Plasma turbulence simulations of the tokamak scrape-off layer

Federico David Halpern, Augustin Joëssel, Sébastien Jolliet, Paolo Ricci, Fabio Riva, Trach-Minh Tran, Christoph Wersal
2014
Discussion par affiche

Résumé

In the tokamak scrape-off layer (SOL) the magnetic field lines are open, channeling particles and heat onto plasma facing components, and constraining their lifetime. Therefore, safe operation of future fusion reactors requires an understanding of SOL plasma dynamics. Recently, we have gained deep insights into the tokamak SOL dynamics by means of massively-parallel fluid electromagnetic turbulence simulations carried out with the Global Braginskii Solver (GBS) code. GBS is currently capable of performing full-size SOL simulations of medium-size tokamaks such as TCV or Alcator C-Mod. In the present paper, we emphasize recent numerical developments aimed towards (a) more realistic description of the plasma geometry and (b) simulating larger, reactor-class machines. First, we address the numerical implementation of parallel advection and diffusion operators in finite difference representation. This is a crucial aspect of our computation, as the cost of the simulation can be drastically reduced if the parallel dynamics are discretized adequately taking advantage of the strong anisotropy of the turbulent modes that are aligned to the magnetic field lines. Second, we address the development and implementation of a matrix-free, parallel multigrid solver for the Poisson operator in the vorticity equation. Using this new solver, it is now possible to further parallelize GBS without breaking parallel scalability, an important step towards the realm of 10^4 CPUs. Finally, we summarize the understanding of SOL turbulence obtained through GBS simulations - for instance, the mechanisms regulating the turbulence level and therefore the SOL width, the turbulent regimes, and the mechanisms driving plasma rotation in this region. The simulated non-linear dynamics have been compared with analytical estimates, which have highlighted the key physics mechanisms at play in the SOL, and with experimental measurements taken in a number of tokamak worldwide, showing good agreement.

Official source

https://infoscience.epfl.ch/record/200340?ln=fr

À propos de ce résultat

Cette page est générée automatiquement et peut contenir des informations qui ne sont pas correctes, complètes, à jour ou pertinentes par rapport à votre recherche. Il en va de même pour toutes les autres pages de ce site. Veillez à vérifier les informations auprès des sources officielles de l'EPFL.

Concepts associés

Chargement

Publications associées

Chargement

Federico David Halpern, Augustin Joëssel, Sébastien Jolliet, Paolo Ricci, Fabio Riva, Trach-Minh Tran, Christoph Wersal
2014
Discussion par affiche

Résumé

Official source

https://infoscience.epfl.ch/record/200340?ln=fr

À propos de ce résultat

Concepts associés (22)

Concepts associés

Chargement

Publications associées (133)

Publications associées

Chargement

, , ,

Simulating the most external plasma region of a tokamak, the scrape-off layer (SOL), is of crucial importance in the way towards a fusion reactor as heat load on the vessel wall, impurity generation, and overall plasma confinement, all depend on the plasma dynamics in this region. In the last few years a numerical code, GBS, has been developed for solving the drift-reduced Braginskii equations, and describe turbulence in the tokamak SOL. In the present work, we re-formulate these equations in a general form that enables to treat diverted configuration with X-point. This is obtained by relaxing the requirement of using a coordinate system that follows the magnetic field lines. Within this alternative coordinate system, the drift-reduced Braginskii equations are then solved by using an high order numerical scheme. We show initial simulations carried out with GBS that describe well the expected field aligned turbulence structures.

2016

Chargement

, , ,

The turbulent plasma dynamics in the periphery of a tokamak plays a key role in determining its overall performances. In particular, it governs the confinement properties of the device, through the formation of a transport barrier associated with the L-H transition, and it controls the heat load on the vessel walls. GBS [1] is a three-dimensional two-fluid turbulence code, based on the drift-reduced Braginskii equations, which allows the simulation of plasma turbulence in this tokamak region. A non-field aligned algorithm has been recently implemented in GBS, to allow simulations in diverted configurations, such as the single- and double-null. Furthermore, simulations in innovative exhaust configurations, such as the snowflake, are being performed. The results of GBS simulations in single-null configurations are used to investigate the processes determining the radial electric field at the plasma edge and the related formation of a transport barrier. In particular, we show the presence of two different turbulent transport regimes driven by Kelvin-Helmholtz and resistive ballooning instability, respectively. A transition between the two regimes is obtained by changing the power source, which leads to a strong ExB shear and to the onset of a transport barrier at the plasma edge. The ExB shear provides a saturation mechanism for the resistive ballooning instability while destabilising a Kelvin-Helmholtz mode that becomes the main drive for turbulent transport. The transition between the two regimes leads to a steepening of the pressure profile and improved plasma confinement. We derive an analytical expression for the pressure gradient length in both regimes. The simulation and analytical results are in good agreement. The analysis is then extended to the SOL where we highlight the effect of edge turbulence on the SOL width and therefore on the heat load on the vessel walls. Finally, the results of simulations of alternative divertor configurations, which are considered for DEMO, are analysed. The analysis focuses on four different magnetic configurations: the ideal snowflake, the snowflake plus, the snowflake minus with the secondary X-point in the high field side and the snowflake minus with the secondary X-point in the low field side. For all the different geometries, the SOL width and the heat flux to the vessel walls are computed and the physics behind them analysed. A comparison between the single-null configuration and the four considered advanced configurations is shown.

2019

Chargement

Model-based optimization of magnetic control in the TCV tokamak: design and experiments

Federico Pesamosca

Thermonuclear controlled fusion is a promising answer to the current energy and climate issues, providing a safe carbon-free source of energy which is virtually inexhaustible. In magnetic confinement thermonuclear fusion based on tokamak reactors, hydrogen fuel in the state of plasma is confined using a system of external and self-generated magnetic fields. This thesis contributes to the development of magnetic confinement fusion research by applying techniques derived from control engineering to designing magnetic controllers for tokamak plasmas. Specifically, a new design for feedback control of plasma shape and position in the TCV tokamak is provided, and its efficacy is studied in dedicated simulations and experiments.Elongated plasmas lead to improved plasma performance in tokamaks, which is key to sustaining fusion conditions in reactors. The required magnetic field for shaping the plasma column however results in an unstable equilibrium that makes feedback control of the vertical plasma position mandatory. Active stabilization of the axisymmetric plasma vertical instability is a standard feature of elongated tokamaks and will be a fundamental feature in the ITER magnetic control system, since a loss of vertical control and the subsequent plasma disruption can lead to unacceptable heat loads on the plasma facing components.The TCV tokamak is the ideal benchmark for investigating the effect of plasma shaping on tokamak physics and performance, with its system of 16 independently powered poloidal field coils and a vessel with an elongated cross section. Shape and position control are coupled problems in TCV as they share the same poloidal field coils as actuators, requiring a multivariable approach to designing magnetic controllers.In this thesis, controller design for TCV is based on a model for the coupled plasma-vessel-coils electromagnetic dynamics: the RZIp model. In this axisymmetric model, the plasma current distribution is fixed but is free to move radially and vertically in the poloidal plane of a toroidal reference frame. An extension to this model is suggested, relaxing the assumption of rigid displacement in the radial direction to include plasma shape deformation and leading to a semi-rigid RZIp model which better fits numerical equilibria.An improvement to the existing algorithm for shape and position control in TCV is then proposed. In this new approach, tuning of the plasma position controller, in charge of vertical stabilization, can be performed independently of the shape controller, which itself acts on a stable system. Static decoupling is achieved and the shape controller is designed on the basis of an improved model for the plasma deformation, which includes the plasma contribution to the static magnetic flux perturbation. Simulations in closed loop with the RZIp model are provided to evaluate several optimized schemes.Finally, the vertical controller is optimized including the plasma dynamics as part of the controller design. Structured H-infinity, extending classical H-infinity to fixed-structure control systems, is applied to obtain a controller using all available coils for position control, and in particular a coil combination optimized for vertical stabilization. Closed-loop performance improvement is demonstrated in dedicated TCV experiments, confirming the simulation results and paving the way for the routine integration of the optimized position and shape controller in TCV discharges.

EPFL2021

Chargement

Model-based optimization of magnetic control in the TCV tokamak: design and experiments

Federico Pesamosca

EPFL2021

, , ,

2019