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

M.R.K Wigram,B Labombard,D Brunner,D.G Whyte,M.E Rensink,A.Q Kuang,M.V Umansky,T Golfinopoulos,C.P Ridgers,J.L Terry

doi:10.1088/1741-4326/ab394f

Abstract

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 long-legged divertor geometry proposed for the ARC pilot plant conceptual design—a device with projected heat flux power width () 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 (∼1.4) between main and divertor X-point magnetic flux surfaces. Even greater potential reductions are identified for reducing separations to ∼1 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 of 0.4 mm.

Highlights

The divertor power handling and divertor plasma detachment control remains a major concern for both near-term exper imental fusion devices as well as demonstration fusion power plant scale reactors of the future
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 of 0.4 mm and power exhaust of 93 MW—first for a simplified Super-X divertor configuration (SXD) and for the actual X-point target divertor (XPTD) being proposed
Stable, detached solutions for both the SXD and XPTD grids were obtained at high core exhaust power—in some cases with parallel heat fluxes entering into the divertor of q|| ∼ 15 GW m−2 and heat flux widths of λq|| ∼ 0.4 mm, consistent with the anticipated heat flux width for ARC based on empirical scalings

Summary

Introduction

The divertor power handling and divertor plasma detachment control remains a major concern for both near-term exper imental fusion devices as well as demonstration fusion power plant scale reactors of the future. It is recognised that these techniques are likely to be inadequate to handle the higher heat loads expected from future reactor-level devices like DEMO [6, 7]. In order to suppress target erosion to acceptable levels, fully detached divertor conditions may be required. Added to this requirement is a formidable divertor plasma control challenge—e.g. at no time during high power operation should the divertor plasma be allowed to re-attach to the target, despite inevitable variations in power exhaust that are associated with transients (e.g. confinement transitions)

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