A single layer race track magnet with a large Nb
3Sn conductor has been constructed and tested in order to obtain engineering information on fabrication of a 10T multishell dipole magnet.
A bronze processed Nb
3Sn monolithic wire is used in the wind and react method. The cross section of the conductor is 2.3×6.0mm
2 and the critical current is 4.60kA at 10T and 4.2K. The coil has an inner width of 25mm and an outer width of 181mm with a thickness of 6mm. The length of the straight section is 800mm in the total length of 956mm. The number of turns is 30. Mica glass tapes are sandwiched as insulators between the turns, and mica boards bonded with inorganic adhesives are used between the coil and two shielding iron plates. The coil has not been impregnated with epoxy resin after the heat treatment to keep good thermal contact with a liquid helium.
After the construction of this magnet the following things became clear. A large current monolithic conductor can be wound in a small diameter as 25mm. The coil keeps its shape without an impregnation of epoxy resin. Though the conductor expanded longitudinally about 0.5% after the heat treatment, no local wire looseness and no stress concentration in the windings were induced. A mica glass tape withstands the heat treatment, which is surely useful as an insulator for a Nb
3Sn magnet in the wind and react method.
In the excitation tests, the magnet experienced two quenches at the currents of 5.32kA and 6.82kA, and reached 7.00kA which was the upper limit of our power supply. The central field is 4.65T at 7.00kA. The estimated critical current of the Nb
3Sn conductor is about 11.83kA at 4.65T and 4.2K.
Voltage taps having each interval of 2cm are attached to the wire in every 5 turns of the windings. Propagations of normal zone, temperatures of the conductor after quenches and voltage oscillations were measured using the voltage taps. Such voltage oscillations have been found in excitations of other superconducting magnets and always observed just before quenches during training. Therefore the measurement of the voltage oscillation can be used as a detector of a mechanical disturbance in a magnet.
The starting points of both of two quenches deduced from the normal zone propagations were at the boundaries between the straight section and the end part of the windings, and then the voltage oscillations were surely observed just before the quenches. The cause of these quenches and training is considered that the wires at the end part are not fixed sufficiently and moved by the Lorentz force.
The minimum quench energy of the magnet and the propagation velocities of normal zone were measured using a heater placed on the windings. From the result of these experiments and thermal analyses, it is estimated that there is no degradation of the critical current caused by the winding and heat treatment, and the thermal margin of the magnet is sufficiently large for the local heat disturbances.
抄録全体を表示