L-H transition dynamics in fluid turbulence simulations with neoclassical force balance

Spontaneous transport barrier generation at the edge of a magnetically confined plasma is investigated. To this end, a model of electrostatic turbulence in three-dimensional geometry is extended to account for the impact of friction between trapped and passing particles on the radial electric field. Non-linear flux-driven simulations are carried out, and it is shown that considering the radial and temporal variations of the neoclassical friction coefficients allows for a transport barrier to be generated above a threshold of the input power

Angular momentum transport modeling: achievements of a gyrokinetic quasi-linear approach

QuaLiKiz, a model based on a local gyrokinetic eigenvalue solver is expanded to include momentum flux modeling in addition to heat and particle fluxes. Essential for accurate momentum flux predictions, the parallel asymmetrization of the eigenfunctions is successfully recovered by an analytical fluid model. This is tested against self-consistent gyrokinetic calculations and allows for a correct prediction of the ExB shear impact on the saturated potential amplitude by means of a mixing length rule. Hence, the effect of the ExB shear is recovered on all the transport channels including the induced residual stress. Including these additions, QuaLiKiz remains ~10 000 faster than non-linear gyrokinetic codes allowing for comparisons with experiments without resorting to high performance computing. The example is given of momentum pinch calculations in NBI modulation experiments

Comparison between measured and predicted turbulence frequency spectra in ITG and TEM regimes

The observation of distinct peaks in tokamak core reflectometry measurements - named quasi-coherent-modes (QCMs) - are identified as a signature of Trapped-Electron-Mode (TEM) turbulence [H. Arnichand et al. 2016 Plasma Phys. Control. Fusion 58 014037]. This phenomenon is investigated with detailed linear and nonlinear gyrokinetic simulations using the \gene code. A Tore-Supra density scan is studied, which traverses through a Linear (LOC) to Saturated (SOC) Ohmic Confinement transition. The LOC and SOC phases are both simulated separately. In the LOC phase, where QCMs are observed, TEMs are robustly predicted unstable in linear studies. In the later SOC phase, where QCMs are no longer observed, ITG modes are identified. In nonlinear simulations, in the ITG (SOC) phase, a broadband spectrum is seen. In the TEM (LOC) phase, a clear emergence of a peak at the TEM frequencies is seen. This is due to reduced nonlinear frequency broadening of the underlying linear modes in the TEM regime compared with the ITG regime. A synthetic diagnostic of the nonlinearly simulated frequency spectra reproduces the features observed in the reflectometry measurements. These results support the identification of core QCMs as an experimental marker for TEM turbulenc

Turbulent Transport in Tokamak Plasmas: bridging theory and experiment

In fusion devices such as tokamaks, the achievement of good energy confinement is a key issue. The energy, particle and angular momentum transport is dominated by turbulent mechanims. To understand, model and predict temperature, density and rotation is existing and future tokamaks, a numerical tool, bridging theory and experiments, is introduced. .Nonlinear gyrokinetic codes allow for detailed understanding of turbulent transport. However, their computational demand precludes their use for predictive profile modelling. An alternative approach is required to bridge the gap between theoretical understanding and prediction of experiments. A quasi-linear gyrokinetic model, QuaLiKiz, allows for a 1 million speedup while retaining key physics. Indeed, in the tokamak plasma core, relatively low levels of turbulence are reported, typically below 10%. It is further shown that the nonlinear phase shift is close to the linear phase shift and that the frequency broadening observed in nonlinear simulations typically follows the linear growth rate. Therefore quasilinear gyrokinetic turbulent transport can be used to efficiently model fluxes in integrated modelling platforms. The saturated potential is constructed based on nonlinear simulation results and turbulence measurements. The predicted particle, heat and angular momentum fluxes have been compared successfully to nonlinear fluxes in a wide range of parameters using the quasilinear gyrokinetic code QuaLiKiz. In terms of CPU time, a factor one million is gained compared with nonlinear modelling. This allows for extensive interpretative and predictive applications.The simplified model also stimulates the development of theoretical understanding, since its construction relies on a deep understanding of the nonlinear physical mechanisms. Such work is hence at the cross-roads between experimental observations and detailed theoretical investigations.The quasilinear fluxes are compared to experimental observations at a given time. In particular, experimental observations of trace Nickel transport in the Tore Supra tokamak are successfully compared to the quasilinear predictions. Finally the model transport quantities (heat, particles and angular momentum) are used in a time evolving platform to predict temperature, density and rotation profiles. Predicted temperature and density profiles are successfully compared to experiments carried out on the JET tokamak. The successes and limits of the quasilinear approach are reviewed. Perspectives are given in the discussion section

Nonlinear stabilization of tokamak microturbulence by fast ions

Nonlinear electromagnetic stabilization by suprathermal pressure gradients found in specific regimes is shown to be a key factor in reducing tokamak microturbulence, augmenting significantly the thermal pressure electromagnetic stabilization. Based on nonlinear gyrokinetic simulations investigating a set of ion heat transport experiments on the JET tokamak, described by Mantica et al. [Phys. Rev. Lett. 107 135004 (2011)], this result explains the experimentally observed ion heat flux and stiffness reduction. These findings are expected to improve the extrapolation of advanced tokamak scenarios to reactor relevant regimes.Comment: 5 pages, 5 figure

Propellant Sloshing Torque H ∞ -based Observer Design for Enhanced Attitude Control

International audienceIn this paper a control-oriented LPV model of the sloshing torque arising during attitude maneuvers, supported by Computational Fluid Dynamics results, is presented and used for attitude control design. The proposed strategy essentially relies on the design of a robust LPV-based disturbance torque observer with the help of the structured multi-model H ∞ synthesis framework. The estimated torque is then used to improve a satisfying attitude controller initially designed without sloshing. The stability of the parameter-varying closed-loop system is finally proved with parameter-dependent Lyapunov functions

Angular momentum transport modeling: achievements of a gyrokinetic quasi-linear approach

International audienceQuaLiKiz, a model based on a local gyrokinetic eigenvalue solver is expanded to include momentum flux modeling in addition to heat and particle fluxes. Essential for accurate momentum flux predictions, the parallel asymmetrization of the eigenfunctions is successfully recovered by an analytical fluid model. This is tested against self-consistent gyrokinetic calculations and allows for a correct prediction of the E×B shear impact on the saturated potential amplitude by means of a mixing length rule. Hence, the effect of the E×B shear is recovered on all the transport channels including the induced residual stress. Including these additions, QuaLiKiz remains ∼10 000 faster than non-linear gyrokinetic codes allowing for comparisons with experiments without resorting to high performance computing. The example is given of momentum pinch calculations in NBI modulation experiments

First principle integrated modeling of multi-channel transport including Tungsten in JET

For the first time, over five confinement times, the self-consistent flux driven time evolution of heat, momentum transport and particle fluxes of electrons and multiple ions including Tungsten (W) is modeled within the integrated modeling platform JETTO (Romanelli et al 2014 Plasma Fusion Res. 9 1–4), using first principle-based codes: namely, QuaLiKiz (Bourdelle et al 2016 Plasma Phys. Control. Fusion 58 014036) for turbulent transport and NEO (Belli and Candy 2008 Plasma Phys. Control. Fusion 50 95010) for neoclassical transport. For a JET-ILW pulse, the evolution of measured temperatures, rotation and density profiles are successfully predicted and the observed W central core accumulation is obtained. The poloidal asymmetries of the W density modifying its neoclassical and turbulent transport are accounted for. Actuators of the W core accumulation are studied: removing the central particle source annihilates the central W accumulation whereas the suppression of the torque reduces significantly the W central accumulation. Finally, the presence of W slightly reduces main ion heat turbulent transport through complex nonlinear interplays involving radiation, effective charge impact on ITG and collisionality.EURATOM 63305