Search

An important step on the way to future fusion power plants was the 2021 deuterium–tritium experimental campaign (DTE2) at the Joint European Torus (JET), in which crucial DT physics was investigated. In this study, we have reconstructed the fast-ion deuterium distribution function in JET discharge 99971 which broke the former fusion energy record. It is the first time that the fast-ion distribution has been reconstructed from experimental data in a DT discharge. The reconstruction shows that the fast-ion deuterium distribution is anisotropic, with a bias towards co-going ions ( p > 0). The fast-ion deuterium distribution likely peaks in energy ( E ) at around $E\sim 60$ –70 keV and has a marginal high-energy tail ( $E\gtrsim 180$ keV). Furthermore, an orbit analysis shows that the fast-ion distribution is composed of mostly co-passing orbits ( $50 \%$ ), trapped orbits ( $21 \%$ ) and counter-passing orbits ( $27 \%$ ), as well as a small population of potato orbits ( $1.7 \%$ ) and counter-stagnation orbits ( $0.3 \%$ ). The orbit-type constituents of the neutron measurements are distributed in similar fractions.

Dedicated experiments were performed in JET DTE2 plasmas for obtaining an α -particle bump-on-tail (BOT) distribution aiming at exciting Alfvén eigenmodes (AEs). Neutral beam injection-only heating with modulated power was used so that fusion-born α -particles were the only ions present in the MeV energy range in these DT plasmas. The beam power modulation on a time scale shorter than the α -particle slowing down time was chosen for modulating the α -particle source and thus sustaining a BOT in the α -particle distribution. High-frequency modes in the toroidicity-induced Alfven eigenmode (TAE) frequency range and multiple short-lived modes in a wider frequency range have been detected in these DT discharges with interferometry, soft x-ray cameras, and reflectometry. The modes observed were localised close to the magnetic axis, and were not seen in the Mirnov coils. Analysis with the TRANSP and Fokker-Planck FIDIT codes confirms that α -particle distributions with BOT in energy were achieved during some time intervals in these discharges though no clear correlation was found between the times of the high-frequency mode excitation and the BOT time intervals. The combined magneto-hydrodynamic (MHD) and kinetic modelling studies show that the high-frequency mode in the TAE frequency range is best fitted with a TAE of toroidal mode number n = 9. This mode is driven mostly by the on-axis beam ions while the smaller drive due to the pressure gradient of α -particles allows overcoming the marginal stability and exciting the mode (Oliver et al 2023 Nucl. Fusion submitted). The observed multiple short-lived modes in a wider frequency range are identified as the on-axis kinetic AEs predicted in Rosenbluth and Rutherford (1975 Phys. Rev. Lett. 34 1428).

The recent deuterium–tritium campaign in JET-ILW (DTE2) has provided a unique opportunity to study the isotope dependence of the L-H power threshold in an ITER-like wall environment (Be wall and W divertor). Here we present results from dedicated L-H transition experiments at JET-ILW, documenting the power threshold in tritium and deuterium–tritium plasmas, comparing them with the matching deuterium and hydrogen datasets. From earlier experiments in JET-ILW it is known that as plasma isotopic composition changes from deuterium, through varying deuterium/hydrogen concentrations, to pure hydrogen, the value of the line averaged density at which the threshold is minimum, ${\bar n_{{\text{e}},{\text{min}}}}$ , increases, leading us to expect that ${\bar n_{{\text{e}},{\text{min}}}}$ (T) < ${\bar n_{{\text{e}},{\text{min}}}}$ (DT) < ${\bar n_{{\text{e}},{\text{min}}}}$ (D) < ${\bar n_{{\text{e}},{\text{min}}}}$ (H). The new power threshold data confirms these expectations in most cases, with the corresponding ordering of the minimum power thresholds. We present a comparison of this data to power threshold scalings, used for extrapolation to future devices such as ITER and DEMO.