Superconducting magnet for variable bandwidth traveling wave masers

In order to amplify microwave signals with optimum gain, the ruby crystal of a 4-Gc/s traveling wave maser requires a 3300-G magnetic field uniform to 0.1 percent. By properly varying the magnetic field along the length of the ruby crystal, the bandwidth of the maser amplifier can be increased at the expense of gain. A compact superconducting magnet is described, which provides the required magnetic field. The field is obtained from a short solenoid, which is made to appear infinitely long through the use of a low reluctance enclosure. A portion of the resulting uniform field can be modified by a superconducting trimmer coil imbedded in the low reluctance enclosure. This coil produces an adjustable stepped magnetic field which allows for varying maser bandwidth. Alternatively, it can be used to operate two independent masers in a single magnetic structure. A high degree of reliability is attained. Problems of flux jumping have been avoided by a design in which the diamagnetic (magnetic insulating) properties of superconducting materials play a minor role.