The instability strength of a diverted plasma with vertical elongation is measured for a range of the magnetic-field decay index, −1.5 < n < +0.5, in the standard-D and inverted-D configurations. The range of the instability growth time is from one to four orders of magnitude greater than previously published results. When the plasma current is in the range of 170–310 kA, the instability can be suppressed by passive stabilization due to currents induced in a discrete coil system together with a moderate-power (100 kW) active feedback system. The inverted-D configuration is three times more unstable than the standard-D configuration for the same ellipticity. The inverted-D configuration is destabilized by its negative triangularity and the standard-D is stabilized by its positive triangularity.