Evaluation on the operational state of turboexpanders in a helium refrigerator for nuclear fusion experimental devices using principal component analysis

ORCID　 0000-0002-1989-867XThe operational state of helium turboexpanders has been evaluated using principal component analysis (PCA). For the evaluation, two models which were single and multiple ones were utilized. The single and the multiple models assess a single turboexpander and seven turboexpanders, respectively. In the model development, we used measurement data of helium turboexpanders for a cryogenic system in the Large Helical Device. Using the two models, the changes in the turboexpanders' condition before and after the occurrence of the trouble were clarified. Consequently, the PCA is very useful to monitor the turboexpanders. Comparing the two models, the single is more practical than the multiple one.journal articl

Study on the State Prediction of a Pool-Cooled Large Superconducting Coil Using Machine Learning

ORCID　0000-0002-1989-867XFor the LHD subcooling system composed of pool-cooled large superconducting coils wound with NbTi superconductors, a machine learning technique was introduced to increase the reliability of the system. The machine learning model for the state prediction of the system was developed using the technique, together with the data accumulated in the LHD plasma experimental campaign. Regarding the temperature changes in the system due to coil excitation and discharging, it is possible to make predictions using the model. Especially for the usual coil current waveform in a helical coil operation, which is a trapezoidal waveform, the model achieved high prediction accuracy.journal articl

Investigating the Effect of Coil Length, Alignment Errors and Cooling Down on a Superconducting Magnet With Active Shielding for Rotating Gantry

ORCID　0000-0002-1989-867XA superconducting magnet with active shielding has been proposed for use in a heavy-ion beam rotating gantry in order to decrease weight. The magnet consists of two types of coils, dipole and active shielding coils, both of which are wound using Nb-Ti wires. In this study, three-dimensional coil configurations were designed, based on coil cross-sections designed in a previous study. Using the coil configurations, the magnetic field distribution in the longitudinal and radial directions of the coil and its peak magnetic field were calculated. The results indicated that the effect of the coil end section on magnetic field distributions became greater as the coil length becomes shorter. Also, the difference of the coil length has no impact on the peak field. In addition, the influence of coil alignment errors on the electromagnetic (EM) force, and that of cooling-down on coil deformation were investigated, using a coil cross-sectional model. As a result, EM forces caused by alignment errors of several mm in the shielding coil were negligible, compared to EM forces generated in the coil under a rated operation. Also, the maximum deformation of the coil cross-section was a few millimeters when the coil was cooled from 300 K to 5 K.journal articl

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.journal articl

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

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

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