Exhaust operational space assessment for the European Volumetric Neutron Source (EU-VNS)

The SOLPS-ITER edge plasma numerical model is employed to predict the exhaust operational space in a small-scale tokamak device currently being investigated as the proposed European Volumetric Neutron Source (EU-VNS) with large power-to-size ratio of P/R ∼ 20 MW/m. Previous work is extended by allowing krypton seeding to dissipate and redistribute heat instead of argon. The found finite operational space is limited by both, engineering limits for the peak heat-flux expected at the divertor plasma facing components < 10 MW/m2, and at the same time requiring a low enough effective charge Zeff and high electron temperature Te in the core region to keep up the neutron wall load NWL ∼ T 3/2 / Z eff 2 from beam-target fusion. With krypton seeding the operational space seems to be larger than with argon and at the same time the amount of tritium throughput can be halved compared to the previous study. The required total T-throughput rate is of the order 3-4 * 10 22 s-1 and it is observed that pellet fuelling is deemed to be as efficient and required to fuel both the core density and the separatrix density at the same time. A positive correlation is found between the krypton enrichment in the divertor and the increase of upstream density effectively fuelled by pellets. A low level of Z eff ≈ 1.6, and the observation of the required Greenwald fraction of 50% to achieve this, is compatible with the predictions of the core plasma conditions for the current EU-VNS design point from ASTRA/METIS. Within the model assumptions made the observation of a depression of the core electron temperature below 1 keV is critical and potentially requires more ECRH power in EU-VNS than anticipated.