Magnetosonic-whistler waves may be destabilized by runaway electrons with strongly anisotropic velocity distribution. The unstable wave frequency is well below the non-relativistic electron cyclotron frequency but above the ion cyclotron frequency. The linear instability growth rate of the magnetosonic-whistler wave destabilized by an avalanche of relativistic runaway electrons through the anomalous Doppler-resonance is calculated in a local analysis using the homogenous plasma approximation. The perturbative stability analysis is complemented by numerical solution of the dispersion equation including the full hot plasma dielectric tensor. In the parameter range relevant to disruptions in large tokamaks, the growth rate is largest for nearly perpendicular propagation. By assuming that the dominant damping mechanism in cold post-disruption plasmas is due to collisions, the local threshold of the instability can be shown to depend on the fraction of runaway electrons, the magnetic field and the temperature of the background plasma. The dependence on the magnetic field is consistent with the experimental observations suggesting that there is a critical toroidal magnetic field below which there is no runaway current after a disruption. One reason for this absence of runaways may be that the instability scatters the runaways in pitch-angle and prevents the beam from forming. Indeed, the quasilinear analysis shows that the main result of the instability is pitch-angle scattering of the runaway electrons on a typical time scale of a microsecond.

Disruptions in large tokamaks can lead to the generation of a relativistic runaway electron beam that may cause serious damage to the first wall. To suppress the runaway beam the application of resonant magnetic perturbations (RMP) has been suggested. In this work we investigate the effect of resonant magnetic perturbations on the confinement of runaway electrons by simulating their drift orbits in magnetostatic perturbed fields and calculating the transport and orbit losses for various initial energies and different magnetic perturbation levels. In the simulations we model the ITER RMP configuration and solve the relativistic, gyro-averaged drift equations for the runaway electrons including a time-dependent electric field, radiation losses and collisions. The results indicate that runaway electrons are rapidly lost from regions where the normalised perturbation amplitude δB/B is larger than 0.1% in a properly chosen perturbation geometry. This applies to the region outside the radius corresponding to the normalised toroidal flux ψ = 0.5.

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.