The role of the pedestal position on the pedestal performance has been investigated in AUG, JET-ILW and TCV. When the pedestal is peeling-ballooning (PB) limited, the three machines show a similar behaviour. The outward shift of the pedestal density leads to the outward shift of the pedestal pressure which, in turns, reduces the PB stability, degrades the pedestal confinement and reduces the pedestal width. Once the experimental density position is considered, the EPED model is able to correctly predict the pedestal height. An estimate of the impact of the density position on a ITER baseline scenario shows that the maximum reduction in the pedestal height is 10% while the reduction in the fusion power is between 10% and 40% depending on the assumptions for the core transport model used.When the pedestal is not PB limited, a different behaviour is observed. The outward shift of the density is still empirically correlated with the pedestal degradation but no change in the pressure position is observed and the PB model is not able to correctly predict the pedestal height. On the other hand, the outward shift of the density leads to a significant increase of η e (where η e is the ratio of density to temperature scale lengths, η e = L ne /L Te ) which leads to the increase of the growth rate of microinstabilities (mainly ETG and ITG) by 50%. This suggests that, when the pedestal is not PB limited, the increase in the turbulent transport due to the outward shift of the density might play an important role in the decrease of the pedestal performance.

The spectral broadening of characteristic γ-ray emission peaks from the reaction (12)C((3)He,pγ)(14)N was measured in D((3)He) plasmas of the JET tokamak with ion cyclotron resonance heating tuned to the fundamental harmonic of (3)He. Intensities and detailed spectral shapes of γ-ray emission peaks were successfully reproduced using a physics model combining the kinetics of the reacting ions with a detailed description of the nuclear reaction differential cross sections for populating the L1-L8 (14)N excitation levels yielding the observed γ-ray emission. The results provide a paradigm, which leverages knowledge from areas of physics outside traditional plasma physics, for the development of nuclear radiation based methods for understanding and controlling fusion burning plasmas.

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.

There is currently both considerable interest in the physics of ELM transport in the scrape-off layer (SOL) and concern over the impact of the ELM power and particle loads on the divertor targets of future fusion reactors. This paper describes some experimental observations of the reaction of target floating potentials and currents during ELMs in TCV, relying principally on fast measurements of these parameters using tile embedded Langmuir probe arrays. Clear evidence is presented for rapid modifications in the local target floating potentials occurring long before the characteristic rise of hydrogenic excitation emission due to local recycling provoked by the arrival at the target of the ELM ion flux. This precursor activity appears to be synchronous with the growth of MHD modes in the main chamber. Simple conditional averaging is used to derive a ‘coherent ELM’ and thus generate the radial distribution of the current to probes held at the target potential. At some locations, the coherent ELM can also be used to estimate the time evolution of the local target electron temperature, density and power flux, even though these quantities are not directly measured. The time delays between the reactions of currents, floating potentials and derived temperature are consistent with the expections of recently published kinetic simulations of ELM energy transport down a one-dimensional SOL plasma. The strong potential variations observed during the ELM are the result of current flows at the targets. These currents are generally of opposite sign at inner and outer divertors and are thus consistent, at least in part, with a thermoelectric origin in which the driven current is produced by an in/out divertor temperature asymmetry near the strike point of nearly a factor 2, probably due to the generation of divertor asymmetries by the ELM heat pulse. Such asymmetries are commonly observed in low to medium density L-modes for the particular TCV magnetic equilibrium studied in this paper. The total current balance during the ELM is satisfied only to within a factor 2, so that, whilst some of the driven current flows parallel to field lines in the SOL, there is an apparent additional negative current to the inner divertor during the ELM whose origin remains unexplained. It might, however, be due, in part, to increased plasma–wall interaction in the main chamber during the ELM event.

High-resolution γ -ray measurements were carried out on the Joint European Torus (JET) in an experiment aimed at accelerating 4 He ions in the MeV range by coupling third harmonic radio frequency heating to an injected 4 He beam. For the first time, Doppler broadening of γ -ray peaks from the 12 C(d, pγ ) 13 C and 9 Be(α, nγ ) 12 C reactions was observed and interpreted with dedicated Monte Carlo codes based on the detailed nuclear physics of the processes. Information on the confined 4 He and deuteron energy distribution was inferred, and confined 4 He ions with energies as high as 6 MeV were assessed. A signature of MHD activity in γ -ray traces was also detected. The reported results have a bearing on diagnostics for fast ions in the MeV range in next step fusion devices.