Experimental observation and integrated modelling of proton-beryllium fusion in He and D plasmas at JET
In the paper we present validated integrated modelling of JET experiments in which fusion performance is driven by reactions between fast ions and intrinsically present metal wall impurities. A steady-state L-mode plasma with dominant proton-beryllium fusion and neutron yields of up to ≈ 6 x 1013 s-1 is developed in He and D. The fusion drive is unambiguously confirmed by neutron and γ-ray diagnostics. The experiments are analysed via an integrated modelling framework, developed to model the two-stage proton beryllium-fusion chain and produce high-fidelity fusion product source terms. The modelling chain comprises TRANSP and JETTO for plasma core modelling, LOCUST for full orbit product tracking and collisional slowing-down, DRESS to resolve two- and three-body fusion kinematics, and MCNP for neutron transport calculations. Modelling shows that the primary 9Be(p,n)9B reaction is the dominant neutron emitter at naturally present concentrations of beryllium in these experiments. The yield contribution of secondary reactions between fusion products and beryllium, 9Be(d,n)10B and 9Be(α,n)12C, is found to be negligible. The proton-deuteron knock-on effect in D plasmas is modelled, which is calculated to contribute ≈ 25 % to the total neutron yield. For both He and D discharges the total computed neutron rates match fission chamber measurements within the combined experimental and computational uncertainty, with an average discrepancy of ≈ ± 20 %. Realistic proton-beryllium neutron sources are propagated through JET’s MCNP neutron transport model – we find that the 235U fission chambers’ response is sensitive to p-Be source changes, with up to ≈ 10 % variation compared to a generic D-D neutron source. We show that differences in p-Be spectral tail energy gradients, present because of fast proton distribution variations in the discharges, could be detected via multi-foil neutron activation. The tool is applied to the study of interactions between fast ions and boron impurities, of relevance to ITER – we calculate that in JET conditions a significant alpha source with DT-like energies could be generated through 11B(p,α)2α, and detected via γ-emission in secondary interactions between fast alphas and boron. The work represents an important step towards validating predictive integrated modelling capabilities for non-standard fusion reactions.