Ryuya Ando

Spatial and Temporal Variations of a Screening Current Induced Magnetic Field in a Double-Pancake HTS Insert of an LTS/HTS NMR Magnet

This paper presents experimental and simulation results of a screening current induced magnetic field (SCF) in a high temperature superconductor (HTS) insert that constitutes a low-/high-temperature superconductor (LTS/HTS) NMR magnet. In this experiment, the HTS insert, a stack of 50 double-pancake coils, each wound with Bi2223 tape, was operated at 77 K. A screening current was induced in the HTS insert by three magnetic field sources: 1) a self field from the HTS insert; 2) an external field from a 5-T background magnet; and 3) combinations of 1) and 2). For each field excitation, which induced an SCF, its axial field distribution and temporal variations were measured and compared with simulation results based on the critical state model. Agreement on field profile between experiment and simulation is satisfactory but more work is needed to make the simulation useful for designing shim coils that will cancel the SCF.

Development of a simulation method for dynamic characteristics of fuel injector

A simple and highly accurate one-dimensional simulation method is presented for simulating dynamic characteristics of a fuel injector. Magnetic aftereffect is taken into account in the method. The results confirm that the magnetic aftereffect plays an important role in the dynamic response of an injector.

A Novel Design Method of Shapes of Ferromagnetic Materials for the Superconducting MRI Magnets

We present a novel non-stochastic method for optimizing the shapes of ferromagnetic materials to generate a target static magnetic field. A magnetizing current corresponding to an error field is calculated as an inverse problem and then the current is replaced with an equivalent ferromagnetic material. Iteration of this process leads to an error field of almost zero. It should be noted that the initial condition of the material shape is critical for convergence. Consequently, we propose a combined method: the initial shape of the material is generated with a stochastic optimization algorithm, and then the shape is updated using an explicit method. We show that this method works well with a test 2D axisymmetric magnet.

Design Optimization for Superconducting Bending Magnets using Pareto Front Curve

A novel limit design method for superconducting magnets is presented. It is particularly suitable for ion core magnets such as those used in accelerator magnets. In general, a stochastic optimization whose objective functions consist of values, e.g., the magnetic field, experience field of superconducting coils, current density, and multipole field integral, is often used. However, it is well known that the obtained solution strongly depends on the initial one. Furthermore, once the calculation model is fixed, the range of solutions is also fixed, i.e., there are times when it may be impossible to find the global optimum solution even with a lot of parameter sweeps. In this study, we draw the Pareto front curve to obtain the range and infer whether the solution is an optimum one. In addition, the Pareto front curve indicates the neighborhood solution that is substituted for the initial one. After this process a stochastic optimization is implemented with its initial design parameters. To confirm the validity, we designed a superconducting bending magnet, and it showed that this method works well.

Verification of Method for Calculating Heat in a Cryocooled BSCCO Coil During Energization

A method for calculating the amount of heat in a bismuth-based superconducting (BSCCO) coil linearly increasing an operating current of 0 A to a rating current not close to the critical current is proposed. The proposed calculation method deals with hysteresis loss, transport current loss in a time-varying magnetic field, and eddy-current loss. The hysteresis and transport current losses were formulated on the basis of Brandts critical state model. The three kinds of losses were calculated and verified in energization tests using a cryocooled BSCCO coil with a diameter of 950 mm. Amounts of heat in energization were measured using a calorimetric method based on the temperature variations of the coil. The energization test was performed twice. Amounts of heat in the first energization were larger than those in the second. The amounts of heat in the first energization had differences of 0.2 W at most for calculation results, indicating that the calculation method was able to predict temperature variations in the energization within an error rate of 0.1 K. In addition, the amounts of heat in the second energization experimentally indicated that only transport current loss occurs in the BSCCO coil when both the magnetization direction of the coil and the temporal variation of the magnetic field generated by the operating current are positive.

An Experimental Study on the Superconducting Coil Behaviors Using Electromagnetic Pulse Signals During Ramping

Measurement and the analysis are done for the pulse signals from magnetic sensors (search coils) near superconducting coils and voltage taps on the coils. The superconducting coils are arranged facing each other, and the magnetic sensors and the voltage taps are expected to detect the coil behavior (motion of the coils, motion of coil windings, and so on) as the pulse signals during the ramping up. One of the causes of the superconducting coil quenching is the occurrence of a small mechanical disturbance around the coil windings. Therefore, information about the coil behavior from these pulses should be useful to get a better understanding of the quenching phenomena. The results of the pulse analysis suggest that the signals can be classified into just two patterns. In addition, only one of them is observed from the signals just before all quenchings; these are supposed as signals of the trigger disturbance of quenching. This fact suggests that the disturbance which causes the quenching has a strong relationship with the specific motion of the coils.

A Study on the Functional Characteristics of Superconducting Loops for External Magnetic Flux Suppression

The functional characteristics of superconducting loops passively driven by an external magnetic flux have been experimentally investigated. The loops can be used for external magnetic flux suppression in superconducting magnets which need to generate highly stable magnetic fields. Two one-turn-loops were installed between two superconducting coils which generate a strong magnetic field, and a Hall sensor and a search coil were set at the center of the system. The effect of external magnetic flux suppression of the one-turn-loops was measured while changing the strength of the magnetic field on the loops. Contrary to expectation, as the magnetic field strength on the loops increased, the suppression rate decreased. The reason for the degradation was identified as a micrometer-order wire motion caused by the electromagnetic force on the loops. Then, the loops were remade and ultra-tightly fixed, before testing again. This time no degradation was observed.

Development of a Design Method for Superconducting Electromagnets Using Racetrack Coils

A new design method for superconducting magnets with a strong, homogeneous magnetic field at their center and a small stray field area outside the magnets has been developed. The method can design magnets with noncircular racetrack coils and field correction iron pieces. The simulated annealing method is adopted to optimize the arrangement of the coils. The minimized objective function is a combination of penalty terms, which are thickness of field correction iron pieces and constraints. The design method was used to prepare some trial designs for wide bore magnets with noncircular shaped coils. It was found that even with such noncircular coils, a homogeneous magnetic field is possible.