Dielectronic recombination of W20+ (4d(10)4 f(8)): addressing the half-open f shell

A recent measurement of the dielectronic recombination (DR) of W20+ [Schippers et al., Phys. Rev. A 83, 012711 (2011)] found an exceptionally large contribution from near-threshold resonances (less than or similar to 1 eV). This still affected the Maxwellian rate coefficient at much higher temperatures. The experimental result was found to be higher by a factor of 4 or more than that currently in use in the 100- to 300-eV range, which is of relevance for modeling magnetic fusion plasmas. We have carried out DR calculations with AUTOSTRUCTURE which include all significant single-electron promotions. Our intermediate-coupling (IC) results are more than a factor of 4 larger than our LS-coupling ones at 1 eV but still lie a factor of 3 below experiment here. If we assume complete (chaotic) mixing of near-threshold autoionizing states, then our results come into agreement (to within 20%) with experiment below less than or similar to 2 eV. Our total IC Maxwellian rate coefficients are 50%-30% smaller than those based on experiment over 100-300 eV.