By any chance, did you shut down with the battery cable loose or off? Is the engine ground strap loose, dirty or damaged?

When the engine shuts down, a powerful reverse current forms in the alternator coils. It dissipates into the battery if everything is in good shape. I think you'll find that there's a weak connection somewhere between the battery and the alternator. If so, the inductive kick would have nowhere to go, and the stator may be toast.

What you need to do is clean and tighten the battery cables, including the battery ground at the point it bolts to the body. Also get under the car, find the engine ground strap and make sure the connection points are clean. Then take the alternator back and act outraged at having received a dud.