  We present new experimentally measured and theoretically calculated rate
coefficients for the electron-ion recombination of W$^{18+}$([Kr] $4d^{10}$
$4f^{10}$) forming W$^{17+}$. At low electron-ion collision energies, the
merged-beam rate coefficient is dominated by strong, mutually overlapping,
recombination resonances. In the temperature range where the fractional
abundance of W$^{18+}$ is expected to peak in a fusion plasma, the
experimentally derived Maxwellian recombination rate coefficient is 5 to 10
times larger than that which is currently recommended for plasma modeling. The
complexity of the atomic structure of the open-$4f$-system under study makes
the theoretical calculations extremely demanding. Nevertheless, the results of
new Breit-Wigner partitioned dielectronic recombination calculations agree
reasonably well with the experimental findings. This also gives confidence in
the ability of the theory to generate sufficiently accurate atomic data for the
plasma modeling of other complex ions.
