Fifth Technical Meeting on Divertor Concepts

First attempts to establish an X-point radiator regime on MAST-U in N-seeded H-modes

29 Oct 2025, 13:50

20m

Press Room (M-Building) (IAEA Headquarters)

Oral X-point and other Radiator Regimes X-point and other Radiator Regimes

Speaker

Stuart Henderson (UKAEA)

Description

This presentation summarises the first attempts to develop an X-point radiator (XPR) regime on MAST-U. The XPR, a localised radiating structure near the magnetic X-point, is a promising power exhaust solution for future fusion reactors. While XPR regimes have been demonstrated on most operating devices, MAST-U remains one of the last major tokamaks yet to access this regime. The work presented will further aim to additionally assess the effect of open versus closed divertor geometries and the influence of magnetic topology (DN vs. LSN) on XPR formation and stability.

The experiments have so far focused on double-null plasmas with conventional divertor geometry, using N2 seeding into beam heated ELMy H-mode scenarios. Figure 1 presents time traces from a one-beam (1.5 MW) scenario which was recently developed with improved initial confinement (H₉₈ ~0.8). As the gas is injected, H₉₈ decreases slightly to ~0.75. The ELMs transition from Type I to smaller grassy Type III ELMs. As shown by the 2D poloidal image of total radiation in Figure 1, this transition coincides with inner divertor detachment and radiation localised strongly within the X-point and weakly along the outer divertor leg, suggesting incomplete outer leg detachment. As the scenario evolves, ELMs become fully suppressed at t=0.27 s; however, the loss of the density pedestal indicates a transition not to a sustained ELM-free H-mode, but to an H-L back transition. This is likely triggered by the arrival of the q = 1 surface, causing rotation locking between the q = 1 and q = 2 surfaces. Notably, the confinement only decreases significantly after the mode completely locks during the ELM phase at t=0.32.

Seeding has also been attempted in a two-beam (~3 MW) scenario, albeit this older scenario had poorer initial confinement (H₉₈ ~0.7). The radiation measurements in this scenario indicate complete detachment of both divertors, with radiation strongly emitting around the X-point and along the inner core separatrix. During the nitrogen seeding phase, ELM behaviour transitions from Type III to small grassy ELMs, while H-mode is sustained with a strong density pedestal. The H₉₈ factor decreases modestly from ~0.7 to ~0.6. Ongoing experiments aim to further investigate XPR access and stability in the high-performance scenario using two-beam heating.