As part of a programme of intercomparison of eddy-resolving and one-dimensional (1-D) boundary-layer models, a convective boundary-layer filled with radiatively active 'smoke' was simulated. The programme is sponsored by the Global Energy and Water Experiment Cloud Systems Study. Cloud-top-cooling rates weire chosen to be comparable with those observed in marine stratocumulus, while avoiding evaporative feedbacks on entrainment and turbulence that are also important in liquid-water clouds. The radiative-cooling rate hacl a specified dependence on the smoke profile, so that differences between simulations could only be a result of different numerical representations of fluid motion and subgrid-scale turbulence. At a workshop in De Bilt, The Netherlands in August 1995, results from numerous groups around the world were compared with each other and with a previously investigated laboratory analogue to the smoke cloud.The intercomparison results show that models must be run with higher vertical resolution in the inversion than is customary at present, in order to accurately simulate the entrainment rate into cloud-topped boundaylayers under strong inversions. In three-dimensional (3-D) models using a vertical grid spacing of 5-12.5 m, sufficient to resolve the horizontal variability of inversion height, entrainment rates were 10-50% larger than ]:he range consistent with the laboratory experiments. With a larger vertical grid spacing of 25 m, 1-D, 2-D and 3-D models all overestimated the entrainment rate by more than 50%. 3-D models with monotone advection-schemes overestimated entrainment slightly more than those with non-monotone schemes, at least when 25 m vertical gridspacing was used. However, results from non-monotone schemes had several undesirable features associated hith the generation of undershoots and overshoots, most notably spurious turbulent mixing above the smoke layer. The 1 -D models tended to underestimate turbulent kinetic energy (TKE) but performed reasonably well given tkeir simplicity. 2-D models produced too much entrainment and considerably overestimated TKE, compared with 3-D models with the same numerical formulation.Based on a simple scaling-argument, we propose that the minimum vertical grid-spacing required to obtain an accurate entrainment-rate is of the order of the horizontal fluctuations in inversion height, which is proportional to the layer-averaged TKE and inversely proportional to the inversion strength.

No abstract

We present an ultrafast neural network (NN) model, QLKNN, which predicts core tokamak transport heat and particle fluxes. QLKNN is a surrogate model based on a database of 300 million flux calculations of the quasilinear gyrokinetic transport model QuaLiKiz. The database covers a wide range of realistic tokamak core parameters. Physical features such as the existence of a critical gradient for the onset of turbulent transport were integrated into the neural network training methodology. We have coupled QLKNN to the tokamak modelling framework JINTRAC and rapid control-oriented tokamak transport solver RAPTOR. The coupled frameworks are demonstrated and validated through application to three JET shots covering a representative spread of H-mode operating space, predicting turbulent transport of energy and particles in the plasma core. JINTRAC-QLKNN and RAPTOR-QLKNN are able to accurately reproduce JINTRAC-QuaLiKiz T i,e and n e profiles, but 3 to 5 orders of magnitude faster. Simulations which take hours are reduced down to only a few tens of seconds. The discrepancy in the final source-driven predicted profiles between QLKNN and QuaLiKiz is on the order 1%-15%. Also the dynamic behaviour was well captured by QLKNN, with differences of only 4%-10% compared to JINTRAC-QuaLiKiz observed at mid-radius, for a study of density buildup following the L-H transition. Deployment of neural network surrogate models in multi-physics integrated tokamak modelling is a promising route towards enabling accurate and fast tokamak scenario optimization, Uncertainty Quantification, and control applications.

No abstract

A power-balance model, with radiation losses from impurities and neutrals, gives a unified description of the density limit (DL) of the stellarator, the L-mode tokamak, and the reversed field pinch (RFP). The model predicts a Sudo-like scaling for the stellarator, a Greenwald-like scaling, , for the RFP and the ohmic tokamak, a mixed scaling, , for the additionally heated L-mode tokamak. In a previous paper (Zanca et al 2017 Nucl. Fusion 57 056010) the model was compared with ohmic tokamak, RFP and stellarator experiments. Here, we address the issue of the DL dependence on heating power in the L-mode tokamak. Experimental data from high-density disrupted L-mode discharges performed at JET, as well as in other machines, are taken as a term of comparison. The model fits the observed maximum densities better than the pure Greenwald limit.

The 2014–2016 JET results are reviewed in the light of their significance for optimising the ITER research plan for the active and non-active operation. More than 60 h of plasma operation with ITER first wall materials successfully took place since its installation in 2011. New multi-machine scaling of the type I-ELM divertor energy flux density to ITER is supported by first principle modelling. ITER relevant disruption experiments and first principle modelling are reported with a set of three disruption mitigation valves mimicking the ITER setup. Insights of the L–H power threshold in Deuterium and Hydrogen are given, stressing the importance of the magnetic configurations and the recent measurements of fine-scale structures in the edge radial electric. Dimensionless scans of the core and pedestal confinement provide new information to elucidate the importance of the first wall material on the fusion performance. H-mode plasmas at ITER triangularity (H  =  1 at βN ~ 1.8 and n/nGW ~ 0.6) have been sustained at 2 MA during 5 s. The ITER neutronics codes have been validated on high performance experiments. Prospects for the coming D–T campaign and 14 MeV neutron calibration strategy are reviewed.

As part of a programme of intercomparison of eddy-resolving and one-dimensional (1-D) boundary-layer models, a convective boundary-layer filled with radiatively active ‘smoke’ was simulated. the programme is sponsored by the Global Energy and Water Experiment Cloud Systems Study. Cloud-top-cooling rates weire chosen to be comparable with those observed in marine stratocumulus, while avoiding evaporative feedbacks on entrainment and turbulence that are also important in liquid-water clouds. the radiative-cooling rate hacl a specified dependence on the smoke profile, so that differences between simulations could only be a result of different numerical representations of fluid motion and subgrid-scale turbulence. At a workshop in De Bilt, the Netherlands in August 1995, results from numerous groups around the world were compared with each other and with a previously investigated laboratory analogue to the smoke cloud. The intercomparison results show that models must be run with higher vertical resolution in the inversion than is customary at present, in order to accurately simulate the entrainment rate into cloud-topped bounday-layers under strong inversions. In three-dimensional (3-D) models using a vertical grid spacing of 5-12.5 m, sufficient to resolve the horizontal variability of inversion height, entrainment rates were 10-50% larger than: he range consistent with the laboratory experiments. With a larger vertical grid spacing of 25 m, 1-D, 2-D and 3-D models all overestimated the entrainment rate by more than 50%. 3-D models with monotone advection-schemes overestimated entrainment slightly more than those with non-monotone schemes, at least when 25 m vertical grid-spacing was used. However, results from non-monotone schemes had several undesirable features associated hith the generation of undershoots and overshoots, most notably spurious turbulent mixing above the smoke layer. the 1-D models tended to underestimate turbulent kinetic energy (TKE) but performed reasonably well given tkeir simplicity. 2-D models produced too much entrainment and considerably overestimated TKE, compared with 3-D models with the same numerical formulation. Based on a simple scaling-argument, we propose that the minimum vertical grid-spacing required to obtain an accurate entrainment-rate is of the order of the horizontal fluctuations in inversion height, which is proportional to the layer-averaged TKE and inversely proportional to the inversion strength

Since the installation of an ITER-like wall, the JET programme has focused on the consolidation of ITER design choices and the preparation for ITER operation, with a specific emphasis given to the bulk tungsten melt experiment, which has been crucial for the final decision on the material choice for the day-one tungsten divertor in ITER. Integrated scenarios have been progressed with the re-establishment of long-pulse, high-confinement H-modes by optimizing the magnetic configuration and the use of ICRH to avoid tungsten impurity accumulation. Stationary discharges with detached divertor conditions and small edge localized modes have been demonstrated by nitrogen seeding. The differences in confinement and pedestal behaviour before and after the ITER-like wall installation have been better characterized towards the development of high fusion yield scenarios in DT. Post-mortem analyses of the plasma-facing components have confirmed the previously reported low fuel retention obtained by gas balance and shown that the pattern of deposition within the divertor has changed significantly with respect to the JET carbon wall campaigns due to the absence of thermally activated chemical erosion of beryllium in contrast to carbon. Transport to remote areas is almost absent and two orders of magnitude less material is found in the divertor.