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The Swiss Plasma Center is upgrading the TCV tokamak to test a tightly baffled, long-legged divertor (TBLLD), a novel concept that promises to enhance power exhaust capabilities with minimal modification to the magnetic configuration [1,2].
Simulations using the SOLPS-ITER code indicate that a TBLLD can improve TCV’s power exhaust capability by an order of magnitude compared to the unbaffled configuration [2]. Tight baffling sustains a high poloidal neutral density gradient, thereby, increasing the neutral density in front of the divertor target and enhancing volumetric power dissipation. In addition to a lower detachment threshold, extended leg length and tight baffling provide a large detachment window and a mechanism for passive detachment front stability. The neutral cushion, futhermore, provides a reservoir of potential energy that can temporarily buffer transient loads.
The simulations informed the design of a proof-of-principle TBLLD for the outer TCV divertor. A straight, vertical divertor design enables diagnostic access via TCV’s reciprocating divertor probe array, while maintaining engineering simplicity. Compatibility with neutral beam heated, high-power plasma scenarios constrains the baffled leg length to 0.34m. A trade-off between predicted plasma plugging and excessive recycling at the outer baffle yield a divertor width of 0.11m. The ability to expand the polodial flux along the divertor leg provides a means to vary both. The required gas tightness limits diagnostics access. Foreseen are poloidally distributed Langmuir probes, thermocouples, pressure gauges, and spectrometric lines of sight, to provide measurements of target fluxes, neutral density distribution, position and dynamics of the detachment front, which are critical to assess the TBLLD concept.
The main concerns for the proof-of-principle are the open inner divertor, which may limit the benefits of the closed outer divertor, and potential self-baffling of dense divertor plasmas. Recent SOLPS-ITER simulations also identified thermo-electric currents resulting from vastly different conditions in the inner and outer divertors as a potential project limitation. These concerns are addressed through modelling and, ultimately, through experiments.
A dedicated experimental campaign with the proof-of-principle TBLLD is planned for 2026. Following a successful validation of the TBLLD concept, a second phase of upgrades will optimise the baffle geometry, extend the exhaust solution to the inner divertor, address particle exhaust, through pump ducts at the top of the TBLLD, and integrate the plasma exhaust solution with an attractive core plasma scenario.
[1] M.V. Umansky, et al., Phys. Plasmas 24 (2017) 056112.
[2] G. Sun, et al., Nucl. Fusion 63 (2023) 096011.