Abstract Samples of JET ILW divertor and limiter tiles extracted from JET after the third ILW campaign underwent baking at elevated temperatures (350°C and 240°C, respectively) for 60-140 hours, to simulate the effects of baking as previously planned for ITER divertor and first wall. A combination of thermal desorption spectroscopy (TDS) and ion beam analysis (IBA) was used to characterize the dynamics of deuterium outgassing during baking and the resulting changes in deuterium depth distribution. It is shown that deuterium release during baking follow a power law time dependence with the average power law exponent of ~0.85 for divertor samples and ~0.68 for limiter ones. For divertor samples the average total fuel removal efficiency is ~47.5% and the near-surface one is ~21%, while for limiter samples they are ~3.8% and 97%, respectively. Spatial redistribution of deuterium during prolonged baking is demonstrated. The combination of TDS and IBA reveal a fundamental difference in deuterium redistribution behaviour between divertor and limiter baking conditions, with predominant diffusion towards the surface followed by desorption in the former and predominant inward diffusion in the latter. Possible promotion of fuel outgassing by a loose morphology of thick co-deposit layers is also observed.
Abstract Achieving tritium self-sufficiency is a critical requirement for the viability of future fusion power plants. Here, a comprehensive assessment of the operational lifetimes of ceramic breeder materials under neutron irradiation within a tokamak environment is resented, focusing on the evolution of the tritium breeding ratio (TBR) over reactor-relevant timescales. This is defined here as the operational timeframe at which the TBR is maintained above a critical threshold of ≈1.10. Using the depletion capabilities within the OpenMC neutronics code, both conventional ceramic breeders (Li2TiO3, Li4SiO4, and advanced ceramic breeder – ACB) and emerging hybrid breeder-multiplier ceramics (Li8PbO6, Li2Be2O3) are assessed for operational lifetime strategies. Initial optimisation identifies breeder-to-multiplier volumetric ratios that maximise TBR at time zero, followed by time-dependent simulations to quantify TBR degradation due to lithium depletion during service. It is shown that while higher initial multiplier fractions enhance TBR, they also accelerate lithium depletion, reducing blanket lifetime. Conversely, hybrid ceramics with intrinsic neutron multiplication show reduced sensitivity to additional multipliers. A novel layer-wise optimisation approach, varying breeder and multiplier distributions radially and poloidally, is shown to extend blanket lifetimes and maintain TBR above the critical threshold for longer periods demonstrating lifetime limitations may be elsewhere within the breeder blanket. Verification against FISPACT-II confirms the accuracy of the OpenMC depletion methodology utilised. The results highlight the importance of spatially optimised material distributions and lithium inventory management in maximising breeder blanket longevity. This work provides robust framework for the design of next-generation solid breeder blankets, offering practical strategies to minimise replacement frequency and support sustainable fusion energy.
Abstract Dispersion strengthened tungsten (DSW) alloys are promising materials for plasma facing components, displaying improved mechanical properties and microstructure stability compared to pure tungsten. In this work, the performance of DSW reinforced with 1 wt% transition-metal carbide (TaC, TiC, or ZrC) has been examined under H-mode plasma in the DIII-D tokamak. DSW samples were mounted both flush with the divertor floor and at a 10° angle towards the incoming plasma fluxes to further increase the heat and particle loading. To simulate the impact of prolonged tokamak operation, similar plasma exposures were repeated on two additional sets of DSW samples: one set pre-implanted with 10 22 m -2 helium, and another set heated in-situ to an additional 500°C. Flush mounted samples exhibited good surface stability, with no evidence of melting or cracking regardless of composition or treatment. Carbon redeposition was observed on most samples, likely due to the graphite environment of DIII-D. Under coupled thermal loading and helium implantation, surface nanostructuring developed in some DSW samples; however, DSW containing tantalum dispersion phases showed strong resistance to such surface modification. In contrast, obvious surface melting was observed on 10° angled samples containing TaC and ZrC dispersoids, likely associated with dispersoid oxidation during fabrication, which reduced thermal conductivity and promoted elevated surface temperatures during plasma exposure. The grain morphology of DSW containing TiC remained largely unchanged, although near-surface dispersoids on the angled TiC sample were missing after plasma exposure. The loss is associated with oxidation of the dispersoids during fabrication, followed by sublimation during plasma exposure. Through systematic comparison of DSW performance under H-mode plasma conditions, this study identifies key degradation mechanisms and highlights strategies to improve the performance of DSW as plasma facing materials.
We report the first observation of ion cyclotron emission (ICE) during lower hybrid wave (LHW) and electron cyclotron resonance heating in a tokamak plasma, and subsequently interpret the resulting ICE spectrum using linear kinetic plasma theory to infer characteristics of the bulk plasma and of the fast-ion distribution. ICE spectra from deuterium plasmas in the EAST tokamak exhibit peaks at successive deuterium cyclotron harmonics, consistent with the collective relaxation of energetic deuterium ions through the magnetosonic cyclotron instability (MCI). By carrying out a multi-dimensional parameter scan of MCI growth rates in high-resolution two-dimensional (k//, k⊥) wavenumber space, we obtain best-fit values for the magnetic field, plasma density, background electron and ion temperature, concentration of fast deuterons, and the fast-ion pitch and bulk velocities parallel and perpendicular to the applied magnetic field, which are consistent with the observations within the assumptions of the model. The spectrum is consistent with emission from deuterons with an indicative energy of 770 keV, plausibly accelerated through LHW–ion interactions. Owing to the simplifying assumptions of the model — including an infinite, homogeneous plasma, a ring-beam fast-ion distribution, and the restriction to linear theory — the derived parameters should be regarded as indicative and qualitative rather than as definitive measurements. These results nevertheless suggest that ICE may serve as a passive diagnostic of energetic ion populations and warrant further investigation of the ICE–LHW connection, which may assist alpha-channeling scenarios for ash removal and boosting fusion gain.
In magnetically confined nuclear fusion, the achievement of ignition necessitates the tokamak plasma in a high confinement state known as H-mode. A major challenge encountered in the standard H-mode is the abrupt collapse of its ultrathin edge layer. Here, we report a first experimental demonstration that a stable H-mode can be sustained by a staircase plasma profile. The basic physical picture is: by staircasing the plasma profile, the potential free energy stored in one single large edge layer is distributed into multiple mesoscale layers throughout the entire plasma volume. Consequently, the large scale edge collapse is converted into small scale desynchronized fluctuations, effectively reducing the risk of violent relaxation event. This finding uncovers a new benign high confinement regime of the tokamak plasma.
