de saturation. Un procédé mathématique rigoureux, qui utilise des techniques perturbative, est développé pour résoudre le problème non-linéaire de la saturation. Cette procédure permet des prévisions fiables de la largeur nale de l'île magnétique liée à l'instabilité. Trois régimes de plasma sont étudiés, selon le modèle pour l'évolution de la résistivité, qui peut être affecté par la croissance du mode. Dans la section finale du travail, une partie des résultats analytiques obtenus est utilisée pour interpréter le Tearing Mode dans un cadre physique approprié pour des plasmas de fusion. En particulier, le modèle mathématique est étendu pour inclure des effets liés à la description Néoclassique du Tokamak. La complexité du problème non-linéaire ne permet pas une approche analytique directe, et doit être exploré avec des calculs numériques. Une recherche numérique systématique sur la saturation du Tearing Mode Néoclassique est présentée, dont résultats sont interprétés avec un modèle théorique simplifiéAIX-MARSEILLE1-BU Sci.St Charles (130552104) / SudocSudocFranceF