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Description
The magnetic field topology has been observed to systematically affect the power threshold for the L–H transition in several tokamaks. In particular, a lower (higher) power threshold is found when the ion-$\nabla B$ drift points toward (away from) the X-point, indicating a "favorable" ("unfavorable") configuration for the transition. Early explanations for this behavior were proposed by [LaBombard et al.,2005], who experimentally linked topology-dependent scrape-off layer (SOL) parallel flows to changes in core toroidal rotation and, ultimately, to the L–H transition. Later studies suggested a possible contribution from edge magnetic shear–induced Reynolds stresses, dependent on the drift configuration[Fedorczak et al.,2012]. More recent experiments, however, do not fully support these interpretations[Plank et al.2023] and instead show that the observed differences between the two drift configurations (especially those involving the edge radial electric field profile[Vermare et al.,2022) are not consistently captured by the existing explanations. This indicates that the phenomenon is not yet completely understood, and given the central importance of reliably accessing H-mode in future fusion reactors, further investigation is needed.
In this contribution, 3D turbulent simulations of the COMPASS tokamak boundary plasma in both favorable and unfavorable configurations are carried out using the first-principle drift-reduced fluid code SOLEDGE3X. The magnetic equilibrium reproduces the shot #15487 (ohmic L-mode D-shaped discharge[Cavalier et al.,2019]) and the two different drift configurations are obtained by intentionally reversing the direction of the toroidal magnetic field $B_T$. Neutrals are self-consistently included through the coupling with the kinetic code EIRENE.
The study compares edge and SOL plasma dynamics across two otherwise identical L-mode scenarios, differing only in the direction of $B_T$. The upstream Mach number profile indicates a displacement of the stagnation point from below (favorable) to above (unfavorable) the outer mid-plane (OMP), corresponding downstream to the relocation of the denser and colder divertor leg respectively from the inner to the outer target. Near-sonic SOL parallel flows in the high-field side are driven by ballooning-like transport and the edge plasma poloidal rotation reverses direction. Remarkably, in the favorable configuration the OMP radial electric field well becomes noticeably steeper and the positive peak in the SOL nearly doubles, pointing also a link to divertor physics. The analysis of the interplay between these equilibrium changes and turbulence itself will highlight the impact of the toroidal field direction on intensity, structure and propagation of plasma fluctuations.
A detailed discussion on the relation between these results and the existing theoretical explanations is presented.