Neodymium Iron Boron Magnets for Launch Sleds
An N52 magnet has a maximum energy product of 50 mega-gauss Oersteds, which is 50*7958 J/m3 or 398 KJ/m³ . NIB density is 7.5, or 7500 kg/m³ so that works out to 53 J/kg.
A high strength NIB magnet should be kept below 80C; launch loop magnet sleds should be refrigerated before use, perhaps to liquid nitrogen temperatures, as they will be heated by hypersonic passage through the thin upper atmosphere. Some grades of NIB (N38 EH) can tolerate 200C, but are 36 mega-gauss Oersteds, 38 J/kg.
A reusable launch sled containing one tonne of N38 EH NIB might be 10 square meters and 1.3 cm thick, 38 KJ.
If the one tonne sled is pushing itself and 5 tonnes of payload with an acceleration of 30 m/s², the total thrust on the sled is 150 KN, or 15 KN/m².
Custom Shaped Field Magnets can be made to concentrate flux on the motor side.
A permanent magnet can be modelled as a ribbon of current around the edges. It can also be modelled as an array of dipole magnets.
FEMM Finite Element Magnet Modelling
http://www.femm.info/wiki/Documentation/
NiB alternatives
University of Minnesota claims anisotropic Iron Nitride magnets Fe16N2 can theoretically produce 134 MGOe, have made 10 MGOe so far.