Abstract 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 60-70 keV and has a marginal high-energy tail (approx. above 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.
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