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.

The research challenges for electric propulsion technologies are examined in the context of s-curve development cycles. It is shown that the need for research is driven both by the application as well as relative maturity of the technology. For flight qualified systems such as moderately-powered Hall thrusters and gridded ion thrusters, there are open questions related to testing fidelity and predictive modeling. For less developed technologies like large-scale electrospray arrays and pulsed inductive thrusters, the challenges include scalability and realizing theoretical performance. Strategies are discussed to address the challenges of both mature and developed technologies. With the aid of targeted numerical and experimental facility effects studies, the application of data-driven analyses, and the development of advanced power systems, many of these hurdles can be overcome in the near future.

The time-resolved cross-field electron anomalous collision frequency in a Hall thruster is inferred from minimally invasive laserbased measurements. This diagnostic is employed to characterize the relationship between the dominant low-frequency "breathing" oscillations and anomalous electron transport mechanisms. The ion Boltzmann equation combined with a generalized Ohm's law is used to infer key quantities including the ionization rate and axial electric field strength which are necessary in computing the total electron cross-field collision frequency. This is accomplished by numerically integrating functions of velocity moments of the ion velocity distribution function measured with laser-induced fluorescence, in conjunction with current density measurements at a spatial boundary. Estimates of neutral density are used to compute the classical collision frequency profile and the difference in the total collision frequency, and this quantity describes the anomalous collision frequency. This technique reveals the anticipated trends in electron transport: few collisions in the acceleration region but a collision frequency approaching the cyclotron frequency farther downstream. The time-resolved transport profiles indicate that the anomalous collision frequency fluctuates by several orders of magnitude during a breathing cycle. At troughs in the discharge current, classical collisions may dominate; at peaks in the discharge current, anomalous collisions dominate. These results show that the breathing mode and electron transport are directly correlated. This finding is discussed with regard to existing numerical models for the breathing mode and interpretations of anomalous electron transport.