Development of 7 T Cryogen-free Superconducting Magnet for Gyrotron

We developed a special type of 7 T 240 mm vertical bore cryogen-free superconducting magnet system, which will be used for gyrotron. The magnet system consists of three sets of coils which are charged separately. A set of main coils is wound with NbTi conductor and produces 7 T at the center of the bore tube. A set of gun coils is also wound with NbTi conductor and located below the main coil. It produces up to -1.4 T at a lower part of the bore tube and controls the magnetic field where a gun is located. A set of sweep coils are located inside of the main coil. The sweep coil produces only +/- 0.2 T but is charged and discharged within several seconds so as to control electron trajectory. To avoid quench with the rise of the temperature from the large AC loss, this set of coils are wound with Nb₃Sn conductor. Also to reduce the induced current in the main coil, the sweep coil is actively shielded. This magnet system will contribute to the fast control of the gyrotron oscillation frequency.