Superconducting magnet for a traveling wave maser

A superconducting magnet has been developed to supply the magnetic field necessary to operate a 4 Gc traveling wave ruby maser. The field required by the maser is about 3000 oersteds uniform throughout the volume of the ruby to one part in a thousand. The field is directed across a gap of about 0.8 inches whose cross section is 5 inches by 0.5 inches. A close approach to the ideal magnetic circuit has been achieved by placing superconducting shields adjacent to the pole pieces. Circulating currents are set up in the shields which prevent the existence of leakage and fringing flux. Since there is no loss mechanism in the shields, the currents persist and the air-gap field remains everywhere uniform. With the absence of leakage and fringing flux, the size and weight of the magnetic circuit is considerably reduced. Superconducting coils allow the use of a persistent current to generate the field. A heater element encased in a glass sleeve is used to initiate the persistent current. The switching procedure is simple and involves no moving parts in the cryostat.