  Absolute cross sections for electron-impact single ionisation (EISI) of
multiply charged tungsten ions (W$^{q+}$) with charge states in the range $ 11
\leq q \leq 18$ in the electron-ion collision energy ranges from below the
respective ionisation thresholds up to 1000~eV were measured employing the
electron-ion crossed-beams method. In order to extend the results to higher
energies, cross section calculations were performed using the
subconfiguration-averaged distorted-wave (SCADW) method for electron-ion
collision energies up to 150~keV. From the combined experimental and scaled
theoretical cross sections rate coefficients were derived which are compared
with the ones contained in the ADAS database and which are based on the
configuration-averaged distorted wave (CADW) calculations of Loch et al. [Phys.
Rev. A 72, 052716 (2005)]. Significant discrepancies were found at the
temperatures where the ions investigated here are expected to form in
collisionally ionised plasmas. These discrepancies are attributed to the
limitations of the CADW approach and also the more detailed SCADW treatment
which do not allow for a sufficiently accurate description of the EISI cross
sections particularly at the ionisation thresholds.
