Assessment of alternative divertor configurations as an exhaust solution for DEMO

The European roadmap for fusion energy has identified plasma exhaust as a major challenge towards the realisation of magnetic confinement fusion. To mitigate the risk that the single null divertor (SND) with a high radiation fraction in the scrape-of-layer (SOL) adopted for ITER will not extrapolate to a DEMO reactor, the EUROfusion consortium is assessing potential benefits and engineering challenges of alternative divertor configurations. A range of alternative configurations that could be readily adopted in a DEMO design has been identified. They include the X divertor (XD), the Super-X divertor (SXD) and the Snowflake divertor (SFD). The flux flaring towards the divertor target of the XD is limited by the minimum grazing angle at the target. The characteristic increase of the target radius in the SXD is a trade-off with the increased TF coil volume, but, ultimately, also limited by forces onto coils. Engineering constraints also limit XD and SXD characteristics to the outer divertor leg with a solution for the inner leg requiring up-down symmetric configurations. Boundary models with varying degrees of complexity have been used to predict the beneficial effect of the alternative configurations on exhaust performance. Desired effects are an easier access to detachment, reluctance of the detachment front to displace along the divertor leg and increased divertor radiation without excessive core confinement degradation. Based on the extended 2-point model and achievable geometric variations the SOL, the radiation required for the onset of detachment in the outer divertor decreases in the SXD and SFD with the tolerable residual power being 30-40% larger than for the SND. Additional improvements are expected from the ability to increase the divertor radiation without adverse effects on the core performance and through SOL broadening as postulated for the SFD. A systematic study of the alternative configurations using the divertor transport codes TECXY and SOLEDGE2D are qualitatively consistent but provide different predicted gains. They also predict a performance increase of the outer target of the XD, albeit at the cost of a higher load on its inner target.