High-order multipole transitions in radiative opacities

The opacity code CASSANDRA, developed at AWE, has recently been modified to include all electric and magnetic multipole transitions of up to multipolarity L=5 in both the bound-bound and bound-free spectra. It is found that the inclusion of these transitions increases the Rosseland mean opacity of moderate-Z elements by a few percent and that the largest additional contribution arises from electric quadrupole transitions. At low densities and moderate temperatures, the increase in the Rosseland mean opacity is predominantly due to new bound-bound transition arrays while, at high temperatures and densities, the increase is due to a larger bound-free contribution