Design and Loading Test Results of 21 T@4.2 K Magnet Using RHQT ${\rm Nb}_{3}{\rm Al}$ Insert Coils

In this experiment, we designed, constructed and tested a 21 T superconducting magnet to be operated at 4.2 K, consisting of a Nb₃Al inner coil with an exclusive power supply, and a series of Nb₃Al middle, Nb₃Sn outer-1 and Nb-Ti outer-2 coils with a common power supply. The flux jumps of the Nb₃Al middle coil occurred in low magnetic fields and caused frequent shutdowns of an overvoltage-sensitive common power supply; however, we discovered this could be avoided by reducing the current ramp rate. The outer-middle coil unexpectedly quenched at 17.7 T, which was less than the expected 19.3 T. Simultaneous excitation of the inner coil was scarcely effective in increasing the quench current of the outer-middle coil, indicating that the Ic degraded location is relatively far from the inner coil. However, energization of the inner coil to a quench, in a backup field of 17.6 T by the outer-middle coil generated a central field of 20 T in a 25.4 mm clear bore at 4.2 K.