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.

No abstract

A new regime of runaway discharges has been found in TCABR (Tokamak Chauffage Alfvén Brésilien). This regime is obtained by initiating the discharge with low filling pressure and, after the initial current rise, maintaining a large filling rate. The line density reaches a maximum value around 2 × 10 19 m −3 , during the current ramp-up phase, and then drops by a factor of around four in the quasi-stationary phase of the discharge, when a new regime is achieved. The most distinctive features of this regime, as compared to 'conventional' runaway discharges reported in the literature, are (i) maintenance of the runaway discharge, with the plasma current almost entirely provided by the runaway beam, in a cold background plasma and with strong neutral gas injection; (ii) enhancement of the relaxation instability with strong spikes in the Hα emission and loop voltage correlated with sawtooth relaxation of the line density; and (iii) plasma detachment from the limiter. A simple phenomenological model, based upon straightforward particle and energy balance calculations, is proposed to explain the experimental observations. According to this model, the plasma is rather cold and the short pulses of gas ionization and the related density spikes are due to sudden plasma heating caused by the relaxation instability. Furthermore, it seems that the runaway generation for the conditions of the experiments can be explained only if the secondary generation process is invoked.