Effects of the equilibrium model on impurity transport in tokamaks

Gyrokinetic simulations of ion temperature gradient mode and trapped electron mode driven impurity transport in a realistic tokamak geometry are presented and compared with results using simplified geometries. The gyrokinetic results, obtained with the GENE code in both linear and non-linear modes are compared with data and analysis for a dedicated impurity injection discharge at JET. The impact of several factors on heat and particle transport is discussed, lending special focus to tokamak geometry and rotational shear. To this end, results using s-alpha and concentric circular equilibria are compared with results with magnetic geometry from a JET experiment. To further approach experimental conditions, non-linear gyrokinetic simulations are performed with collisions and a carbon background included. The impurity peaking factors, computed by finding local density gradients corresponding to zero particle flux, are discussed. The impurity peaking factors are seen to be reduced by a factor of ~2 in realistic geometry compared with the simplified geometries, due to a reduction of the convective pinch. It is also seen that collisions reduce the peaking factor for low-Z impurities, while increasing it for high charge numbers, which is attributed to a shift in the transport spectra towards higher wavenumbers with the addition of collisions. With the addition of roto-diffusion, an overall reduction of the peaking factors is observed, but this decrease is not sufficient to explain the flat carbon profiles seen at JET.Comment: 19 pages, 9 figures (17 subfigures

Fluid and gyrokinetic modelling of particle transport in plasmas with hollow density profiles

Hollow density profiles occur in connection with pellet fuelling and L to H transitions. A positive density gradient could potentially stabilize the turbulence or change the relation between convective and diffusive fluxes, thereby reducing the turbulent transport of particles towards the center, making the fuelling scheme inefficient. In the present work, the particle transport driven by ITG/TE mode turbulence in regions of hollow density profiles is studied by fluid as well as gyrokinetic simulations. The fluid model used, an extended version of the Weiland transport model, Extended Drift Wave Model (EDWM), incorporates an arbitrary number of ion species in a multi-fluid description, and an extended wavelength spectrum. The fluid model, which is fast and hence suitable for use in predictive simulations, is compared to gyrokinetic simulations using the code GENE. Typical tokamak parameters are used based on the Cyclone Base Case. Parameter scans in key plasma parameters like plasma β, R/LT , and magnetic shear are investigated. It is found that β in particular has a stabilizing effect in the negative R/Ln region, both nonlinear GENE and EDWM show a decrease in inward flux for negative R/Ln and a change of direction from inward to outward for positive R/Ln . This might have serious consequences for pellet fuelling of high β plasmas

Particle transport in ion and electron scale turbulence

Micro turbulent modes have important and non-trivial effects on transport in tokamaks. This paper deals with transport of main ions and impurities in ion and electron scale turbulence, driven by ion and electron temperature gradients, and trapped electrons. Using the gyrokinetic Vlasov code GENE, results are obtained from both nonlinear and quasi-linear simulations. The transport properties are quantified by calculating the gradient of zero particle flux for steady state in source free regions of the plasma. The results are compared and contrasted with results obtained using a computationally efficient fluid model. Of particular interest are conditions of steep gradients, relevant to e.g. transport barrier conditions. Further, results from a simple s–α geometry are compared with results obtained using a JET-like magnetic equilibrium, and the effects on transport investigated

EUROfusion Integrated Modelling (EU-IM) capabilities and selected physics applications

International audienceRecent developments and achievements of the EUROfusion Code Development for Integrated Modelling project (WPCD), which aim is to provide a validated integrated modelling suite for the simulation and prediction of complete plasma discharges in any tokamak, are presented. WPCD develops generic complex integrated simulations, workflows, for physics applications, using the standardized European Integrated Modelling (EU-IM) framework. Selected physics applications of EU-IM workflows are illustrated in this paper

Gyrokinetic simulations of microturbulence and transport in tokamak plasmas

Fusion power is one of few viable and sustainable means of energy production. The tokamak is arguable the most mature technology to magnetically confine fusion plasmas. In these devices, heat and particle transport is dominated by small-scale turbulent fluctuations. Using high performance computing resources these phenomena can be studied in detail through numerical experiments. The Joint European Torus (JET) is currently the largest tokamak in operation. Recently, the plasma facing components of JET were changed from carbon to metal — beryllium and tungsten. This in order to better align with the design foreseen for ITER, a next-generation device under construction in Cadarache in France. With this new wall, new impurities were introduced into the plasma. Impurities, any ion that is not a reactant in the fusion reactions, are detrimental to the fusion power as they dilute the plasma and can radiate energy. It is therefore important to study the transport of impurities and how it is affected by different operational parameters. The change of wall material has also led to a degradation in energy confinement for certain types of discharges at JET. Energy confinement must be optimized in future fusion devices for them to be economically viable. Another important issue for ITER is the refuelling of the plasma through pellet injection. The frozen hydrogen pellets are injected at high speed into the plasma. When they ablate, they perturb the density and temperature profiles. This changes the properties of the microturbulence which might hinder the particles from reaching the core of the plasma. The present thesis aims at an improved understanding of these urgent issues by means of gyrokinetic simulations of particle and heat transport driven by Ion Temperature Gradient (ITG) and Trapped Electron (TE) mode turbulence

Gyrokinetic simulations of turbulent particle and heat transport in tokamaks

Fusion power is one of few viable sustainable means of energy production. The tokamak is arguable the most mature technology to magnetically confine fusion plasmas. In these devices, heat and particle transport is dominated by small-scale turbulent fluctuations. Recent advances in high performance computing have made it possible to study these phenomena in detail. The Joint European Torus (JET) is currently the largest tokamak in operation. Recently, the plasma facing components of JET were changed from carbon to metal — beryllium and tungsten. This in order to better align with the design foreseen for ITER, a next-generation device under construction in Cadarache in France. The change to this so-called ITER-like wall at JET has had several consequences. Firstly, it introduces new impurities into the plasma. Impurities, any ion that is not a reactant in the fusion reactions, are detrimental to the fusion power as they dilute the plasma and can radiate energy. It is therefore important to study the transport of impurities and how it is affected by different operational parameters, such as the cross-sectional shape of the plasma. Secondly, the change of wall material has led to a degradation in energy confinement for certain types of discharges at JET. Energy confinement must be optimized in future fusion devices in order for them to be economically viable. The present thesis aims at an improved understanding of these urgent issues by means of gyrokinetic simulations of particle and heat transport driven by Ion Temperature Gradient (ITG) and Trapped Electron (TE) mode turbulence

Gyrokinetic simulations of turbulent particle and heat transport in tokamaks

Fusion power is one of few viable sustainable means of energy production. The tokamak is arguable the most mature technology to magnetically confine fusion plasmas. In these devices, heat and particle transport is dominated by small-scale turbulent fluctuations. Recent advances in high performance computing have made it possible to study these phenomena in detail. The Joint European Torus (JET) is currently the largest tokamak in operation. Recently, the plasma facing components of JET were changed from carbon to metal — beryllium and tungsten. This in order to better align with the design foreseen for ITER, a next-generation device under construction in Cadarache in France. The change to this so-called ITER-like wall at JET has had several consequences. Firstly, it introduces new impurities into the plasma. Impurities, any ion that is not a reactant in the fusion reactions, are detrimental to the fusion power as they dilute the plasma and can radiate energy. It is therefore important to study the transport of impurities and how it is affected by different operational parameters, such as the cross-sectional shape of the plasma. Secondly, the change of wall material has led to a degradation in energy confinement for certain types of discharges at JET. Energy confinement must be optimized in future fusion devices in order for them to be economically viable. The present thesis aims at an improved understanding of these urgent issues by means of gyrokinetic simulations of particle and heat transport driven by Ion Temperature Gradient (ITG) and Trapped Electron (TE) mode turbulence

Comparative Gyrokinetic Analysis of JET Baseline H-mode Core Plasmas with Carbon Wall and ITER-Like Wall

Following the change of plasma facing components at JET from a carbon wall (CW) to a metal ITER-like wall (ILW) a deterioration of global confinement has been observed for H-mode baseline experiments. The deterioration has been correlated with a degradation of pedestal confinement with lower electron temperatures at the top of the edge barrier region. In order to investigate the change in core confinement, heat transport due to Ion Temperature Gradient (ITG)/Trapped Electron Mode (TEM) turbulence is investigated using the gyrokinetic code GENE. Two pairs of CW and ILW discharges that are matched according to several global parameters are simulated at mid radius. The simulations included effects of collisions, finite β, realistic geometries, and impurities. A sensitivity study is performed with respect to the key dimensionless parameters in the matched pairs. The combined effect of the relative change in these parameters is that the ITG mode is destabilized in the ILW discharges compared to the CW discharges. This is also reflected in nonlinear simulations where the ILW discharges show higher normalized ion and electron heat fluxes and larger stiffness. The ion energy confinement time within ρ = 0.5 is found to be comparable while the electron confinement time is shorter for the ILW discharges. The core confinement in the ILW discharges is expected to improve if the edge pedestal is recovered since that would favourably change the key plasma parameters that now serve to destabilize them