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.

Magnetic fields are produced in the 10–15 megagauss range by use of high explosives which compress the flux obtained from initial fields of approximately a hundred thousand gauss. The fields described here occupy a cylindrical volume and are essentially axial. A typical field might have these general characteristics: Peak field 14 megagauss; 2 μsec duration from 10–14 megagauss; field volume around peak, 6 mm diameter, 50 mm estimated length.

Mueller et al. Reply: The Comment [1] by Mazin et al.interprets the de Haas-van Alphen (dHvA) measurements [2] of Fowler et al. making the interesting suggestion that the separation between the low 0.53 and 0.78 kT peaks in the power spectral density and also the barely resolved two higher peaks at 3.51 ±0.11 kT are due to the effect of spin splitting. They obtain good agreement with the extremal cross-sectional areas if they shift their chainlike band downward by about 60 meV.In presenting our analysis in Ref.[2] we had restricted [3] ourselves to three independent Lifshitz-Kosevich (LK) frequencies only. It is not clear to us whether the dHvA Y-Ba-Cu-O (YBCO) data were taken in a normal metallic, superconducting, or induced ferromagnetic region of the YBCO phase diagram [2]. Few experiments on cuprate superconductors have been carried out in 100-T fields. This question remains unresolved and is one on which several laboratories including ours are actively working.Here we adopt the point of view of Mazin et al. and have fitted the data of Ref.[2] using two base cross sections, each Zeeman split. Because we are close to the quantum limit (index «=1), dHvA oscillations are no longer exactly periodic in \/B (the cross section itself is a function of external B). This effect has been observed [4] in the needles of Zn. It means that each dHvA Landau frequency is split in the power spectral density into two "bumps" or peaks, in contrast to an interpretation based on strict adherence to LK theory which yields only a single peak.In Table I we list the power spectral density peak frequencies. A fitted recomparison with the Fowler et al. magnetization data similar to Fig. 1 of [2] suggests that the quality of fit using two spin-split frequencies was improved by about a factor of 3. Yu et al. can also [5] fit these results using their electronic band structure if they shift their bands by about 20 meV. TABLE I. YBCO frequencies F in kilotesla, quantum index (F/B) in 100-T field B, cyclotronic masses in electronic mass, and scattering Dingle temperatures in kelvin. F 0.53 ±0.02 0.78 ±0.02 3.41 ±0.10 3.62 ±0.10 Quantum index 5 7 33 37 Mass 7.0 ±2.5 7.2 ±2.5 7.3 ±2.6 7.5 ±2.7 Dingle temperature 1.7 ±0.6 2.1 ±0.7 3.3 ±1.2 3.5 ±1.3 Shift sign + -+ -It appears that spin-split cyclotron orbits offer a valid interpretation of the dHvA results of Fowler et al. in the 100-T regime. dHvA analysis in the quantum limit.[5] J.