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.

We explore the possibility of using a relativistic plasma density wave as a wiggler for producing free-electron laser radiation. Such a wiggler is a purely electric wiggler with frequency w p (plasma frequency) and wavenumber k,,. If an electron beam is injected parallel to the plasma wave wavefront, it is wiggled transversely with an apparent wiggler wavelength X, = 27rc/wp. Using plasma densities in the 10" (cm-') range, X, of order 100 pm may be obtained, thereby permitting generation of short wavelength radiation with modest energy beams. The effective wiggler strength a, = e A / m c 2-0.5 can be extremely large. We discuss the excitation methods for such wigglers and examine the constraints imposed by the plasma medium on FEL gain in this scheme.

A crossed ion-laser beam apparatus has been used to obtain Cs and K cross sections for photodetachment into the ground ($') and first excited state ('P) of the neutral atom in the photon energy range near the threshold for production of 'P neutrals. A total cross section is determined from the cross section for production of neutral atoms and the partial cross sections are obtained from the cross section for production of photoelectrons transmitted through a low-pass kinetic-energy filter. The filter characteristics are determined empirically and the discrimination between groundand excited-state detachment channels is approximately 1000. It js apparent from the data that the detachment channels are strongly coupled in the threshold region, a result attributed to iong-range forces between the detached electron and the neutral atom. The data are compared with a semiempirical multichannel photodetachment model developed by Lee; the model is quite successful in its prediction of the energy-dependent cross section for production of Cs Pi /2 3/2 from knowledge of the previously measured Cs total photodetachment cross section. We have produced an approximate fit to the K data with this model. The following electron affinities have been determined: for Cs, 0.471S(3) eV, and for K, O.S0147(10).