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.

A two-field model for collisionless trapped electron mode turbulence has both finite amplitude-induced stability and instability, depending on wave number. Effects usually identified with nonlinear plasma instability (self-trapping, kinetics, 3D mode structure, magnetic shear) are absent. Nonlinear stability and instability reside in ExB advection of density. It drives modes of a purely damped branch of the dispersion relation to finite amplitude and changes the rate at which free energy is released into the turbulence by shifting the density-potential cross phase. Analysis shows that modes of the purely damped branch cannot be ignored in saturation, and that the linear growth rate is a poor indicator of driving at finite amplitude, invalidating mixing length and quasilinear approximations. Using statistical closure theory, the nonlinear eigenmode and growth rate are determined from the saturation level of modes on all branches, stable and unstable, and the nonlinear cross phase that governs finite-amplitude instability

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

Transport theory is applied to magnetic helicity injection into plasmas with toroidal geometry. Magnetic relaxation during helicity injection can be described as hyper-resistive diffusion of the current. By using the generalized Balescu-Lenard extension of quasi-linear transport theory, it is shown that hyper-resistive diffusion is generally slow compared with heat transport. It follows that magnetic relaxation due to such turbulence tends to flatten the temperature profile, as observed in reversed-field pinches. Given flattened temperature profiles, Taylor's minimum principle for magnetic relaxation is usefully reformulated as minimum dissipation, yielding circuit equations for electrostatic helicity injection in laboratory devices such as spheromaks and tokamaks. A favorable heat pinch could benefit helicity injection into tokamaks. These results are also relevant to natural phenomena involving the generation of fields by magnetic relaxation.