Culham Science Centre

Abstract At JET, Deuterium-Helium-3 (D- 3 He) experiments using ion cyclotron resonance heating (ICRH) with the three-ion scheme were conducted, involving thermal Deuterium (D), thermal Helium-3 ( 3 He), and a fast D beam injected via Neutral Beam Injection (NBI) and further heated using ICRH. In these plasmas, intense neutron emission at the level of ≈ 10 16 n/s was observed, and a non-thermal neutron spectrum was measured with the JET time of flight neutron spectrometer. Furthermore, the spatial profile of ≈ 16.4 MeV gamma rays originating from the 3 He(D,γ) 5 Li reaction was observed. This is a weak branch (≈ 10 -5 relative probability) of the more common 3 He(D,p)α reaction and it can be used to assess the production of alpha particles in these experiments. In this work we analyse a set of data from nuclear diagnostics in D-D NBI - 3 He experiments at JET and compare them with synthetic diagnostics based on TRANSP simulations of the fast deuterium distribution function, using the so-called "kick operator" to model power transfer from the wave to the beam fast deuterons. The neutron rate predicted by the TRANSP simulations broadly agrees with data measured with fission chambers. The shape of the neutron emission spectrum is also well understood, once finite Larmor radius effects are introduced into calculations. Data from the gamma ray camera are used to infer the alpha birth profile through tomographic reconstructions based on the TREVISO code. These were compared with corresponding calculations starting from TRANSP results and showing an overall good agreement with the experimental tomographies. The validated TRANSP results are finally used to discuss the modifications of the fast deuterium distribution function and of the alpha birth profile in a set of discharges.

The results of numerical studies of optimal operational conditions for efficient negative ions production in the SPIDER negative ion source are presented. Analysis are done by means of the numerical code FSFS2D which gives self-consistent two-dimensional description of the source, including neutral gas flow, plasma chemistry, RF coupling in the driver and plasma transport through the magnetic filter. Different particle species are described by separate continuity equations and the electron temperature is governed by the electron energy equation whereas the plasma potential is described by the Poisson equation.Parametric investigations have been performed assessing the influence of the negative ions production yield, magnetic filter field, gas pressure, plasma grid potential and RF heating power on the negative ions performance. It has been found that the increase of almost all the considered operational parameters leads to the enhancement of the negative ions current.

Abstract This article presents an in-depth study of the sequence of events leading to density limit disruption in J-TEXT tokamak plasmas, with an emphasis on boundary turbulent transport and the high-field-side high-density (HFSHD) front. These phenomena have been extensively investigated using a Langmuir probe and Polarimeter–interferometer diagnostics. The research reveals a consistent pattern of events as the plasma density ramps up: the collapse of the sheared radial electric field, the enhancement of the boundary quasi-coherent turbulence ( 50 ∼ 80 kHz ), the increase in boundary particle transport induced by this turbulence, edge cooling and the emergence of the HFSHD front. These phenomena occur once the plasma density exceeds a critical value. Importantly, by exploring plasmas with varying edge safety factor ( q a ) , it is revealed that the density thresholds for these phenomena are all inversely proportional to q a . The findings offer valuable insights into the mechanisms underlying density limit disruptions in tokamak plasmas, suggesting that the enhancement of edge turbulent transport plays a crucial role in the edge cooling and triggering the HFSHD front. For the first time, a strong link between the edge turbulent transport and the HFSHD front has been observed. In addition, the evolution of the boundary electron temperature across various q a plasmas can further validate the link between the edge turbulent transport and the HFSHD formation.

The EUropean DEMOnstration power plant (EU-DEMO) project, a EUROfusion initiative, seeks to advance fusion power technology by developing a reliable Electron Cyclotron (EC) heating and current drive system capable of delivering up to 130 MW of auxiliary heating power presently through six equatorial ports. This paper introduces an innovative alternative to the baseline ex-vessel waveguide (EW) system design, which could be major for effective EC operations. The proposed design reduces vacuum-sealed interfaces significantly — from 173 to seven — minimizing failure points and reducing the need for extensive monitoring in confined port spaces. Enhanced for remote maintainability, the design simplifies maintenance tasks and mitigates vacuum breach risks. The vacuum dimple-plated casing, acting as the primary confinement boundary, ensures nuclear safety and enables integration with current infrastructure, while addressing challenges like dynamic loads and vessel movements to maintain structural integrity. These advancements indicate that the modular and sealless waveguide approach offers a more reliable, maintainable solution. • Modular design approach ensures remote maintainability under high-radiation conditions. • Sealless waveguides cut vacuum-sealed interfaces by 96% for minimal failure points. • Dimpled-plate casing streamlines integration within the tokamak environment.

Intermetallic dispersion-strengthening (IDS) using nano-scale coherent intermetallic precipitates offers a potent strategy to produce high-strength and radiation-resistant steels, whilst addressing the manufacturability challenges of analogous oxide dispersion-strengthened (ODS) steels. However, their performance with intermetallic stability under irradiation damage, such as radiation-induced hardening (RIH), whilst hypothesised, is undemonstrated. Here, we report on a model IDS α(A2) + α’(L2 1 ) Fe-Ni-Al-Ti ferritic superalloy, which exhibits exceptional resistance to RIH with near-zero hardening after irradiation at 300°C 1 dpa, in contrast to significant RIH in a counterpart coarse precipitate alloy (increase in nano-hardness of 1.0 GPa) and Eurofer97 (0.7 GPa). This irradiation resistance is attributed to the high density of semi-coherent precipitate-matrix interfaces, and partial-disordering L2 1 ->B2 which causes a decrease in anti-phase boundary energy. High interface density with localised interfacial strain offers effective sinks, suppressing defect populations compared to the counterpart with lower interface density. Meanwhile, atomic resolution spectroscopy and irradiation with in-situ transmission electron microscopy show that the disordering stems from Al-rich and Ti-rich sublattices mixing in the initial L2 1 -Ni 2 AlTi structure below 500°C, forming metastable B2-Ni(Al,Ti). Combined, the high interface density and radiation-induced intermetallic disordering underpin the remarkable radiation tolerance, demonstrating the IDS concept as a promising radiation-resistant materials design strategy.

Power exhaust is a crucial issue for future fusion reactors. Divertor detachment and the required power dissipation fractions of about 95% are foreseen to be achieved by impurity seeding. In a tokamak, at high seeding levels the radiation often concentrates in a small region inside the confined plasma near the X-point. In early observations the so-called X-point radiator (XPR) often led to back-transitions to L-mode or disruptions. In metal tokamaks or with higher available heating power, these regimes can be stabilized and are now established on AUG, JET, TCV, KSTAR and WEST. The XPR is a cold, dense plasma inside the confined region in the vicinity of the X-point, that breaks the paradigm of poloidal symmetry of density and temperature on closed flux surfaces. On AUG, the poloidal extent of the XPR is a few centimeters and it is observed up to 15 c m above the X-point. The long connection length in this region and the access of neutral particles from the divertor region facilitate the creation of the XPR, as predicted by an analytical model. Numerical simulations with SOLPS-ITER match the observations at AUG and TCV and allow predictions towards a power plant, where a lower impurity concentration is required to trigger an XPR. Since the XPR greatly reduces power and particle fluxes to the targets, simpler and more efficient divertor concepts, such as the compact radiative divertor, can be envisaged for future devices. A scenario with an XPR, however, comes at the cost of an increased impurity concentration and a potential reduction in confinement, which has to be further quantified. The XPR location can be well detected by various diagnostics, enabling responsive real-time control, even through large transients like an LH transition. The active control helped to access a new regime of ELM suppression at AUG, which is now also observed at TCV and JET. The observation of the XPR on multiple tokamaks, the demonstration of its active control, and the emergence of theoretical models that scale favourably towards fusion reactors have opened up a new phase of advanced power exhaust research. • X-point radiators (XPR) are observed at almost all operational tokamaks. • Earlier observations did not identify it as a stable operational regime. • In JET D–T discharges, an XPR could be created and successfully be controlled. • Future reactors would benefit from its controllable full detachment and ELM suppression. • The XPR enables detached ramp-up & -down and the novel compact radiative divertor (CRD).