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

EUROfusion considers a volumetric neutron source (EU-VNS) to generate a neutron wall load of about 0.5MW/m² to qualify tritium breeding blankets early in support of EU-DEMO that mitigates the risk of a late testing for required nuclear technology. The envisaged small-scale R = 2.5m D-beam / T-target driven fusion device (P_fus ≈ 30MW) must exhaust helium particles and dissipate sufficient energy from the large auxiliary power required (P_aux ≈ 50MW) entering in large parts the edge. A SOLPS-ITER assessment demonstrates that with argon seeding a finite divertor operational window exists allowing to avoid core dilution by helium and to reduce the peak heat-flux density below 10MW/m². It is shown that an extra constraint of Z_eff < 2 − 3, required to sustain good core performance to produce the required amount of fusion neutrons, can also be met if the Greenwald-fraction f_GW ≈ 0.5 is maintained with total T-throughputs at about half the ITER value. However, it will also be demonstrated that for a EU-VNS design study the exhaust operational window can be enlarged by choosing other seeding species like krypton helping to reduce the amount of T-throughputs by a factor of 2. Further optimizations are possible by refining pellet- to gas-fuelling, and pursuing integrated core-edge modelling.