This paper reports recent progress in the field of γ-ray diagnosis of fast ions in the JET tokamak. The γ-rays, born in nuclear reactions between fast ions and main plasma impurities and/or plasma fuel ions, are analysed with a new modelling tool (the GAMMOD code) that has been developed for a quantitative analysis of the measured γ-ray energy spectra. The analysis of the γ-ray energy spectra identifies the different fast ions giving rise to the γ-ray emission and assesses the effective tail temperatures and relative concentrations of these fast ions. This assessment is possible, since the excitation functions for the different nuclear reactions are well established and exhibit a threshold or/and a resonant nature. The capabilities of the γ-ray spectral analysis are illustrated with the examples from the recent γ-ray diagnostic measurements of 4He, 3He, deuterium and hydrogen ions accelerated by ion-cyclotron resonance frequency heating in JET. Simultaneous measurements of several fast ion species, including highly energetic α-particles, are demonstrated. In addition to the γ-spectroscopy, tomographic reconstructions of the radial profile of the γ-ray emission are performed using the JET neutron profile monitor, thus providing direct measurements of the radial profiles of fast ions in JET.

Gamma-ray images of fast D- and 4He-ions accelerated with third-harmonic ion-cyclotron-resonance heating of 4He-beam were simultaneously recorded for the first time in JET tokamak experiments dedicated to the investigation of burning plasmas with 3.5 MeV fusion alpha (α) particles. Gamma (γ) rays, born as a result of nuclear reactions, 9Be(4He, nγ)12C and 12C(D, pγ)13C, between the fast ions and the main plasma impurities, are measured using a two-dimensional multicollimator spectrometer array, which distinguishes the γ-rays from accelerated D- and 4He-ions. Tomographic reconstruction of the γ-ray emission profiles gives images of the fast-ion population in the poloidal cross-section. The potential of this technique to visualize several energetic ion species and to determine their behaviour in different plasma scenarios is demonstrated.

In 2003, the performance of the ‘hybrid’ regime was successfully validated in JET experiments up to βN = 2.8 at low toroidal field (1.7 T), with plasma triangularity and normalized Larmor radius (ρ*) corresponding to identical ASDEX Upgrade discharges. Stationary conditions have been achieved with the fusion figure of merit ( ) reaching 0.42 at q95 = 3.9. The JET discharges show similar MHD, edge and current profile behaviour, when compared with the ASDEX Upgrade. In addition, the JET experiments have extended the hybrid scenario operation at higher toroidal field of 2.4 T and lower ρ* towards the projected ITER values. Using this database, transport and confinement properties are characterized with respect to the standard H-mode regime. Moreover, trace tritium has been injected to assess the diffusion and convective coefficients of the fusion fuel. The maximization of confinement and stability properties provides, to this scenario, a good probability of achieving a high fusion gain at reduced plasma current for durations of up to 2000 s in ITER.

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.

An overview is given of the experimental method, the analysis technique and the results for trace tritium experiments conducted on the JET tokamak in 2003. Observations associated with events such as sawtooth collapses, neo-classical tearing modes and edge localized modes are described. Tritium transport is seen to approach neo-classical levels in the plasma core at high density and low q 95 , and in the transport barrier region of internal transport barrier (ITB) discharges. Tritium transport remains well above neo-classical levels in all other cases. The correlation of the measured tritium diffusion coefficient and convection velocity for normalized minor radii r/a = [0.65, 0.80] with the controllable parameters q 95 and plasma density are found to be consistent for all operational regimes (ELMy H-mode discharges with or without ion cyclotron frequency resonance heating, hybrid scenario and ITB discharges). Scaling with local physics parameters is best described by gyro-Bohm scaling with an additional inverse beta dependence.

Gamma-ray spectra from nuclear reactions between fusion-born alpha (alpha) particles and Be impurities were measured for the first time in deuterium-tritium plasmas in the Joint European Torus. The time dependence of the measured spectra allowed the determination of the density evolution of fast alpha particles. Correlation between the decay time of the gamma-ray emission and the plasma parameters in different plasma scenarios was established. Results are consistent with classical slowing down of the alpha particles in discharges with high plasma currents and monotonic q-profiles. In low plasma current discharges and in the discharges with large on-axis current holes (extreme reversal central magnetic shear), the gamma-ray emission decay times are shorter than the classical slowing down times, indicating an alpha-particle confinement degradation in such discharges in line with theoretical predictions.

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 JET 2019-2020 scientific and technological programme exploited the results of years of concerted scientific and engineering work, including the ITER-like wall (ILW: Be wall and W divertor) installed in 2010, improved diagnostic capabilities now fully available, a major Neutral Beam Injection (NBI) upgrade providing record power in 2019-2020, and tested the technical & procedural preparation for safe operation with tritium. Research along three complementary axes yielded a wealth of new results. Firstly, the JET plasma programme delivered scenarios suitable for high fusion power and alpha particle physics in the coming D-T campaign (DTE2), with record sustained neutron rates, as well as plasmas for clarifying the impact of isotope mass on plasma core, edge and plasma-wall interactions, and for ITER pre-fusion power operation. The efficacy of the newly installed Shattered Pellet Injector for mitigating disruption forces and runaway electrons was demonstrated. Secondly, research on the consequences of long-term exposure to JET-ILW plasma was completed, with emphasis on wall damage and fuel retention, and with analyses of wall materials and dust particles that will help validate assumptions and codes for design & operation of ITER and DEMO. Thirdly, the nuclear technology programme aiming to deliver maximum technological return from operations in D, T and D-T benefited from the highest D-D neutron yield in years, securing results for validating radiation transport and activation codes, and nuclear data for ITER.