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.

This paper presents results of high-density plasma production experiments carried out using an optimally designed and excited slotted helical antenna. The slotted helical antenna was excited by linearly polarized, right-hand polarized (RHP) and left-hand polarized (LHP) CW microwaves (f = 2.45 GHz) with power up to 2.5 kW. Two different discharges-the resonant and the non-resonant discharges-were studied. In the resonant discharges the magnetic field at the edge of the helical antenna was equal to the ECR field, so that a resonant coupling of the electrons and the microwaves could occur. In non-resonant discharges the value of the magnetic field was greater than the resonant field throughout the mirror. It was seen that the linearly polarized and RHP waves can produce about 100% ionization (at pressures ≈ 1-2 × 10 −4 Torr; power ≤ 2.5 kW) for both resonant and non-resonant discharges. In most cases an enhancement over the neutral particle density could be observed due to improved confinement. The resonant discharge using LHP waves also yielded similar results as obtained using the RHP and the linearly polarized waves. This result is somewhat unexpected since the LHP waves are not expected to undergo resonance at electron cyclotron resonance. The non-resonant discharge using LHP waves was found to be very weak (underdense). Finally, an attempt has been made to understand the unexpected nature of the resonant discharges using LHP waves, in terms of the modes of plasma loaded helices and waveguides.

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This paper presents results of plasma production in a mirror machine with the microwaves being coupled from a side port, in the ordinary-mode polarization. In this method of coupling, contamination due to the launching antenna is avoided; also, there is very little window degradation since plasma bombardment on the side port is negligible. The coupling of the microwaves to the plasma in this method relies on the existence of certain surface waves that exist at low values of the magnetic field (below that for electron cyclotron resonance) and have a dominant E z component. The existence of such surface waves is predicted by a recent theory (Ganguli A et al 1998 Phys. Plasmas 5 1178). The plasma characterization at microwave power levels of about 200 W, indicates moderately overdense plasmas (density ≈10 11 cm −3 ), with fairly uniform radial and axial profiles. From the data, the mechanism of plasma production and confinement in the mirror has been analysed. To gain insight into the nature of the waves in the plasma-loaded waveguide, the wavelengths of the waves within the plasma and the radial profiles of the wave electric field were measured. The results are compared with the theory of Ganguli et al for wave identification.

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Absorption of high-frequency guided waves in a plasma-loaded waveguide