1,068 research outputs found
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
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
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
L to H mode transition: parametric dependencies of the temperature threshold
The L to H mode transition occurs at a critical power which depends on various parameters, such as the magnetic field, the
density, etc. Experimental evidence on various tokamaks (JET, ASDEX-Upgrade, DIII-D, Alcator C-Mod) points towards the
existence of a critical temperature characterizing the transition. This criterion for the L-H transition is local and is therefore
easier to be compared to theoretical approaches. In order to shed light on the mechanisms of the transition, simple theoretical
ideas are used to derive a temperature threshold (Tth). They are based on the stabilization of the underlying turbulence by a
mean radial electric field shear. The nature of the turbulence varies as the collisionality decreases, from resistive ballooning modes to ion temperature gradient and trapped electron modes. The obtained parametric dependencies of the derived Tth are tested versus magnetic field, density, effective charge. Various robust experimental observations are reproduced, in particular Tth increases with magnetic field B and increases with density below the density roll-over observed on the power threshold.EURATOM 63305
Discriminating the trapped electron modes contribution in density fluctuation spectra
Quasi-coherent (QC) modes have been reported for more than 10 years in reflectometry
fluctuations spectra in the core region of fusion plasmas. They have characteristics in-between
coherent and broadband fluctuations as they oscillate at a marked frequency but have a wide
spectrum. This work presents further evidences of the link recently established between QC
modes and the trapped electron modes (TEM) instabilities (Arnichand et al 2014 Nucl. Fusion
54 123017). In electron cyclotron resonance heated discharges of Tore Supra, an enhancement
of QC modes amplitude is observed in a region where TEM cause impurity transport and
turbulence. In JET Ohmic plasmas, QC modes disappear during density ramp-up and current
ramp-down. This is reminiscent of Tore Supra and TEXTOR observations during transitions
from the linear Ohmic confinement (LOC) to the saturated Ohmic confinement (SOC)
regimes. Evidencing TEM activity then becomes experimentally possible via analysis of
fluctuation spectra.EURATOM 63305
Real-Time multichannel tokamak plasma profile simulations using the RAPTOR code and the QLKNN first-principle transport model
Neural network surrogate of QuaLiKiz using JET experimental data to populate training space
Within integrated tokamak plasma modeling, turbulent transport codes are typically the computational bottleneck limiting their routine use outside of post-discharge analysis. Neural network (NN) surrogates have been used to accelerate these calculations while retaining the desired accuracy of the physics-based models. This paper extends a previous NN model, known as QLKNN-hyper-10D, by incorporating the impact of impurities, plasma rotation, and magnetic equilibrium effects. This is achieved by adding a light impurity fractional density (n imp,light/n e) and its normalized gradient, the normalized pressure gradient (α), the toroidal Mach number (M tor), and the normalized toroidal flow velocity gradient. The input space was sampled based on experimental data from the JET tokamak to avoid the curse of dimensionality. The resulting networks, named QLKNN-jetexp-15D, show good agreement with the original QuaLiKiz model, both by comparing individual transport quantity predictions and by comparing its impact within the integrated model, JINTRAC. The profile-averaged RMS of the integrated modeling simulations is <10% for each of the five scenarios tested. This is non-trivial given the potential numerical instabilities present within the highly nonlinear system of equations governing plasma transport, especially considering the novel addition of momentum flux predictions to the model proposed here. An evaluation of all 25 NN output quantities at one radial location takes ∼0.1 ms, 104 times faster than the original QuaLiKiz model. Within the JINTRAC integrated modeling tests performed in this study, using QLKNN-jetexp-15D resulted in a speed increase of only 60–100 as other physics modules outside of turbulent transport become the bottleneck.</p
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