The electron density at the separatrix (n_e,sep) plays a central role in balancing energy confinement, detachment achievement, and ELM suppression in tokamaks, thereby influencing core-edge integration. To study what determines this key parameter, a database of H-mode separatrix density measurements from Alcator C-Mod, ASDEX Upgrade, and JET tokamaks has been assembled using a consistent analysis method across all devices. This dataset is used to derive a regression scaling expression based solely on engineering parameters, and the results are compared to predictions from the two-point model. The agreement found is remarkable: both the regression and model provide similar parameter dependencies and tokamak-specific multiplicative constants. Building on this agreement, a fully predictive formula that combines the regression dependencies and the two-point model multiplicative constant is proposed. This formula is able to estimate n_e,sep across the three machines within a factor of 1.5, and provides projections to next-step devices (ITER, SPARC, DTT, JT-60SA and COMPASS-U) that are in agreement with available SOLPS simulations.
In JET-ILW, beam-target reactions contribute to a large fraction of the fusion power generated in deuterium-tritium (D-T) plasmas, with core ion temperatures of 10-12 keV and large neutral-beam injection (NBI) power. Previous modelling done in preparation for the recent D-T campaigns in JET have shown that injecting D beam ions with energies of similar to 120 keV in T-rich plasmas produces larger 14 MeV fusion yield than in 50:50 D:T plasmas, but such scenario had never been tested in past D-T experiments. In addition, the simulations showed that fundamental ion cyclotron resonance heating (ICRH) of the D ions can significantly boost the net fusion reactivity, since both the D-bulk ions and the fast D-beam ions are accelerated to energy ranges that are optimal for the D-T reactions to take place. In the last JET D-T campaigns (DTE2 and DTE3), dedicated experiments confirmed-for the first time-the improved fusion performance of T-rich plasmas with high D-NBI power and highlighted the key impact of fundamental D ICRH on the fusion performance. This new scenario led to the world-wide D-T fusion energy record ever achieved in a fusion device and allowed to sustain more than 12 MW of fusion power averaged over 5 s. The main results of these unprecedented experiments will be presented and the NBI + ICRF physics responsible for the high fusion performance achieved will be highlighted through numerical modelling.
We investigated laser-driven shock propagation in low-density foams through coordinated experiments and simulations. At the Vulcan laser facility, a nanosecond laser pulse launched a shock from a plastic ablator–titanium pusher assembly into a 0.1 g cm–3 trimethylolpropane triacrylate foam, diagnosed by side-on, time-resolved X-ray radiography using a picosecond Cu backlighter. This enabled direct tracking of the pusher motion and the shock front. Comparison with one-dimensional and two-dimensional hydrodynamic simulations (MULTI and FLASH) shows good agreement when assuming an effective on-target intensity of (3–5) × 1013 W cm–2, lower than nominal laser estimates, underscoring the influence of coupling losses. An analytical model based on impedance-matched shock relations connects the ablation pressure, pusher dynamics and transmitted shock strength. Overall, the combined results indicate that the foam behaves as an effectively homogeneous medium under these conditions, supporting modelling assumptions for foam-based fusion targets and establishing a validated platform for future shock-propagation studies in low-density materials.
This study investigates impurity behaviour during ion cyclotron resonance heating (ICRF) experiments, focusing on the application of two different three-ion heating schemes in H–D and D–T plasmas at JET. In the D-( ^3 He)-H scenario, the phasing of the ICRF antenna straps was varied to modify the launched parallel wave number k _|| , enabling a systematic study of its effect on fast-ion generation, plasma dynamics and impurity transport. The results indicate dependence of impurity behaviour on antenna phasing, particularly for the nickel (Ni) in the main plasmas and for the beryllium (Be) source, measured in the far-field of the antenna, at a location not magnetically connected to it. Analysis of sawtooth oscillations using the symmetrised dot pattern method reveals correlations between sawtooth frequency and crash intensity, the applied ICRF power and antenna phasing. The lowest impurity levels are obtainedwith +90° antenna phasing, corresponding to maximised fast-ion generation. A comparative study of impurity behaviour in D–T plasmas is also presented for the three-ion T-( ^9 Be)-D and hydrogen minority heating scenarios. The three-ion scheme produces the largest increases in ion temperature T _i , while hydrogen minority heating yields higher electron temperatures T _e and slightly reduced impurity levels. These results suggest that impurity behaviour in three-ion ICRF scenarios depends on the chosen heating optimisation (fast-ion generation versus ion heating) and can be further controlled through appropriate selection of ICRF antenna phasing.
The realization of p-type doping in wide-band-gap oxide semiconductors remains a major challenge, particularly in β-Ga_2O_3 where nitrogen has long been considered a potential acceptor dopant but has consistently failed to produce hole conductivity. Here we investigate the microscopic configuration of implanted nitrogen in (100) β-Ga_2O_3 using temperature-dependent N K-edge x-ray absorption spectroscopy. The spectra reveal a pronounced π^* resonance characteristic of molecular nitrogen, which becomes increasingly dominant upon thermal annealing. First-principles calculations and multiple-scattering simulations reveal a pronounced tendency for nitrogen atoms to form N-N bonded configurations in the Ga_2O_3 matrix, particularly in defect-rich environments created by ion implantation, reproducing the characteristic spectral features observed in the N K-edge XANES spectra. Structural analysis further indicates that implantation induces a defect-rich near-surface layer with local β-to-γ-like structural motifs, highlighting the strongly nonequilibrium structural environment in which nitrogen incorporation occurs. Reported results show that implanted nitrogen preferentially forms molecular N_2-like configurations rather than substitutional acceptors. Our results provide a microscopic explanation for the long-standing failure of nitrogen acceptor doping in β-Ga_2O_3 and reveal dopant molecularization as a previously overlooked pathway for impurity incorporation under strongly nonequilibrium implantation conditions.
