688 research outputs found
Estimate of convection-diffusion coefficients from modulated perturbative experiments as an inverse problem
The estimate of coefficients of the Convection-Diffusion Equation (CDE) from
experimental measurements belongs in the category of inverse problems, which
are known to come with issues of ill-conditioning or singularity. Here we
concentrate on a particular class that can be reduced to a linear algebraic
problem, with explicit solution. Ill-conditioning of the problem corresponds to
the vanishing of one eigenvalue of the matrix to be inverted. The comparison
with algorithms based upon matching experimental data against numerical
integration of the CDE sheds light on the accuracy of the parameter estimation
procedures, and suggests a path for a more precise assessment of the profiles
and of the related uncertainty. Several instances of the implementation of the
algorithm to real data are presented.Comment: Extended version of an invited talk presented at the 2012 EPS
Conference. To appear in Plasma Physics and Controlled Fusio
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
Real time plasma current and elongation control using ECRH actuators
Experiments have been carried out on TCV using Electron Cyclotron Resonance Heating (ECRH) actuators to control the plasma current and elongation in real time. In fully non-inductive plasmas, the plasma current may be driven entirely by ECCD on TCV. By replacing the Ohmic coils with the gyrotron power supplies, we were able to control the plasma current in real time. In tokamak plasmas, the elongation is usually controlled by the quadrupole magnetic field from the poloidal field coils. Applying off-axis ECRH, the current profile is flattened reducing the plasma internal inductance, and at constant quadrupole field, the plasma elongates (κ ~ 2.4) [1]. This paper reports on experiments to control the plasma elongation using a real time elongation observer and the ECRH power actuator to control the current profile. As the plasma elongates, the ECRH deposition becomes more central. Therefore a deposition tracking control was developed which tracked the ECRH deposition at constant ρ by controlling the ECRH launcher mirror position
Erratum: "Gyrokinetic simulations including the centrifugal force in a rotating tokamak plasma" [Phys. Plasmas 17, 102305 (2010)]
Validation of gyrokinetic modelling of light impurity transport including rotation in ASDEX Upgrade
Upgraded spectroscopic hardware and an improved impurity concentration
calculation allow accurate determination of boron density in the ASDEX Upgrade
tokamak. A database of boron measurements is compared to quasilinear and
nonlinear gyrokinetic simulations including Coriolis and centrifugal rotational
effects over a range of H-mode plasma regimes. The peaking of the measured
boron profiles shows a strong anti-correlation with the plasma rotation
gradient, via a relationship explained and reproduced by the theory. It is
demonstrated that the rotodiffusive impurity flux driven by the rotation
gradient is required for the modelling to reproduce the hollow boron profiles
at higher rotation gradients. The nonlinear simulations validate the
quasilinear approach, and, with the addition of perpendicular flow shear,
demonstrate that each symmetry breaking mechanism that causes momentum
transport also couples to rotodiffusion. At lower rotation gradients, the
parallel compressive convection is required to match the most peaked boron
profiles. The sensitivities of both datasets to possible errors is
investigated, and quantitative agreement is found within the estimated
uncertainties. The approach used can be considered a template for mitigating
uncertainty in quantitative comparisons between simulation and experiment.Comment: 19 pages, 11 figures, accepted in Nuclear Fusio
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
Isotope dependence of energy, momentum and particle confinement in tokamaks
The isotope dependence of plasma transport will have a significant impact on the performance of future D-T experiments in JET and ITER and eventually on the fusion gain and economics of future reactors. In preparation for future D-T operation on JET, dedicated experiments and comprehensive transport analyses were performed in H, D and H-D mixed plasmas. The analysis of the data has demonstrated an unexpectedly strong and favourable dependence of the global confinement of energy, momentum and particles in ELMy H-mode plasmas on the atomic mass of the main ion species, the energy confinement time scaling as τE∼A0.5 (Maggi et al., Plasma Phys. Control. Fusion, vol. 60, 2018, 014045; JET Team, Nucl. Fusion, vol. 39, 1999, pp. 1227–1244), i.e. opposite to the expectations based only on local gyro-Bohm (GB) scaling, τE∼A−0.5 , and stronger than in the commonly used H-mode scaling for the energy confinement (Saibene et al., Nucl. Fusion, vol. 39, 1999, 1133; ITER Physics Basis, Nucl. Fusion, vol. 39, 1999, 2175). The scaling of momentum transport and particle confinement with isotope mass is very similar to that of energy transport. Nonlinear local GENE gyrokinetic analysis shows that the observed anti-GB heat flux is accounted for if collisions, E × B shear and plasma dilution with low-Z impurities (9Be) are included in the analysis (E and B are, respectively the electric and magnetic fields). For L-mode plasmas a weaker positive isotope scaling τE∼A0.14 has been found in JET (Maggi et al., Plasma Phys. Control. Fusion, vol. 60, 2018, 014045), similar to ITER97-L scaling (Kaye et al., Nucl. Fusion, vol. 37, 1997, 1303). Flux-driven quasi-linear gyrofluid calculations using JETTO-TGLF in L-mode show that local GB scaling is not followed when stiff transport (as is generally the case for ion temperature gradient modes) is combined with an imposed boundary condition taken from the experiment, in this case predicting no isotope dependence. A dimensionless identity plasma pair in hydrogen and deuterium L-mode plasmas has demonstrated scale invariance, confirming that core transport physics is governed, as expected, by the 4 dimensionless parameters ρ*, ν*, β, q (normalised ion Larmor radius, collisionality, plasma pressure and safety factor) consistently with global quasi-linear gyrokinetic TGLF calculations (Maggi et al., Nucl. Fusion, vol. 59, 2019, 076028). We compare findings in JET with those in different devices and discuss the possible reasons for the different isotope scalings reported from different devices. The diversity of observations suggests that the differences may result not only from differences affecting the core, e.g. heating schemes, but are to a large part due to differences in device-specific edge and wall conditions, pointing to the importance of better understanding and controlling pedestal and edge processes.EUROfusion Consortium grant agreement No 63305
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
The Problem of Marginality in Model Reductions of Turbulence
Reduced quasilinear (QL) and nonlinear (gradient-driven) models with scale
separations, commonly used to interpret experiments and to forecast turbulent
transport levels in magnetised plasmas are tested against nonlinear models
without scale separations (flux-driven). Two distinct regimes of turbulence --
either far above threshold or near marginal stability -- are investigated with
Boltzmann electrons. The success of reduced models especially hinges on the
reproduction of nonlinear fluxes. Good agreement between models is found above
threshold whilst reduced models would significantly underpredict fluxes near
marginality, overlooking mesoscale flow organisation and turbulence
self-advection. Constructive prescriptions whereby to improve reduced models is
discussed
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