R-matrix calculations in support of diagnostics for fusion and astrophysical plasmas

In this thesis we develop and systematically analyse models which describe collisional processes involving neutral molybdenum (Mo I), neutral tungsten (W I), singly ionised iron (Fe II), and neutral iron (Fe I). While atomic data for the neutral stages of molybdenum and tungsten finds an important application when investigating the influx of impurities into magnetically confined fusion plasmas, atomic data for the low ion stages of iron facilitates the study of astrophysical plasmas across every corner of the universe. To study the atomic structures of Mo I, W I, and Fe II we make use of sophisticated configuration interaction methods implemented by the well known computer packages GRASP0 and CIV3. In addition, to accurately describe electron and photon collisions with these species we make use of relativistic and semi-relativistic R-matrix scattering methods implemented by the parallel Breit-Pauli R-matrix and Dirac R-matrix codes. We then employ collisional-radiative models, exploiting results from both the atomic structure and R-matrix calculations, to provide us with valuable diagnostic information about the plasma. The underlying theory of these atomic structure, scattering, and collisional-radiative methods will be discussed at length and in particular, we will note significant developments to the Dirac R-matrix codes which were necessary to handle the calculations presented throughout this thesis. Detailed level-resolved atomic structure models for Mo I, W I, and Fe II will be discussed and energies and radiative transition rates will be presented. Furthermore, results from large scale R-matrix scattering calculations exploring the electron-impact excitation of Mo I, W I and Fe II and the photoionisation of Fe I will also be presented. Alongside our results we will provide extensive com- parisons with existing theoretical calculations and experimental measurements with a particular focus on transitions most relevant for applications to fusion and astrophysical plasmas.