High Frequency Geodesic Acoustic Modes in Electron Scale Turbulence

In this work the finite $\beta$-effects of an electron branch of the geodesic acoustic mode (el-GAM) driven by electron temperature gradient (ETG) modes is presented. The work is based on a fluid description of the ETG mode retaining non-adiabatic ions and the dispersion relation for el-GAMs driven non-linearly by ETG modes is derived. The ETG growth rate from the fluid model is compared to the results found from gyrokinetic simulations with good agreement. A new saturation mechanism for ETG turbulence through the interaction with el-GAMs is found, resulting in a significantly enhanced ETG turbulence saturation level compared to the mixing length estimate. It is shown that the el-GAM may be stabilized by an increase in finite $\beta$ as well as by increasing non-adiabaticity. The decreased GAM growth rates is due to the inclusion of the Maxwell stress.Comment: 15 pages, 4 figures. Presented at the IAEA conference 201

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

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

Impact of fast ions on density peaking in JET: fluid and gyrokinetic modeling

The effect of fast ions on turbulent particle transport, driven by ion temperature gradient (ITG)/trapped electron mode turbulence, is studied. Two neutral beam injection (NBI) heated JET discharges in different regimes are analyzed at the radial position rho(t) = 0.6, one of them an L-mode and the other one an H-mode discharge. Results obtained from the computationally efficient fluid model EDWM and the gyro-fluid model TGLF are compared to linear and nonlinear gyrokinetic GENE simulations as well as the experimentally obtained density peaking. In these models, the fast ions are treated as a dynamic species with a Maxwellian background distribution. The dependence of the zero particle flux density gradient (peaking factor) on fast ion density, temperature and corresponding gradients, is investigated. The simulations show that the inclusion of a fast ion species has a stabilizing influence on the ITG mode and reduces the peaking of the main ion and electron density profiles in the absence of sources. The models mostly reproduce the experimentally obtained density peaking for the L-mode discharge whereas the H-mode density peaking is significantly underpredicted, indicating the importance of the NBI particle source for the H-mode density profile

Gyrokinetic simulations of turbulent transport in tokamak plasmas

With the enormous growth of high performance computing (HPC) over the last few decades, plasma physicists have gained access to a valuable instrument for investigating turbulent plasma behaviour. In this thesis, these tools are utilised for the study of particle transport in fusion devices of the tokamak variety.The transport properties of impurities is a major part of the work. This is of high relevance for the performance and optimisation of magnetic fusion devices. For instance, the possible accumulation of He ash in the core of the reactor plasma will serve to dilute the fuel, thus lowering fusion power. Heavier impurity species, originating from the plasma-facing surfaces, may also accumulate in the core, and wall-impurities of relatively low density may lead to unacceptable energy losses in the form of radiation. In an operational power plant, such as the ITER device, both impurities of low and high charge numbers will be present.This thesis studies turbulent particle transport driven by two different modes of drift wave turbulence: the trapped electron (TE) and ion temperature gradient (ITG) modes. Results for ITG mode driven impurity transport are also compared with experimental results from the Joint European Torus.Principal focus is on the balance of convective and diffusive transport, as quantified by the stationary density gradient of zero flux (“peaking factor”, PF). Quasi- and nonlinear results are obtained using the gyrokinetic code GENE, and compared with results from a computationally efficient multi-fluid model. The results are scalings of PF with the driving background gradients of temperature and density, and other parameters, including plasma shape and sheared toroidal rotation

Turbulent impurity transport in tokamak fusion plasmas

With the enormous growth of high performance computing (HPC) over the last few decades, plasma physicists have gained access to a valuable instrument for investigating turbulent plasma behaviour. In this thesis, these tools are utilised for the study of particle transport in fusion devices of the tokamak variety, focusing in particular on the transport of impurities.The transport properties of impurities is of high relevance for the performance and optimisation of magnetic fusion devices. For instance, the possible accumulation of He ash in the core of the reactor plasma will serve to dilute the fuel, thus lowering fusion power. Heavier impurity species, originating from the plasma-facing surfaces, may also accumulate in the core, and wall-impurities of relatively low density may lead to unacceptable energy losses in the form of radiation. In an operational power plant, such as the ITER device, both impurities of low and high charge numbers will be present.This thesis studies turbulent impurity transport driven by two different modes of drift wave turbulence: the trapped electron (TE) and ion temperature gradient (ITG) modes. Principal focus is on the balance of convective and diffusive impurity transport, as quantified by the impurity density gradient of zero flux (“peaking factor”, PF ). The results are scalings of PF with impurity charge number, as well as with the driving background gradients of temperature and density, as well as other plasma parameters.Quasi- and nonlinear results are obtained using the gyrokinetic code GENE, and compared with results from a computationally efficient multi-fluid model. In general, the three models show a good qualitative agreement. Results for ITG mode driven impurity transport are also compared with experimental results from the Joint European Torus, and also in this case a good qualitative agreement is obtained

Turbulent impurity transport in tokamak fusion plasmas

With the enormous growth of high performance computing (HPC) over the last few decades, plasma physicists have gained access to a valuable instrument for investigating turbulent plasma behaviour. In this thesis, these tools are utilised for the study of particle transport in fusion devices of the tokamak variety, focusing in particular on the transport of impurities.The transport properties of impurities is of high relevance for the performance and optimisation of magnetic fusion devices. For instance, the possible accumulation of He ash in the core of the reactor plasma will serve to dilute the fuel, thus lowering fusion power. Heavier impurity species, originating from the plasma-facing surfaces, may also accumulate in the core, and wall-impurities of relatively low density may lead to unacceptable energy losses in the form of radiation. In an operational power plant, such as the ITER device, both impurities of low and high charge numbers will be present.This thesis studies turbulent impurity transport driven by two different modes of drift wave turbulence: the trapped electron (TE) and ion temperature gradient (ITG) modes. Principal focus is on the balance of convective and diffusive impurity transport, as quantified by the impurity density gradient of zero flux (“peaking factor”, PF ). The results are scalings of PF with impurity charge number, as well as with the driving background gradients of temperature and density, as well as other plasma parameters.Quasi- and nonlinear results are obtained using the gyrokinetic code GENE, and compared with results from a computationally efficient multi-fluid model. In general, the three models show a good qualitative agreement. Results for ITG mode driven impurity transport are also compared with experimental results from the Joint European Torus, and also in this case a good qualitative agreement is obtained

Turbulence, Fusion and Clean Energy

In 1926 Sir Arthur Eddington published his treatiseThe Internal Constitution of the Stars, the first comprehensive work on fusion, and with its publication the vision of fusion as a power source was kindled. Since then, taming the nuclear furnace and bringing the power of the Sun to Earth has been the ambition of generations of physicists and engineers. With the ITER experiment (www.iter.org) planned for 2020, the goal seems within reach, appropriately around the centennial of Sir Arthur’s theory

Core transport studies in fusion devices

Comprehensive first principles modelling of fusion plasmascontains many numerical and theoretical challenges: a complicated magnetic geometry, long range electromagnetic interactions, collective modes, and extreme density and temperature gradients driving turbulent fluctuations, to name just a few. HPC provides the tools for tackling these challenges and is the focus of a major undertaking within the European fusion community. In this work, the turbulent transport of trace impurities in a tokamak device has been studied using quasi-linear and non-linear gyrokinetic simulations from the GENE code. The results are quantitative and qualitative assessments of the transport properties of several impurity species, and the dependence thereof on various plasma parameters