Protection and stabilisation of large superconducting coils

Large superconducting coils, with stored energies from 108J upwards, must be stable and have a predictable performance. This is achieved by combining the superconductor with a good ordinary conductor (substrate) to form a composite and by providing cooling throughout the winding. In addition, large superconducting coils must be protected against overheating and voltage breakdown which might be caused by some failure, notably of the cooling system. We show that the use of an external resistor with a switch is a suitable method of protection and is preferable to the use of a secondary winding. A composite conductor is again the solution, and we show how to determine the minimum quantity of substrate which will satisfy the requirements both of stabilisation and of protection. We conclude that such large superconducting coils can be made stable and be protected.