AbstractReliable superconducting magnets and cryostats have been designed and built for use with several different types of high-power gyrotron tube operating in the frequency range from 60 to 140 GHz. Methods for controlling and monitoring the axial field profiles are described. Ways of minimising the radial field variation and achieving geometrical alignment of the magnetic field and cryostat are discussed. Transverse ‘beam steering’ coils can be fitted.A full set of magnet power supplies and monitoring electronics has been developed. The system may be operated and monitored by computer.Efficient liquid nitrogen shielded, liquid helium cooled cryostats with long hold times have been developed. For installations involving a small number of gyrotrons, simple enthalpy cooled cryostats are often an economic solution.For multiple gyrotron installations, there is a requirement for reliable and economic long-hold cryostats which require little attention during use. To address this problem, the incorporation of a two-stage closed-cycle cooler is described. The closed-cycle cooler may be removed for routing maintenance without the need to warm the cryostat.