Sparks coming out of SH Alt.

1 - A bit of conductive scrap (a strand of wire, metal filings, etc) got inside the alternator and has shorted across a heat sink, the brush holders, etc.

2 - If the alternator is used, the brushes may be worn out. When the brush springs contact the slip rings they'll burn. Sparks/bits of molten metal will then fall out of the backside of alternator.

3 - Could be you somehow got it wired up wrong and the sparking was the regulator doing a "melt down" thing. This however is unlikely since the alternator did work briefly.

FWIW I was stopped by a state trooper late at night a few years ago to advise he observed sparks falling from beneath my van. Turned out the alternator was on it's last legs due to worn out brushes (the alternator failed completely only a a few more miles down the road).

If they or the brush holder assembly (on a Delco anyway) are worn enough they can either hang up and not contact properly or as Bob suggested if worn away the spring will come in contact with the slip rings.