The Oil Drum

An important property of a superconductor is its critical current density (i.e. the maximum current density it can carry while still remaining a superconductor). This is a particular problem for superconducting magnets since the critical current density decreases in the presence of a magnetic field. While high temperature superconductors exist (~110K) they don't have the critical current densities needed for magnets producing 13 Tesla fields where as for low temperature metal superconductors (e.g. Nb3Sn) it is just about possible. People are talking about using high temperature superconductors in Maglev trains some time in the future but the field strengths would be a lot lower than a tokamak. It should also be noted that because of all this the generally held notion that if we could get a viable fusion reactor working it would be perfectly safe is probably wrong. A full scale tokamak that produced useful amounts of energy would be huge (football stadium size) and hense the associated magnetic field would not only have a very large field strength but would encompass a massive volume. Playing around with the equation E = B^2.V/(2mu) you should be able to convince yourself that the energy stored in the field would be enormous and therefore a magnet coolant failure could be catastrophic.