Progress in thermonuclear fusion energy research based on deuterium plasmas magnetically confi ned in toroidal tokamak devices requires the development of effi cient current drive methods. Previous experiments have shown that plasma current can be driven effectively by externally launched radio frequency power coupled to lower hybrid plasma waves. However, at the high plasma densities required for fusion power plants, the coupled radio frequency power does not penetrate into the plasma core, possibly because of strong wave interactions with the plasma edge. Here we show experiments performed on FTU (Frascati Tokamak Upgrade) based on theoretical predictions that nonlinear interactions diminish when the peripheral plasma electron temperature is high, allowing signifi cant wave penetration at high density. The results show that the coupled radio frequency power can penetrate into high-density plasmas due to weaker plasma edge effects, thus extending the effective range of lower hybrid current drive towards the domain relevant for fusion reactors.

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.

Alpha particles with energies on the order of megaelectronvolts will be the main source of plasma heating in future magnetic confinement fusion reactors. Instead of heating fuel ions, most of the energy of alpha particles is transferred to electrons in the plasma. Furthermore, alpha particles can also excite Alfvénic instabilities, which were previously considered to be detrimental to the performance of the fusion device. Here we report improved thermal ion confinement in the presence of megaelectronvolts ions and strong fast ion-driven Alfvénic instabilities in recent experiments on the Joint European Torus. Detailed transport analysis of these experiments reveals turbulence suppression through a complex multi-scale mechanism that generates large-scale zonal flows. This holds promise for more economical operation of fusion reactors with dominant alpha particle heating and ultimately cheaper fusion electricity.

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Spontaneous increases in plasma density, up to ∼1.6 times the Greenwald value, are observed in FTU with lithized walls. These plasmas are characterized by profile peaking up to the highest obtained densities. The transport analysis of these discharges shows a 20% enhancement of the energy confinement time, with respect to the ITER97 L-mode scaling, correlated with a threshold in the peaking factor. It has been found that 0.4 MW of ECRH power, coupled at q = 2 surface, are sufficient to avoid disruptions in 0.5 MA discharges. Direct heating of magnetic islands produced by MHD modes determines current quench delay or avoidance. Supra-thermal electrons generated by 0.5 MW of lower hybrid power are sufficient to trigger precursors of the electron-fishbone instability. Evidence of spatial redistribution of fast electrons, on the ∼100 µs typical mode timescale, is shown by the fast electrons bremsstrahlung diagnostic. From the presence of new magnetic island induced accumulation points in the continuous spectrum of the shear Alfvén wave spectrum, the existence of new magnetic island induced Alfvén eigenmodes (MiAE) is suggested. Due to the frequency dependence on the magnetic island size, the feasibility of utilizing MiAE continuum effects as a novel magnetic island diagnostic is also discussed. Langmuir probes have been used on FTU to identify hypervelocity (10 km s−1), micrometre size, dust grains. The Thomson scattering diagnostic was also used to characterize the dust grains, present in the FTU vacuum chamber, following a disruption. Analysis of the broad emitted light spectrum was carried out and a model taking into account the particle vaporization is compared with the data. A new oblique ECE diagnostic has been installed and the first results, both in the presence of lower hybrid or electron cyclotron waves, are being compared with code predictions. A time-of-flight refractometer at 60 GHz, which could be a good candidate for the ITER density feedback control system, has also been tested.

High density plasmas (n0 ≈ 8 × 10 20 m −3) featuring steady improved core confinement have been obtained in FTU up to the maximum nominal toroidal field (8 T) by deep multiple pellet injection. These plasmas also feature high purity efficient electron-ion coupling and peaked density profiles sustained for several confinement times. Neutron yields in excess of 1 × 10 13 n/s are measured, consistent with the reduction of the ion transport to neoclassical levels.

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Steady internal transport barriers (ITBs) are obtained in FTU at ITER-relevant magnetic field and density (ne0 ≥ 1.3 × 1020 m−3) in almost full non-inductive discharges, sustained by lower hybrid (LH) and electron cyclotron (EC) RF waves sources. Similarly to ITER, only electrons are directly heated which in turn heat ions via collisions and no momentum is injected. Collisions do not affect the mechanisms of turbulence suppression and energy transport. At the highest densities the ion thermal conductivity remains ≤ the ohmic level, while the energy confinement time exceeds the ITER 97-L scaling by about 1.6 times. The ITB radius can be varied in the range 0.2 ≤ r/a ≤ 0.65 modifying the radial profile of the LH driven current, acting mainly on the safety factor q. A liquid lithium limiter (LLL) of innovative design, composed of a mesh of porous capillaries, has been tested successfully for the first time on a medium size tokamak. The LLL surface showed no damage up to the maximum thermal load of 5 MW m−2. With LLL cleaner plasmas are obtained and the particle recycling strongly drops; new interesting regimes of particle transport arise at high density, with highly peaked profiles. Significant progress in disruption mitigation by means of EC power has shown that they can be avoided when absorption occurs directly on the MHD islands driving the disruption. Feedback control/suppression of MHD tearing modes (TM, m = 2) with EC waves has been achieved relying on a real-time detection of the TM and of its radial location. Testing the collective Thomson scattering in ITER-relevant configuration has stressed that avoiding backscattered radiation to the source is very crucial. The theory of the evolution of fishbone-like instabilities driven by LH generated supra-thermal electrons in FTU is outlined, and its relation to the trapped α particles dynamics is stressed.