of around 0.5 miles (800 m). This is traditionally pictured as a circle, though in practice it could be more like a rounded rectangle. In either case it would require access to a significant amount of land to house the installation. Manufacturing There are two manufacturing issues around SMES. The first is the fabrication of bulk cable suitable to carry the current. The HTSC superconducting materials found to date are relatively delicate ceramics, making it difficult to use established techniques to draw extended lengths of superconducting wire. Much research has focused on layer deposit techniques, applying a thin film of material onto a stable substrate, but this is currently only suitable for small-scale electrical circuits. Infrastructure The second problem is the infrastructure required for an installation. Until room-temperature superconductors are found, the 0.5 mile (800 m) loop of wire would have to be contained within a vacuum flask of liquid nitrogen. This in turn would require stable support, most commonly envisioned by burying the installation. Critical magnetic field Above a certain field strength, known as the critical field, the superconducting state is destroyed. This means that there exists a maximum charging rate for the superconducting material, given that the magnitude of the magnetic field determines the flux captured by the superconducting coil. Critical current In general power systems look to maximize the current they are able to handle. This makes any losses due to inefficiencies in the system relatively insignificant. Unfortunately, large currents may generate magnetic fields greater than the critical field due to Ampere's Law. Current materials struggle, therefore, to carry sufficient current to make a commercial storage facility economically viable. Several issues at the onset of the technology have hindered its proliferation: Expensive refrigeration units and high power cost to maintain operating temperatures Existence and continued development of adequate technologies using normal conductors These still pose problems for superconducting applications but are improving over time. Advances have been made in the performance of superconducting materials. Furthermore, the reliability and efficiency of refrigeration systems has improved significantly. Long precooling time At the moment it takes four months to cool the coil from room temperature to its operating temperature. This also means that the SMES takes equally long to return to operating temperature after maintenance and when restarting after operating failures. Protection Due to the large amount of energy stored, certain measures need to be taken to protect the coils