302 research outputs found
Drift reduced Landau fluid model for magnetized plasma turbulence simulations in BOUT++ framework
Recently the drift-reduced Landau fluid six-field turbulence model within the
BOUT++ framework has been upgraded. In particular, this new model employs a new
normalization, adds a volumetric flux-driven source option, the Landau fluid
closure for parallel heat flux and a Laplacian inversion solver which is able
to capture n=0 axisymmetric mode evolution in realistic tokamak configurations.
These improvements substantially extended model's capability to study a wider
range of tokamak edge phenomena, and are essential to build a fully
self-consistent edge turbulence model capable of both transient (e.g., ELM,
disruption) and transport time-scale simulations.Comment: 26 pages, 14 figure
Recent EUROfusion Achievements in Support of Computationally Demanding Multiscale Fusion Physics Simulations and Integrated Modeling
Integrated modeling (IM) of present experiments and future tokamak reactors requires the provision of computational resources and numerical tools capable of simulating multiscale spatial phenomena as well as fast transient events and relatively slow plasma evolution within a reasonably short computational time. Recent progress in the implementation of the new computational resources for fusion applications in Europe based on modern supercomputer technologies (supercomputer MARCONI-FUSION), in the optimization and speedup of the EU fusion-related first-principle codes, and in the development of a basis for physics codes/modules integration into a centrally maintained suite of IM tools achieved within the EUROfusion Consortium is presented. Physics phenomena that can now be reasonably modelled in various areas (core turbulence and magnetic reconnection, edge and scrape-off layer physics, radio-frequency heating and current drive, magnetohydrodynamic model, reflectometry simulations) following successful code optimizations and parallelization are briefly described. Development activities in support to IM are summarized. They include support to (1) the local deployment of the IM infrastructure and access to experimental data at various host sites, (2) the management of releases for sophisticated IM workflows involving a large number of components, and (3) the performance optimization of complex IM workflows.This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014 to 2018 under grant agreement 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission or ITER.Peer ReviewedPostprint (published version
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Electostatic plasma edge turbulence and anomalous transport in SOL plasmas
textControlling the scrape-off layer (SOL) properties in order to limit divertor erosion and extend component lifetime will be crucial to successful operation of ITER and devices that follow, where intermittent thermal loads on the order of GW/m² are expected. Steady state transport in the edge region is generally turbulent with large, order unity, fluctuations and is convection dominated. Owing to the success of the past fifty years of progress in magnetically confining hot plasmas, in this work we examine convective transport phenomena in the SOL that occur in the relatively "slow", drift-ordered fluid limit, most applicable to plasmas near MHD equilibrium. Diamagnetic charge separation in an inhomogeneous magnetic field is the principal energy transfer mechanism powering turbulence and convective transport examined in this work. Two possibilities are explored for controlling SOL conditions. In chapter 2 we review basic physics underlying the equations used to model interchange turbulence in the SOL and use a subset of equations that includes electron temperature and externally applied potential bias to examine the possibility of suppressing interchange driven turbulence in the Texas Helimak. Simulated scans in E₀×B₀ flow shear, driven by changes in the potential bias on the endplates appears to alter turbulence levels as measured by the mean amplitude of fluctuations. In broad agreement with experiment negative biasing generally decreases the fluctuation amplitude. Interaction between flow shear and interchange instability appears to be important, with the interchange rate forming a natural pivot point for observed shear rates. In chapter 3 we examine the possibility of resonant magnetic perturbations (RMPs) or more generally magnetic field-line chaos to decrease the maximum particle flux incident on the divertor. Naturally occurring error fields as well as RMPs applied for stability control are known to cause magnetic field-line chaos in the SOL region of tokamaks. In chapter 3 2D simulations are used to investigate the effect of the field-line chaos on the SOL and in particular on its width and peak particle flux. The chaos enters the SOL dynamics through the connection length, which is evaluated using a Poincaré map. The variation of experimentally relevant quantities, such as the SOL gradient length scale and the intermittency of the particle flux in the SOL, is described as a function of the strength of the magnetic perturbation. It is found that the effect of the chaos is to broaden the profile of the sheath-loss coefficient, which is proportional to the inverse connection length. That is, the SOL transport in a chaotic field is equivalent to that in a model where the sheathloss coefficient is replaced by its average over the unperturbed flux surfaces. Both fully chaotic and the flux-surface averaged approximation of RMP application significantly lower maximum parallel particle flux incident on the divertor.Physic
The JOREK non-linear extended MHD code and applications to large-scale instabilities and their control in magnetically confined fusion plasmas
JOREK is a massively parallel fully implicit non-linear extended magneto-hydrodynamic (MHD) code for realistic tokamak X-point plasmas. It has become a widely used versatile simulation code for studying large-scale plasma instabilities and their control and is continuously developed in an international community with strong involvements in the European fusion research programme and ITER organization. This article gives a comprehensive overview of the physics models implemented, numerical methods applied for solving the equations and physics studies performed with the code. A dedicated section highlights some of the verification work done for the code. A hierarchy of different physics models is available including a free boundary and resistive wall extension and hybrid kinetic-fluid models. The code allows for flux-surface aligned iso-parametric finite element grids in single and double X-point plasmas which can be extended to the true physical walls and uses a robust fully implicit time stepping. Particular focus is laid on plasma edge and scrape-off layer (SOL) physics as well as disruption related phenomena. Among the key results obtained with JOREK regarding plasma edge and SOL, are deep insights into the dynamics of edge localized modes (ELMs), ELM cycles, and ELM control by resonant magnetic perturbations, pellet injection, as well as by vertical magnetic kicks. Also ELM free regimes, detachment physics, the generation and transport of impurities during an ELM, and electrostatic turbulence in the pedestal region are investigated. Regarding disruptions, the focus is on the dynamics of the thermal quench (TQ) and current quench triggered by massive gas injection and shattered pellet injection, runaway electron (RE) dynamics as well as the RE interaction with MHD modes, and vertical displacement events. Also the seeding and suppression of tearing modes (TMs), the dynamics of naturally occurring TQs triggered by locked modes, and radiative collapses are being studied.Peer ReviewedPostprint (published version
Landau-fluid closures and numerical implementation in BOUT++
Fluid models are used to quantitatively describe many phenomena in plasmas, providing a reduced description of the lower dimensionality in comparison to kinetic models. Often, fluid models are more amenable to numerical and analytical analysis including nonlinear effects. The principal drawback of fluid models is the inability to describe kinetic effects which are important in the long mean free path regimes. However, a linear closure can be introduced to model kinetic effects, such as Landau damping. Such closures for three- and four-moment fluid model [G.W. Hammett and F.W. Perkins, Physical Review Letters 64, 3019(1990)] are known to be able to model plasma response function (with the decent accuracy) and kinetic effects of plasma microinstabilities (such as ion-temperature gradient instability). One of the results of this work is the derivation of the exact linear closure for the set of one-dimensional plasma fluid equations. The exact linear expression for the heat flux is obtained thus replacing the infinite hierarchy of fluid moments with a finite set of equations that incorporate kinetic effects of thermal motion into a fluid model. It is shown that the obtained exact closure in the limit case is reduced to the closure derived previously by Hammett and Perkins. Another goal of this work is to show how such fluid model with the kinetic closure can be modeled numerically using a recently developed non-Fourier method [A. Dimits, et. al., Phys Plasmas, 21 (5) 2014]. The method is based on the approximation of a Fourier image by a sum of Lorentzian functions allowing fast conversion into the configuration (real) space. With this approach, the one-dimensional model which includes evolution equation for the energy was implemented using the BOUT++ framework. The numerical implementation was verified in the series of test simulations of the plasma response function. Additionally, a self-consistent model of the ion Landau damping was implemented. It is shown that the damping rate for the ion Landau damping model agrees well with the exact kinetic result
3D Simulations of Scrape-Off Layer Filaments
In the Scrape-Off Layer (SOL) of magnetic confinement devices, cross-field transport of particles is dominated by the convection of filamentary plasma structures via self-generated E×B velocity fields. This thesis investigates the dynamics of such filaments using three dimensional simulations to further theoretical understanding of SOL transport. A new 3D SOL simulation code called STORM3D has been developed using the BOUT++ framework to implement an isothermal drift-reduced fluid model in a slab geometry. Verification and validation exercises are documented to demonstrate that the code has been implemented correctly and that the physical model adequately reproduces experimental observations.
A comprehensive characterisation of how a filament’s initial geometry affects its subsequent dynamics is provided via a series of 3D simulations of isolated filaments. In particular the size of a filament in the plane perpendicular to the magnetic field, δ⊥, is shown to have a strong influence on its motions, as it determines which currents balance the filament’s pressure-driven diamagnetic currents, which in turn determines its E×B velocity. At small δ⊥, this balance is predominantly provided by polarisation currents and the filament’s radial velocity is observed to increase with δ⊥. In contrast, at large δ⊥, parallel currents closing through the target are found to be dominant, and the radial velocity decreases with δ⊥.
Comparisons are made between 3D simulations and 2D simulations using different parallel closures; namely the sheath dissipation closure, which neglects parallel gradients, and the vorticity advection closure, which neglects the influence of parallel currents. The vorticity advection closure is found not to replicate the 3D perpendicular dynamics well and overestimates the initial radial velocity of all filaments studied. A more satisfactory comparison is obtained with the sheath dissipation closure, even in the presence of significant parallel gradients, where the closure is no longer valid. The vorticity advection closure’s poor performance occurs because in the 3D case parallel currents closing through the sheath play an important role in reducing the extent to which polarisation currents are driven. In a conduction-limited or detached SOL regime however, low plasma temperatures and high neutral densities near the divertor will produce significantly higher resistivity values in the region than that used in the aforementioned 3D simulations.
Therefore the effect of increasing the normalised plasma resistivity in the last quarter of the domain nearest the targets is examined using 3D simulations. Whilst small δ⊥ filaments are observed to be relatively unaffected by this quantity, large δ⊥ filaments exhibit faster radial velocities at higher resistivity values due to two mechanisms. Firstly, parallel currents are reduced meaning that polarisation currents are necessarily enhanced and secondly, a potential difference forms along the parallel direction so that higher potentials are produced in the region of the filament for the same amount of current to flow into the sheath. This indicates that broader SOL profiles could be produced at higher values of normalised resistivity, and hence at larger reference SOL densities and at colder temperatures
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
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