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An impurity seeding scan has been simulated in three different divertor configurations of MAST-U, showing the effect of the divertor geometry on the transition to detachment. The three configurations include a conventional short divertor, a Super-X divertor with large poloidal flux expansion and a Super-X divertor with small poloidal flux expansion. Both Super-X configurations detach at the same impurity seeding rate, while the detachment threshold in the conventional divertor is much higher. A comparison of SOLPS results for all configurations is presented in a detaching plasma with target temperatures around 5 eV. The total power balance, the distribution of radiation and the divertor closure with respect to neutrals and impurities is analyzed. [ABSTRACT FROM AUTHOR]

A density scan is performed in SOLPS5.0 for two divertor configurations of MAST Upgrade: (i) a short divertor silimiar to configurations in present-day tokamaks, (ii) the Super-X divertor. In the simulation, a clear roll-over of the ion flux and plasma density at the target is observed as the plasma detaches. The separatrix density at which the transition occurs is estimated. In addition, we investigate the ability of the long-legged divertor to reduce the target power loads and enhance the radiated power at higher collisionalities. We also study how distribution of power losses in the divertor region changes with the modification of geometry and we separately analyze neutral radiation and carbon ion line radiation. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

A 1D code modelling scrape-off layer (SOL) transport parallel to the magnetic field (SOLF1D) is benchmarked with 2D simulations of MAST-U SOL performed via the SOLPS code for two different collisionalities. Based on this comparison, SOLF1D is then used to model the effects of divertor leg stretching in 1D, in support of the planned Super-X divertor on MAST. The aim is to separate magnetic flux expansion from volumetric power losses due to recycling neutrals by stretching the divertor leg either vertically (∇B∥ = 0 in the divertor) or radially (B α 1/R). [ABSTRACT FROM AUTHOR]

In the presented contribution two groups of techniques of computational physics were used for the study of sheath structure in the DC glow discharge in argon plasma – the fluid modelling describing macroscopic plasma phenomena and the particle modelling providing more detailed insight into the plasma processes. A comparison of different computational methods is given with attention to the efficiency of computer codes in two dimensions. Another point of interest is the inclusion of external magnetic field into the models and its effect on the sheath structure. [ABSTRACT FROM AUTHOR]

One of the first studies of MAST Upgrade divertor configurations with scrape-off layer plasma simulation (SOLPS) 5.0 are presented. We focus on understanding the main prospects associated with the novel geometry of the Super-X divertor (SXD). This includes a discussion of the effect of magnetic flux expansion and volumetric power losses on the reduction of target power loads, the effect of divertor geometry on the divertor closure and the distribution of neutral species and radiation in the divertor, and the role of the connection length in broadening the target wetted area. A comparison in conditions typical for MAST inter-ELM H-mode plasmas confirms the improved performance of the Super-X topology resulting in significantly better divertor closure with respect to neutrals (the atomic flux from the target increased by a factor of 6, but the atomic flux from the divertor to the upper SOL reduced by a factor of 2), increased radiation volume and increased total power loss (a factor of 2) and a reduction of target power loads through both magnetic flux expansion and larger volumetric power loss in the divertor (a factor of 5–10 in attached plasmas). The reduction of the target power load by SXD further increases with collisionality (high density or detached regimes) thanks to the larger importance of volumetric power losses. It is found that a cold divertor plasma leads to stronger parallel temperature gradients in the SOL which drive more parallel heat flux, meaning that the effectiveness of perpendicular transport in spreading the power at the target can be reduced, and this needs to be taken into account in any optimization. [ABSTRACT FROM AUTHOR]

Abstract: As part of preparations for the new Super-X divertor (SXD) on MAST, the influence of long divertor leg is modelled by SOL transport codes. The SOLPS5.0 package is used to study the effect of the expanded divertor in two dimensions and a comparison between the conventional geometry (standard short divertor) and the SXD geometry is presented. We focus on plasma parameters in the divertor and total particle and energy balance in the SOL. Finally, a one-dimensional SOLF1D code is benchmarked with SOLPS and used to study the effects of longer connection length L ∥ and magnetic flux expansion ∇∥ B separately. [Copyright &y& Elsevier]

Abstract: The effect of plasma fluctuations due to turbulence at the outboard midplane on parallel transport properties is investigated. Time-dependent fluctuating signals at different radial locations are used to study the effect of signal statistics. Further, a computational analysis of parallel transport in the scrape-off layer (SOL) taking into account these fluctuations is presented. Plasma transport in the SOL along the magnetic field between two targets is calculated by a one-dimensional fluid code in order to estimate the response to transient conditions along the SOL and the attention is given to a comparison of steady-state and time-dependent modelling. [Copyright &y& Elsevier]

Non-local closures allow kinetic effects on parallel transport to be included in fluid simulations. This is especially important in the scrape-off layer, but to be useful there the non-local model requires consistent kinetic boundary conditions at the sheath. A non-local closure scheme based on solution of a kinetic equation using a diagonalized moment expansion has been previously reported. We derive a method for imposing kinetic boundary conditions in this scheme and discuss their implementation in BOUT++. To make it feasible to implement the boundary conditions in the code, we are lead to transform the non-local model to a different moment basis, better adapted to describe parallel dynamics. The new basis has the additional benefit of enabling substantial optimization of the closure calculation, resulting in an O ( 10 ) speedup of the non-local code. [ABSTRACT FROM AUTHOR]

This paper presents simulations of isolated 3D filaments in a slab geometry obtained using a newly developed 3D reduced fluid code, written using the BOUT++ framework. First, systematic scans were performed to investigate how the dynamics of a filament are affected by its amplitude, perpendicular size, and parallel extent. The perpendicular size of the filament was found to have a strong influence on its motions, as it determined the relative importance of parallel currents to polarization and viscous currents, whilst drift-wave instabilities were observed if the initial amplitude of the blob was increased sufficiently. Next, the 3D simulations were compared to 2D simulations using different parallel closures; namely, the sheath dissipation closure, which neglects parallel gradients, and the vorticity advection closure, which neglects the influence of parallel currents. The vorticity advection closure was found to not replicate the 3D perpendicular dynamics and overestimated the initial radial acceleration of all the filaments studied. In contrast, a more satisfactory comparison with the sheath dissipation closure was obtained, even in the presence of significant parallel gradients, where the closure is no longer valid. Specifically, it captured the contrasting dynamics of filaments with different perpendicular sizes that were observed in the 3D simulations which the vorticity advection closure failed to replicate. However, neither closure successfully replicated the Boltzmann spinning effects nor the associated poloidal drift of the blob that was observed in the 3D simulations. Although the sheath dissipation closure was concluded to be more successful in replicating the 3D dynamics, it is emphasized that the vorticity advection closure may still be relevant for situations where the parallel current is inhibited from closing through the sheath due to effects such as strong magnetic shear around X points or increased resistivity near the targets. [ABSTRACT FROM AUTHOR]