Comparison of different approaches for modelling the beam-target reactivity in fusion plasmas

Abstract: Fusion research has been an ongoing research endeavour for many decades and it has the potential of providing an important part in the future of energy production. An important part of this process is to understand and control the various heating systems of the reactor where one of the most important systems being the Neutral Beam Injector (NBI) which injects particles at high velocities into the plasma. The number of fusion reactions caused by this process depends on the beam-target reactivity where the beam can be modelled in various ways. The aim of this report was to compare the results of two different approaches in modelling the beam-target reactivity. This was done using two models, Stix and TRANSP which has different strengths and weaknesses but they use the same input parameters. Hence it is interesting to assess how much accuracy is sacrificed when using the simpler model (Stix). The aim of this report was to compare beam-target reactivty calculations based on two different approaches for modelling the distribution of the beam ions. One approach (TRANSP) performs very detailed modelling of both the energy distribution and the distribution in pitch (i.e. the direction of the velocity with respect to the plasma magnetic field), but is also very time consuming. The other approach (Stix) is more simplistic but can only provide the distribution in energy (no pitch information). It was found that the beam-target reactivities calculated from these two modelling approaches typically differed by about 10-20 percent (a bit more a the very edge of the fusion plasma, but this is not a great concerns since very few fusion reactions happen in this region). The difference in reactivity values could be attributed to differences in the modelled energy distributions and the fact that the Stix model does not model the pitch distribution. These results provide useful insights about the merits and drawbacks of the TRANSP and Stix models. In particular, it can be useful for quantifying the uncertainty introduced by using the Stix model instead of TRANSP if the beam-target reactivity is used as input for interpreting neutron measurements from fusion plasmas.