  We present images of radio recombination lines (RRLs) at wavelengths around
17 cm from the star-forming region W49A to determine the kinematics of ionized
gas in the THOR survey (The HI/OH/Recombination line survey of the inner Milky
Way) at an angular resolution of 16.8"x13.8". The distribution of ionized gas
appears to be affected by feedback processes from the star clusters in W49A.
The velocity structure of the RRLs shows a complex behavior with respect to the
molecular gas. We find a shell-like distribution of ionized gas as traced by
RRL emission surrounding the central cluster of OB stars in W49A. We describe
the evolution of the shell with the recent feedback model code WARPFIELD that
includes the important physical processes and has previously been applied to
the 30 Doradus region in the Large Magellanic Cloud. The cloud structure and
dynamics of W49A are in agreement with a feedback-driven shell that is
re-collapsing. The shell may have triggered star formation in other parts of
W49A. We suggest that W49A is a potential candidate for star formation
regulated by feedback-driven and re-collapsing shells.
