The potential role of cryogenics in insertion magnets

Most of the insertion magnets that are used for storage rings and free-electron lasers are room temperature permanent magnets. Superconducting wiggler and undulator magnets have been built, but their performance has been limited by the engineering current density and stability of the coils. Superconducting undulators must have a small gap and cell length, which can be a hindrance even when the beam vacuum chamber is at room temperature. Beam heating is also an issue. To control, the heat leak at low temperature, the beam vacuum chamber should be cooled to a temperature between 20 and 40 K. Permanent magnets fabricated from Nd-Fe-B can be cooled to cryogenic temperatures with an increase in the magnetic field within the magnet gap. This permits the magnet gap to be reduced considerably when the vacuum chamber is at the same temperature as the permanent magnet. This paper discusses the cryogenic cooling of both superconducting and cryogenic permanent magnet wigglers and undulators.