Since you mentioned 12 volts before, if you wind that coil with 20
gauge film insulated magnet wire( about 1000 feet; coil will be
approximately 26 layers at 88 turns per layer), you'll have about 12
watts into the coil (will draw about 1 amp and if you care, the
inductance will be about 84 milliHenry) and get about 400 gauss at the
center of the coil with no iron in it. Iron will increase the field.
This should be in a safe continuous duty temperature range. To push
things you can drop the wire to 18 gauge which increases the center
field to about 800 gauss, but will put 45 watts into the coil. That
baby will likely get HOT! If this is too weak then you've got to go
with winding the coil with tiny copper tubing use low voltage and
heavy current and run water through it to take the heat away. Any way
to remove heat will help such as fans or heat sinks. But the problem
is the heat is going into the center of the coil where air currents or
heat sinks can't carry it away. Why not use another permanent magnet
as a "pusher"? They'll be plenty strong.
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