Magnetic coils in the shape of a circle, but more often the letter D, placed vertically and creating a toroidal magnetic field, the basic component of the magnetic field in tokamaks. The toroidal coils are strung on the donut-like vacuum chamber of the tokamak. They can be made of copper, but for larger tokamaks requiring a stronger magnetic field, the use of superconductors is essential. To maintain superconductivity, toroidal coils must be cooled to a temperature of 4 K.