Modelling tokamak power exhaust and scrape-off-layer thermal transport in high-power fusion devices

Managing the steady-state power loading onto the divertor target plates remains a major unresolved challenge facing tokamak fusion energy, that will be crucial for the success of the next generation of high-power reactor-level devices. This thesis will tackle two topics within this wide research area: assessing the performance of advanced divertor geometries in the context of the ARC reactor concept, and studying the impact of 'nonlocal' thermal transport on modelling predictions for the ITER tokamak SOL. Numerical simulations are performed using UEDGE for the Super-X divertor (SXD) and X-point target divertor (XPTD) configurations proposed for the ARC reactor design. The SXD, combined with 0.5% fixed-fraction neon impurity concentration, produced passively stable, detached divertor regimes for power exhausts in the range of 80-108 MW. The XPTD configuration is found to reduce the strike-point temperature by a factor of ~10 compared to the SXD for small X-point radial separations (~1.4lambda_{q||}). Even greater potential reductions are identified for separations of ≤1lambda_{q||}. Raising the separatrix density by a factor 1.5, stable detached divertor solutions were obtained that fully accommodate the ARC exhaust power without impurity seeding. In the presence of steep temperature gradients, classical local transport theory breaks down, and thermal transport becomes nonlocal, depending on conditions in distant regions of the plasma. An advanced nonlocal thermal transport model is implemented into the 'SD1D' complex SOL code to create 'SD1D-nonlocal', and applied to study typical ITER steady-state conditions. Results suggest that nonlocal transport effects will have importance for the ITER SOL, with discrepancies observed between nonlocal/local transport model predictions in low-density scenarios. Heat flux models employing global flux limiters are shown to be inadequate to capture the spatially/temporally changing SOL conditions. An analysis of SOL collisionality and nonlocality suggests nonlocal effects will be significant for future devices such as DEMO and ARC as well

Scaling laws for electron kinetic effects in tokamak scrape-off layer plasmas

Tokamak edge (scrape-off layer) plasmas can exhibit non-local transport in the direction parallel to the magnetic field due to steep temperature gradients. This effect along with its consequences has been explored at equilibrium for a range of conditions, from sheath-limited to detached, using the 1D kinetic electron code SOL-KiT, where the electrons are treated kinetically and compared to a self-consistent fluid model. Line-averaged suppression of the kinetic heat flux (compared to Spitzer-Harm) of up to 50% is observed, contrasting with up to 98% enhancement of the sheath heat transmission coefficient, $\gamma_e$. Simple scaling laws in terms of basic SOL parameters for both effects are presented. By implementing these scalings as corrections to the fluid model, we find good agreement with the kinetic model for target electron temperatures. It is found that the strongest kinetic effects in $\gamma_e$ are observed at low-intermediate collisionalities, and tend to increase at increasing upstream densities and temperatures. On the other hand, the heat flux suppression is found to increase monotonically as upstream collisionality decreases. The conditions simulated encompass collisionalities relevant to current and future tokamaks.Comment: 24 pages, 14 figure

Incorporating nonlocal parallel thermal transport in 1D ITER SOL modelling

Accurate modelling of the thermal transport in the ‘Scrape-Off-Layer’ (SOL) is of great importance for assessing the divertor exhaust power handling in future high-power tokamak devices. In conditions of low collisionality and/or steep temperature gradients that will be charateristic of such devices, classical local diffusive transport theory breaks down, and the thermal transport becomes nonlocal, depending on conditions in distant regions of the plasma. An advanced nonlocal thermal transport model is implemented into a 1D SOL code ‘SD1D’ to create ‘SD1D-nonlocal’, for the study of nonlocal transport in tokamak SOL plasmas. The code is applied to study typical ITER steady-state conditions, to assess the relevance of nonlocality for ITER operating scenarios. Results suggest that nonlocal effects will be present in the ITER SOL, with strong sensitivity in simulation outputs observed for small changes in upstream density conditions, and drastically different temperature profiles predicted using local/nonlocal transport models in some cases. Global flux limiters are shown to be inadequate to capture the spatially and temporally changing SOL conditions. Introducing impurity seeding, under conditions where detached divertor operation is achieved using the flux-limited Spitzer-H ̈arm models used in standard SOL codes, simulations using the nonlocal thermal transport model under equivalent conditions were found to not reach detachment. An analysis of the connection between SOL collisionality and nonlocality suggests that nonlocal effects will be significant for future devices such as DEMO as well. The results motivate further work using nonlocal transport models to study disruption events and low collisionality regimes for ITER, to further improve accuracy of the nonlocal models employed in comparison to kinetic codes, and to identify more appropriate boundary conditions for a nonlocal SOL model

Performance assessment of long-legged tightly-baffled divertor geometries in the ARC reactor concept

Extremely intense power exhaust channels are projected for tokamak-based fusion power reactors; a means to handle them remains to be demonstrated. Advanced divertor configurations have been proposed as potential solutions. Recent modelling of tightly baffled, long-legged divertor geometries for the divertor test tokamak concept, ADX, has shown that these concepts may access passively stable, fully detached regimes over a broad range of parameters. The question remains as to how such divertors may perform in a reactor setting. To explore this, numerical simulations are performed with UEDGE for the longlegged divertor geometry proposed for the ARC pilot plant conceptual design-a device with projected heat flux power width (λq∥) of 0.4 mm and power exhaust of 93 MW-first for a simplified Super-X divertor configuration (SXD) and then for the actual X-point target divertor (XPTD) being proposed. It is found that the SXD, combined with 0.5% fixed-fraction neon impurity concentration, can produce passively stable, detached divertor regimes for power exhausts in the range of 80-108 MW-fully accommodating ARC's power exhaust. The XPTD configuration is found to reduce the strike-point temperature by a factor of ∼10 compared to the SXD for small separations (∼1.4λ [subscript]q [subcript]∥) between main and divertor X-point magnetic flux surfaces. Even greater potential reductions are identified for reducing separations to ∼1λ [subscript]q [subscript]∥ or less. The power handling response is found to be insensitive to the level of cross-field convective or diffusive transport assumed in the divertor leg. By raising the separatrix density by a factor of 1.5, stable fully detached divertor solutions are obtained that fully accommodate the ARC exhaust power without impurity seeding. To our knowledge, this is the first time an impurity-free divertor power handling scenario has been obtained in edge modelling for a tokamak fusion power reactor with λ [subscript]q [subcript]∥ of 0.4 mm. ©2019US DoE cooperative agreement DE-SC0014264EPSRC Fusion Centre for Doctoral Training (Training grant number EP/LO1663X/1)DoE Contract DE-AC52-07NA2734

6 toros 6.

Suplemento de actualidad taurina de 6 Toros

Bilbao en fiestas.

Incluye el art.: Bilbao en fiestas : Astenagusia 1999