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.

Neutral particles play a crucial role in the plasma edge due to their strong interaction with the plasma. To capture all physics details most often a kinetic equation is solved using a Monte Carlo (MC) approach. Unfortunately, the MC noise hampers the convergence assessment of the solution of the coupled plasma-neutral equations. Moreover, the MC calculation time increases for high-collisional detached cases. In these high-collisional cases, however, the kinetic model may be well-approximated by a fluid neutral model, which can be solved deterministically. In this paper, we assess different fluid neutral models by comparing the resulting plasma sources to the sources from an MC simulation of the kinetic equation. The fluid models take into account the microscopic cross-sections and rate coefficients from the AMJUEL-HYDHEL databases, as well as the microscopic TRIM reflection model. We accomplish the latter without the introduction of any user-defined fitting parameters. We show that a pure pressure-diffusion equation provides accurate results for the particle source, but is inaccurate for the parallel momentum and ion energy source. Adding a parallel momentum equation gives maximum errors of about 9% for the momentum and 32% for the energy source. These errors are further reduced to respectively 6% and 14% by adding a separate neutral energy equation.

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