Discrepancy between estimated and measured fusion product rates on MAST using TRANSP/NUBEAM

Experimental evidence is presented of a discrepancy between the predicted and measured D–D fusion product rates on MAST. Both the neutron and proton production rates, measured independently with a neutron camera and charged fusion product detector array, are approximately 40% lower than those predicted by TRANSP/NUBEAM codes. This deficit is scenario independent and cannot be explained by uncertainties in the typical plasma parameters suspected for such discrepancies, such as electron temperature, the plasma effective charge and the injected neutral beam power. Instead, a possible explanation is an overestimate of the neutron emissivity due to the guiding centre approximation used in NUBEAM to model the fast ion orbits.