The introduction of superconducting magnet technology has
allowed physicists to attain higher energies in circular
accelerators. One can obtain higher magnetic fields because
there is no resistance in a superconducting coil. In conventional
electromagnets resistance causes heating of the coil, and so
much energy is lost in the form of heat that the power cost of the
magnets is very high. Superconducting coils have allowed
higher field magnets to be operated reliably and at less power
cost.
The energy of the circular accelerator at Fermilab was doubled by a change to superconducting magnet technology. The proposed Large Hadron Collider will use superconducting magnets to guide the beam around its ring, which even so will be some 8.5 km in diameter!