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.

Energetic ion driven Alfvén eigenmodes (AEs) are believed to be an important element disturbing the transport in a future fusion reactor. The studies of the AE properties in modern toroidal devices have made crucial contributions to the reactor relevant physics. AEs are conventionally studied by magnetic probes (MPs), which provide the poloidal m and toroidal n mode numbers and their spectral characteristics. Heavy ion beam probing (HIBP) has become a new tool to study AEs with high spatial and frequency resolution. HIBP in the TJ-II heliac observes locally (∼1 cm) resolved AEs over the whole radial interval. The set of low-m (m < 8) modes, detected with the high-frequency resolution (<5 kHz), present different types of AEs. AEs are pronounced in the local density, electric potential and poloidal magnetic field oscillations, detected simultaneously by HIBP in the frequency range 50 kHz < f AE < 300 kHz. Various AE modes are visible in the neutral beam injector (NBI)-heated plasma for co-NBI (<450 kW), counter- (<450 kW) and balanced NBI (<900 kW) from the plasma centre to the edge. A high coherence between MP and HIBP data was found for specific AEs. When the density rises, AE frequency decreases, , and the cross-phase between the plasma density, poloidal magnetic field and potential remains constant. The amplitude of the AE potential oscillations δφAE ∼ 10 V was estimated. Poloidally resolved density and potential measurements may provide information about the AE poloidal wavelength and the AE contribution to the poloidal electric field E pol and the turbulent particle flux 𝚪 E × B . The typical range of E pol oscillations for AEs is . Depending on the δn e and δE pol amplitudes and cross-phase, AEs may make a small or a significant contribution to the turbulent particle flux 𝚪 E × B for the observed wavenumbers k θ < 3 cm−1.

Alfvén eigenmodes (AEs) were studied in neutral beam injection (NBI) heated plasmas in the TJ-II stellarator using a heavy ion beam probe (HIBP) in the core, and by Langmuir (LP) and Mirnov probes (MP) at the edge. AEs were detected over the whole plasma radius by the HIBP with a spatial resolution of about 1 cm. AE-induced oscillations were detected in the plasma density n e, electric potential φ and poloidal magnetic field B pol with frequencies 50 kHz < f AE < 300 kHz. The LP, MP and HIBP data showed a high level of coherency for specific branches of AEs. Poloidal mode wave-vectors k θ , mode numbers m(m < 8) and propagation velocities V θ  ∼ 30 km s−1 were detected for various branches of AEs, having different radial locations. When the density rose due to NBI fuelling, the AE frequency decreased as predicted by the Alfvén law . During the AE frequency decay the following new AE features were observed: (i) the poloidal wave-vector k θ and mode number m remained constant, (ii) the cross-phases between the oscillations in B pol, n e and electric potential remained constant, having an individual value for each AE branch, (iii) V θ decreased proportional to the AE frequency. The interaction of the AEs with the bulk (thermal) plasma resulted in clearly pronounced quasi-coherent peaks in the electrostatic turbulent particle flux spectra. Various AE branches exhibited different contributions to the particle flux: outward, inward and also zero, depending on the phase relations between the oscillations in E pol and n e, which are specific for each branch. A comparison with MHD mode modelling indicated that some of the more prominent frequency branches can be identified as radially extended helical AEs.

Excitation of modes in the Alfvénic frequency range, 30 kHz < fAE < 300 kHz, was observed in hydrogen plasma heated by hydrogen neutral beam injection (NBI) in the TJ-II heliac. Co-field and counter-field NBI were injected, and the components of the poloidal magnetic field were varied one by one and in combinations, in order to investigate the beam-driven modes over an extended range of the rotational transform values, . Taking advantage of the unique TJ-II capabilities, a dynamic magnetic configuration experiment with variation during discharges has shown strong effects on the mode frequency via both vacuum changes and induced net plasma current. A drastic frequency increase from ∼50 to ∼250 kHz was observed for some modes when plasma current as low as ±2 kA was induced by small (10%) changes in the vertical field. A comprehensive set of diagnostics including a heavy ion beam probe, magnetic probes and a multi-chord bolometer made it possible to identify the spatial spread of the modes and deduce the internal amplitudes of their plasma density and magnetic field perturbations. A simple analytical model for fAE, based on the local Alfvén eigenmode (AE) dispersion relation, was proposed to characterize the observation. It was shown that all the observations, including vacuum iota and plasma current variations, may be fitted by the model, so the linear mode frequency dependence on (plasma current) and one over square root density dependence present the major features of the NBI-induced AEs in TJ-II, and provide the framework for further experiment-to-theory comparison.