Dirac $R$-matrix and Breit-Pauli distorted wave calculations of the electron-impact excitation of W$^{44+}$

Abstract: With construction of ITER progressing and existing tokamaks carrying out ITER-relevant experiments, accurate fundamental and derived atomic data for numerous ionization stages of tungsten (W) is required to assess the potential effect of this species upon fusion plasmas. The results of fully relativistic, partially radiation damped, Dirac $R$-matrix electron-impact excitation calculations for the W$^{44+}$ ion are presented. These calculations use a configuration interaction and close-coupling expansion that opens-up the 3d-subshell, which does not appear to have been considered before in a collision calculation. As a result, it is possible to investigate the arrays, [3d$^{10}$4s$^2-$3d$^9$4s$^2$4f] and [3d$^{10}$4s$^2-$3d$^9$4s4p4d], which are predicted to contain transitions of diagnostic importance for the soft x-ray region. Our $R$-matrix collision data are compared with previous $R$-matrix results by Ballance and Griffin as well as our own relativistically corrected, Breit-Pauli distorted wave and plane-wave Born calculations. All relevant data are applied to the collisional-radiative modelling of atomic populations, for further comparison. This reveals the paramount nature of the 3d-subshell transitions from the perspectives of radiated power loss and detailed spectroscopy.