Electromagnetic-Structural Analysis of a Superconducting Magnet With Active Shielding for a Rotating Gantry

As a magnet utilized for a rotating gantry in heavy particle radiotherapy, a superconducting magnet with active shielding has been proposed to reduce the magnet weight. The magnet is composed of a dipole coil and an active shielding coil wound with NbTi wires. In this study, an electromagnetic-structural analysis of the magnet with active shielding was conducted using an FEM model. The results of the analysis indicate that the coil deformation, due to an electromagnetic force, slightly affects the field quality but negligible for the operation. The design concept of superconducting magnets with active shielding is valid in terms of magnetic field and the support structure for a rotating gantry. In addition, based on the previous study, the weight of the superconducting magnet with active shielding was revaluated. As a result, the superconducting magnet with active shielding can be one-third lighter than a superconducting magnet surrounded with an iron yoke, in terms of the magnet weight per unit length

Design of Lightweight Superconducting Magnets for a Rotating Gantry With Active Shielding

In order to reduce the weight of superconducting magnets used in a rotating gantry for heavy particle radiotherapy, the design study of a cosine-theta superconducting magnet with active shielding was conducted. The superconducting magnet with active shielding was composed of a dipole coil and an active shield coil. The cross-section of each coil was designed, based on a cosine-theta current distribution made by the arrangement of a superconducting wire with NbTi filaments. The result of the design study indicated that the designed coil cross-sections can meet the requirements regarding the magnetic field distribution and the load factor for the magnet operation. Additionally, the weight of the superconducting magnet with active shielding was evaluated while comparing that of the superconducting magnet with an iron yoke. As a result, the superconducting magnet with the active shielding has a possibility to reduce the weight significantly

Measurements of self-field and voltage for the REBCO stacked tapes assembled in rigid structure (STARS) conductor at 77 K

The measurements of self-fields and voltages for the stacked tapes assembled in rigid structure (STARS) conductor were carried out when the conductor immersed in liquid nitrogen was energized. In the measurements, a 3 m straight line shaped STARS conductor, which is composed of 45 REBCO tapes (15 tapes × 3 rows) embedded in a copper stabilizer, was used as a conductor sample. The measurement results indicate that the ramp rate at the conductor excitation and the one-time thermal cycle of the conductor do not affect the self-fields but the voltages.The current center position in the conductor cross-section during the conductor excitation was analyzed from the measurement results of the self-fields. The current center position always maintains the same position in the middle of the cross-section during the excitation. The analysis indicates that the current distribution of the STARS conductor is stable during the excitation. In addition, two-dimensional magnetic field calculations were conducted using the various models for the cross-sectional configuration of the STARS conductor. As a result, the calculation results agree with the measurement results regarding the self-fields

Effects of Subcooling on Lengths of Propagating Normal Zones in the LHD Helical Coils

Propagation of a short normal zone was observed in a helical coil of the Large Helical Device, when the coil was cooled with subcooled helium, of which the inlet and outlet temperatures are 3.2 K and below 4.0 K, respectively. The normal zone was induced at the bottom position of the coil. It propagated to only the downstream side of the current with recovery from the opposite side, and stopped after passing the outer equator of the torus. The induced balance voltage is obviously lower and the propagating time is shorter than those of propagating normal zones observed in the helical coil cooled with saturated helium at 4.4 K. According to the simulation of the propagation of a normal zone, it is considered that such a short normal zone at the current close to the minimum propagating current propagates without full transition to film boiling

Self-field measurements of an HTS twisted stacked-tape cable conductor

For a twisted stacked-tape cable (TSTC) conductor composed of REBCO tapes, self-field measurements were conducted with Hall sensors. In the measurements, a 650 mm diameter single turn coil wound with the TSTC conductor, which was made with 48 REBCO tapes whose width was 6 mm, was utilized as a test sample. Based on the measurement results, the current distribution of the TSTC conductor was investigated with analytical models. The analytical results indicate the current distribution of the TSTC is uniform under the condition that the operating current is 10 kA and the sample temperature is approximately 30 K. On the other hand, the current distribution is not uniform at the excitation and the degauss of the TSTC conductor with the ramp rate of 50 A/s

Effect of Direction of External Magnetic Field on Minimum Propagation Current of a Composite Conductor for LHD Helical Coils

The conductor for helical coils of the Large Helical Device consists of a Rutherford-type NbTi/Cu cable, a pure aluminum stabilizer, and a copper sheath. The dimensions of the conductor and the stabilizer cross-sections are 18.0 mm × 12.5 mm and 12.4 mm × 5.2 mm, respectively. The measured cold-end recovery current in the magnetic field parallel to the shorter side (B//12.5) is clearly lower than that in the field parallel to the longer side (B//18.0) because of the difference in magnetoresistance by Hall currents. Since the minimum propagation current Imp is important to determine the upper limit of operation current, Imp has been measured for two types of one-turn coil samples, which were bent flatwise (B//18.0) and edgewise (B//12.5) with the inner radius of 0.14 m to extend the length in the uniform background field of the test facility. The measured Imp at B//12.5 is almost the same as that at B//18.0 in spite of the large difference in the steady-state resistance. Imp is considered to be determined by the heat balance before the current diffuses deeply into the stabilizer

Measurement of Decay Time Constant of Shielding Current in ITER-TF Joint Samples

Joint sample tests have been carried out as a qualification test for ITER Toroidal Field (TF) coils. The joint sample comprises two short TF conductors that have "twin-box" joint terminals at both ends. The lower joint is a testing part that is a full size joint of the TF coils. Hall probes are attached on the lower joint box at around the center of the external field coil of the test facility. The magnetic field induced by shielding currents in the joint can be estimated from the difference between the measured magnetic field strength and the magnetic field generated by the external field coil. The magnetic field by the shielding currents during shut-off of the external field coil from -1.0 T is evaluated for six samples. The decay time constants of the shielding currents are gradually elongated with decrease of the shielding currents in all the samples. In comparison with simulation results, it is considered that the main shielding current flows in superconducting cables in the two conductors with crossing the jointed plane and that the joint resistance is decreased at low total current

Evaluation of ITER TF Coil Joint Performance

To evaluate the ITER TF joint performance, the joint test sample, which consists of two short TF conductors and has full size joint, shall be tested using NIFS test facility under the condition of current of 68 kA and external field of 2 T. For high accuracy, the issue of voltage difference between cable and jacket had been anticipated in the evaluation of joint resistance. If a voltage difference exist between them, it is difficult to measure real joint resistance using voltage taps on the jacket. Therefore, the author first calculated the position where voltage of cable and jacket become equipotential and then decided the voltage tap position where the influence of voltage drop could be avoided. Thus, a high accuracy measurement of joint resistance could be achieved and the joint resistance was accurately evaluated as around 1 n Ω , which is well below the ITER requirement of 3 n Ω

Test of ITER-TF Joint Samples With NIFS Test Facilities

Qualification tests of the ITER toroidal field (TF) conductor joints have been carried out by testing joint samples with test facilities in the National Institute for Fusion Science, NINS, Toki, Japan. The joint sample consists of two short TF conductors with the length of 1535 mm, which is restricted by the test facility with 9-T split coils and 100-kA current leads. The sample current is supplied from a dc 75-kA power supply. Each conductor has two joint boxes at both terminals. The lower joint is a testing part that is a full-size joint of the TF coil. The joint resistance of the lower joint is estimated from the increase of the average voltage drop among the six taps on the conductor against the currents. Five joint samples were tested until 2016, and all the samples satisfied the requirement of the joint resistance at less than 3 nΩ. The method of the measurement and the results are summarized, and the voltage distribution among the voltage taps is discussed

Results of All ITER TF Full-Size Joint Sample Tests in Japan

Nine toroidal field (TF) coils have been developed in Japan for the international thermonuclear experimental reactor (ITER). The joint resistance of TF coil should satisfy the requirement of smaller than 3 nano-ohm at 2 T of external magnetic field and 68 kA of transport current. Full-size joint sample (FSJS) tests were performed for joint development and for TF coil manufacture, as part of the process control. 11 FSJS tests are conducted in total. FSJS tests were conducted with assistance from a test faculty in the National Institute for Fusion Science as reported in a previous paper. All FSJS tests successfully satisfied the requirement of resistance less than 3 nΩ at 2 T. Additionally, the TF coil joints are subjected to cyclic electromagnetic force and warm-up/cool-down during the ITER operation. The authors investigated the joint performance for the abovementioned influence. The results showed no degradation in the joint resistance. Thus, the TF joint developed in Japan was qualified successfully