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.

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We have measured total sputtering yields for impact of slow (#100 eV) singly and doubly charged ions on LiF. The minimum potential energy necessary to induce potential sputtering (PS) from LiF was determined to be about 10 eV. This threshold coincides with the energy necessary to produce a cold hole in the valence band of LiF by resonant neutralization. This allows the first unambiguous identification of PS induced by cold holes. Further stepwise increase of the sputtering yield with higher projectile potential energies provides evidence for additional defect-mediated sputtering mechanisms operative in alkali halides. PACS numbers: 79.20.Rf In recent studies on the impact of slow multiply charged ions on insulator surfaces, a dramatic increase of the yields for sputtering [1][2][3][4] and secondary ion emission [5][6][7][8] with projectile charge state has been observed for certain target species as, e.g., LiF and SiO 2 . In contrast to the well established process of kinetically induced sputtering, ablation of target atoms and ions due to the potential energy of the projectile, henceforth called potential sputtering (PS), is largely unexplored. PS can result in high sputter yields at low impact energy and, unlike kinetically induced sputtering, is not accompanied by strong radiation defects in deeper target layers. It has therefore the potential of acquiring considerable technological relevance: Preferential removal of insulating layers (PS is absent for conducting surfaces) could be the basis for novel cleaning procedures for semiconductors (e.g., soft sputtering of SiO 2 from Si wafers). Other applications such as nanostructuring and controlled surface modifications of insulators are also conceivable. A detailed understanding of mechanisms responsible for the conversion of projectile potential energy in PS processes is therefore highly desirable.Presently, several complementary models for different surface materials are being considered to explain PS. For impact of ions in very high charge states q (up to q 70 for Th) on uranium oxide, it has been speculated [3] that a "Coulomb explosion" mechanism [9,10] is responsible for the observed strong increase of ablation and secondary ion yields with q. For comparably highly charged ions on GaAs, a model involving structural instabilities arising from the destabilization of atomic bonds due to a high density of electronic excitations [11] was invoked to explain the observed high sputtering yields [4]. For projectile ions in somewhat lower charge states (q # 27), a large amount of experimental data for various target surfaces (among them alkali halides and SiO 2 ) [1,2,12] are at variance with the Coulomb explosion mechanism [13]. They are, however, consistent with the so-called "defect-mediated desorption" model originally developed for electron-and photon-stimulated desorption [14] for alkali halides. In this model, localized defects (e.g., "self-trapped excitons," STE) are formed following particle-hole excitations in the valence band of insulators with strong ...