7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Improved understanding and performance of power exhaust of alternative
divertor discharges on MAST Upgrade
K. Verhaegh1, J.R. Harrison1, B. Lipschultz2, D. Moulton1, N. Lonigro2,1, N. Osborne3,1, P. Ryan1,T. Wijkamp4,5, C. Theiler6, D. Brida7, C. Cowley2,1, G. Derks5,4, R. Doyle8, F. Federici9, S.
Henderson1, B. Kool5,4, H. Reimerdes6, L. Xiang1, N. Vianello101
United Kingdom Atomic Energy Authority, 2University of York, 3University of Liverpool,
4Eindhoven University of Technology, 5Dutch Institute for Fundamental Energy Research,
6Ecole Polytechnique Fédérale Lausanne, 7Max Planck Institute Garching, 8Dublin City
University, 9Oak Ridge National Laboratory, 10Consorzio RFX
e-mail (speaker): kevin.verhaegh@ukaea.uk
MAST Upgrade supports a range of Alternative
Divertor Configurations (ADC), including a Super-X
Divertor (SXD), in a strongly baffled divertor chamber.
Our results show that the SXD (Bt/Bxpt = 2.3 (ratio ofmagnetic field at target to X-point)) exhibits numerous
advantages over the Conventional Divertor (CD)
(Bt/Bxpt = 1.1), including enhanced detachment accessand reduced particle & heat fluxes through increased
ion & power sinks. The SXD remains detached inbeam-heated scenarios (up to PNBI=2.5 MW) between
25-50% Greenwald fraction (nGW), while the CD only
detaches at 45% nGW without extrinsic seeding. Anintermediate strike point radius (Bt/Bxpt = 1.7)
configuration, the Elongated Divertor (ED), retains
many of the SXD benefits.
The MAST-U SXD and ED demonstrate a reduced
detachment front location sensitivity during heating/fueling perturbations and transients; whilst theconditions in the core and upstream of the detachedregion are identical between the SXD, ED and CD(Figure 1a,b,c). The downstream-end of the ionisation
region (detachment front) remains at a constantpoloidal distance from the X-point, independent of themagnetic geometry below the detachment front,according to both experiments (Figure 1 a,b) andsimulations. Therefore, the enhanced power/particle
exhaust capabilities of the longer legged baffleddivertors (SXD & ED) are caused by the additional
volume downstream of the detachment front up until
the strike point. In this detached region (Figure 1a, b),
plasma-molecular interactions involving molecularions lead to Molecular Activated Recombination(MAR) ion sinks, enhancing particle removal
capabilities [1]. Such molecular interactions have anunprecedented impact on the detachment physics inthe strongly baffled MAST-U SXD [2,3] & ED (Figure1a,b) and their treatment requires modifications toexhaust modelling [4].
Initial H-mode results show that the MAST-U SXDand ED remains detached during the inter-ELM
period. H-mode operation and a larger range of ADCs
will be investigated in the upcoming experimental
campaign (07/2023-) including studies on ELM power
dissipation using novel ultrafast spectroscopy.
References[1] K. Verhaegh, et al. 2021, Nucl. Fusion 61 016014[2] K. Verhaegh, et al. 2023 Nucl. Fusion 63 016014[3] K. Verhaegh, et al. 2023 arXiv: 2304.09109[4] K. Verhaegh, et al. 2023 Nucl. Fusion 63 076015

Figure 1: Ion source & sink profiles in the divertor
chamber as function of poloidal distance to the Xpoint for the SXD (a,d), ED (b,e) and CD (c,f)
configurations at 35 % nGW. The detached region isshaded black (a,b) and the strike point (SP) position for
the three configurations is highlighted (a,b,c)