Long-legged divertors for addressing tokamak PMI challenges*

Recent modeling of a tightly-baffled long-legged divertor demonstrates a detached divertor regime where energy is dissipated on sidewalls of the divertor leg by radial transport and radiation [1]. This fully detached regime is found to be passively stable; it persist for a wide range of input power from the core, and as input power is varied, the location of the detachment front in the leg shifts closer to, or away from, the divertor target. For a sufficiently long divertor leg, the divertor remains detached, with benign power loads on the material surfaces, and the detachment front is located safe distance away from the target plate and the primary X-point. Calculations demonstrate that a long-legged divertor can accommodate up to an order of magnitude larger exhaust power than a standard divertor can, for otherwise similar parameters. This makes a long-legged divertor a potentially attractive option for a high-power tokamak, and it is currently considered for high-field designs (ADX, ARC). Physical mechanisms and sensitivity to model assumptions are examined for the long-legged detached regime, and its implications for fusion energy are discussed. [1] Umansky et al., Phys. Plasmas 25 (2017) 056112.