EUROfusion considers a volumetric neutron source (EU-VNS) to generate a neutron wall load of about 0 . 5 M W / m 2 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 . 5 m D-beam/T-target driven fusion device ( P f u s ≈ 30 M W ) must exhaust helium particles and dissipate sufficient energy from the large auxiliary power required ( P a u x ≈ 50 M W ) entering in large parts the edge. A SOLPS-ITER assessment indicates 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 10 M W / m 2 . It is shown that an extra constraint of Z e f f < 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 G W ≈ 0 . 5 is maintained with total T-throughputs at about half the ITER value. It is reasoned that for a EU-VNS design study the exhaust operational window can be enlarged by choosing other seeding species like Krypton, refining the balance between pellet- to gas-fuelling, and integrated core-edge modelling.
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Wiesen et al. (2025) studied this question.