electromagnet sweeper

I plan to make an electromagnet sweeper but my understanding of electromagnet field very basic. The sweeper condition should be operate by cut off DC supply to release the nails and etc and the piece load of metal about max 200 - 300 gram with range of magnetic field about 500 mm

My question :

1. Any impact of diameter of wire to field of magnetic ? If yes how is corelation
2. Same question with no.1 but for diameter of pole
3. If I use AC generator and recifier, is there any circuit needed to protect the wire and bar get burn ?
4. What's should be the best of metal use as pole for application of heavy duty.
5. To bring more magnet effect, can I apply more layer of wire on each pole ?
6. To cover the magnetic surface wider, can I put paralell bar ?

Thanks and appreciate your coment.

Regards, Zul