Integrated modelling of tokamak core and edge plasma turbulence

The accurate prediction of turbulent transport and its effect on tokamak operation is vital for the performance and development of operational scenarios for present and future fusion devices. For problems of this complexity, a common approach is integrated modelling where multiple, well-benchmarked codes are coupled together to form a code that covers a larger domain and range of physics than each of the constituents. The main goal of this work is to develop such a code that integrates core and edge physics for long-time simulation of the tokamak plasma. Three questions are addressed that contribute to the ultimate end goal of this core/edge coupling, each of which spans a chapter. Firstly, the choice of model for edge and core must be fluid for the time scales of interest, but the validity of a common further simplification to the physics models (i.e. the drift-reduction) is explored for regions of interest within a tokamak. Secondly, maintaining a high computational efficiency in such integrated frameworks is challenging, and increasing this while maintaining accurate simulations is important. The use of sub-grid dissipation models is ubiquitous and useful, so the accuracy of such models is explored. Thirdly, the challenging geometry of a tokamak necessitates the use of a field-aligned coordinate system in the edge plasma, which has limitations. A new coordinate system is developed and tested to improve upon the standard system and remove some of its constraints. Finally, the investigation of these topics culminates in the coupling of an edge and core code (BOUT++ and CENTORI, respectively) to produce a novel, three-dimensional, two-fluid plasma turbulence simulation

Simulation of the interaction between plasma turbulence and neutrals in linear devices

The interaction between plasma and neutrals within a tokamak dominates the behaviour of the edge plasma, especially in the divertor region. This area is not quiescent, but has significant perturbations in the density and temperature due to turbulent fluctuations. Investigating the interaction between the neutrals and plasma is important for accurately simulating and understanding processes such as detachment in tokamaks. For simplicity, yet motivated by tokamak edge plasma, we simulate a linear plasma device and compare the sources and sinks due to ionisation, recombination, and charge exchange for cases with and without turbulence. Interestingly, the turbulence systematically strengthens the interaction, creating stronger sources and sinks for the plasma and neutrals. Not only does the strength of the interactions increase, but the location of these processes also changes. The recombination and charge exchange have relatively short mean free paths, so these processes occur on the scale of the eddy fluctuations, while the ionisation is mostly unaffected by the turbulence

Perturbing microwave beams by plasma density fluctuations

The propagation of microwaves across a turbulent plasma density layer is investigated with full-wave simulations. To properly represent a fusion edge-plasma, drift-wave turbulence is considered based on the Hasegawa-Wakatani model. Scattering and broadening of a microwave beam whose amplitude distribution is of Gaussian shape is studied in detail as a function of certain turbulence properties. Parameters leading to the strongest deterioration of the microwave beam are identified and implications for existing experiments are given

Overview of recent physics results from MAST

New results from MAST are presented that focus on validating models in order to extrapolate to future devices. Measurements during start-up experiments have shown how the bulk ion temperature rise scales with the square of the reconnecting field. During the current ramp-up, models are not able to correctly predict the current diffusion. Experiments have been performed looking at edge and core turbulence. At the edge, detailed studies have revealed how filament characteristics are responsible for determining the near and far scrape off layer density profiles. In the core the intrinsic rotation and electron scale turbulence have been measured. The role that the fast ion gradient has on redistributing fast ions through fishbone modes has led to a redesign of the neutral beam injector on MAST Upgrade. In H-mode the turbulence at the pedestal top has been shown to be consistent with being due to electron temperature gradient modes. A reconnection process appears to occur during edge localized modes (ELMs) and the number of filaments released determines the power profile at the divertor. Resonant magnetic perturbations can mitigate ELMs provided the edge peeling response is maximised and the core kink response minimised. The mitigation of intrinsic error fields with toroidal mode number n  >  1 has been shown to be important for plasma performance

FDTD simulation of a microwave beam propagating across a plasma with density fluctuations

The video shows the absolute value of the wave electric field of an electromagnetic wave propagating across a plasma with density fluctuations. It has been obtained with full-wave simulations using a cold plasma model (for details about the code, see Ref. [1]).<br><br>The geometry was such that a constant background density is taken with half of the cut-off density of the injected microwave. Onto that homogeneous background, density fluctuations are added which are indicated by the white contour lines in the video where each contour line corresponds to an additional increase of 10 % of the cut-off density. Note that only positive density perturbations (with respect the background density) are shown. The density fluctuations are obtained from a Hasegawa-Wakatani drift-wave turbulence model, see Ref. [2]. <br><br>A constant background magnetic field is used, oriented perpendicular to the simulation domain with a strength corresponding to half of the electron cyclotron resonance frequency. An O-mode is injected with a beam radius of 2 times the vacuum wavelength. The number in the lower left corner indicates the wave oscillation periods.<br><br>A quantitative analysis of the beam scattering can be found in Ref. [3].<br><br>[1]: doi <a href="http://dx.doi.org/10.1088/0741-3335/50/8/085018">10.1088/0741-3335/50/8/085018</a><br>[2]: doi <a href="http://dx.doi.org/10.5281/zenodo.47206" target="_blank">10.5281/zenodo.47206</a><br>[3]: <a href="http://arxiv.org/abs/1604.00344">arXiv:1604.00344</a><br><br><br