72 research outputs found
Gyrokinetic studies of core turbulence features in ASDEX Upgrade H-mode plasmas
Gyrokinetic validation studies are crucial in developing confidence in the
model incorporated in numerical simulations and thus improving their predictive
capabilities. As one step in this direction, we simulate an ASDEX Upgrade
discharge with the GENE code, and analyze various fluctuating quantities and
compare them to experimental measurements. The approach taken is the following.
First, linear simulations are performed in order to determine the turbulence
regime. Second, the heat fluxes in nonlinear simulations are matched to
experimental fluxes by varying the logarithmic ion temperature gradient within
the expected experimental error bars. Finally, the dependence of various
quantities with respect to the ion temperature gradient is analyzed in detail.
It is found that density and temperature fluctuations can vary significantly
with small changes in this parameter, thus making comparisons with experiments
very sensitive to uncertainties in the experimental profiles. However,
cross-phases are more robust, indicating that they are better observables for
comparisons between gyrokinetic simulations and experimental measurements
Fast transport simulations with higher-fidelity surrogate models for ITER
A fast and accurate turbulence transport model based on quasilinear gyrokinetics is developed. The model consists of a set of neural networks trained on a bespoke quasilinear GENE dataset, with a saturation rule calibrated to dedicated nonlinear simulations. The resultant neural network is approximately eight orders of magnitude faster than the original GENE quasilinear calculations. ITER predictions with the new model project a fusion gain in line with ITER targets. While the dataset is currently limited to the ITER baseline regime, this approach illustrates a pathway to develop reduced-order turbulence models both faster and more accurate than the current state-of-the-art.</p
Fast transport simulations with higher-fidelity surrogate models for ITER
A fast and accurate turbulence transport model based on quasilinear
gyrokinetics is developed. The model consists of a set of neural networks
trained on a bespoke quasilinear GENE dataset, with a saturation rule
calibrated to dedicated nonlinear simulations. The resultant neural network is
approximately eight orders of magnitude faster than the original GENE
quasilinear calculations. ITER predictions with the new model project a fusion
gain in line with ITER targets. While the dataset is currently limited to the
ITER baseline regime, this approach illustrates a pathway to develop
reduced-order turbulence models both faster and more accurate than the current
state-of-the-art
Electromagnetic stabilization of tokamak microturbulence in a high- regime
The impact of electromagnetic stabilization and flow shear stabilization on
ITG turbulence is investigated. Analysis of a low- JET L-mode discharge
illustrates the relation between ITG stabilization, and proximity to the
electromagnetic instability threshold. This threshold is reduced by
suprathermal pressure gradients, highlighting the effectiveness of fast ions in
ITG stabilization. Extensive linear and nonlinear gyrokinetic simulations are
then carried out for the high- JET hybrid discharge 75225, at two
separate locations at inner and outer radii. It is found that at the inner
radius, nonlinear electromagnetic stabilization is dominant, and is critical
for achieving simulated heat fluxes in agreement with the experiment. The
enhancement of this effect by suprathermal pressure also remains significant.
It is also found that flow shear stabilization is not effective at the inner
radii. However, at outer radii the situation is reversed. Electromagnetic
stabilization is negligible while the flow shear stabilization is significant.
These results constitute the high- generalization of comparable
observations found at low- at JET. This is encouraging for the
extrapolation of electromagnetic ITG stabilization to future devices. An
estimation of the impact of this effect on the ITER hybrid scenario leads to a
20% fusion power improvement.Comment: 10 pages, 13 figures. Paper coupled to invited talk at the 41st EPS
conference, Berlin, 201
Integrated modelling and multiscale gyrokinetic validation study of ETG turbulence in a JET hybrid H-mode scenario
Previous studies with first-principle-based integrated modelling suggested
that ETG turbulence may lead to an anti-GyroBohm isotope scaling in JET
high-performance hybrid H-mode scenarios. A dedicated comparison study against
higher-fidelity turbulence modelling invalidates this claim. Ion-scale
turbulence with magnetic field perturbations included, can match the power
balance fluxes within temperature gradient error margins. Multiscale
gyrokinetic simulations from two distinct codes produce no significant ETG heat
flux, demonstrating that simple rules-of-thumb are insufficient criteria for
its onset
Quasi-symmetry and the nature of radial turbulent transport in quasi-poloidal stellarators
Quasi-symmetric configurations have a better neoclassical confinement compared to that of standard stellarators. The reduction of the neoclassical viscosity along the direction of quasi-symmetry should facilitate the self-generation of zonal flows and, consequently, the mitigation of turbulent fluctuations and the ensuing radial transport. Therefore, it is expected that quasi-symmetries should also result in better confinement properties regarding radial turbulent transport. In this paper we show that, at least for quasi-poloidal configurations, the influence of quasi-symmetry on radial transport exceeds the expected reduction of fluctuation levels and associated effective transport coefficients, and that the intimate nature of transport itself is affected. In particular, radial turbulent transport becomes increasingly subdiffusive as the degree of quasi-symmetry becomes larger. This behavior is somewhat reminiscent of what has been previously reported in tokamaks with strong radially sheared zonal flows. Published by AIP Publishing.Research funded in part by the Spanish National Project Nos. ENE2012-33219 and ENE2012-31753. Research supported in part by the DOE Office of Science Grant No. DE-FG02-04ER5741 at the University of Alaska. Research carried out in part at the InstitĂĽt fĂĽr Plasmaphysik of the Max-Planck InstitĂĽt in Greifswald (Germany), whose hospitality is gratefully acknowledged. Fruitful interactions with members of the ABIGMAP research network, funded by the Spanish National Project No. MAT2015-69777-REDT, is also acknowledged. Gene simulations have been possible thanks in part to a continued grant (Nos. FI-2014-1-0021, FI-2014-2-0026, FI-2014-3-0012, and FI-2015-1-0011) to use resources from the MareNostrum supercomputer at BSC (Barcelona, Spain). Gene and TRACER runs have also been carried out in Uranus, a supercomputer cluster located at Universidad Carlos III de Madrid (Spain) funded jointly by EU FEDER funds and by the Spanish Government via the National Project Nos. UNC313-4E-2361, ENE2009-12213-C03-03, ENE2012-33219, and ENE2012-31753
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