Masashi Kitamura

An optimal design technique for coil configurations in iron-shielded MRI magnets

An optimization method to determine coil positions in iron-shielded MRI (Magnetic Resonance Imaging) magnets has been developed. In our method, strengths of the field harmonics of desired orders, instead of the spatial distribution of the central field, are restricted to obtain a highly homogeneous magnetic field. Details of the optimization procedure and calculation results are presented. >

Cogging torque due to roundness errors of the inner stator core surface

Cogging torque of brushless permanent magnet (PM) motors employing segmented stator cores with narrow slot openings has been investigated. Two types of test motors were examined: one with a segmented Halbach cylinder rotor and the other with a radially oriented ring PM rotor. The measured cogging torque was predicted from the measured dimensional errors for the inner radius of the stator core in both types.

A study on optimal coil configurations in a split-type superconducting MRI magnet

Optimal coil configurations in a split-type superconducting MRI magnet have been investigated, and the feasibility of the compact and high performance magnet is presented. A highly homogeneous magnetic field of about 10 ppm/50 cm diameter spherical volume (DSV) is attainable by eliminating error fields of higher orders and, moreover, the radius of the outermost coil can be reduced to about 70 percent of that of the magnet with positive current simply by alternately arranging coils with positive and negative currents.

Three-dimensional optimization of correction iron pieces for open high field MRI system

We have developed a new magnet for open high field (0.7 T) MRI systems. In order to homogenize magnetic fields in the field of view, an optimization method to determine initial configurations of large correction iron pieces was developed. In our method, we combined linear programming (LP) and 3D magnetic FEA and, using analytic formulae for LP, we computed field harmonics due to the volume of magnetized iron.

A direct search shape optimization based on complex expressions of 2-dimensional magnetic fields and forces

A 2-dimensional shape optimization method for magnetic materials and conductors such as those of induction machines has been developed. The method combines a steady state AC field analysis based on complex expressions of magnetic fields, a method to compute Lorentz force in complex form and the direct search method. As an example, shape optimization of a single sided linear induction motor is reported.

Shielding stray magnetic fields of open high field MRI magnets

We have developed a new magnet for open high field (0.7 T) MRI systems. Since MRI systems are installed in hospital sites, it is important to reduce stray magnetic fields of the magnets. We studied passive, active and hybrid shielding methods and compared them. In order to study feasibility of active shielding, we considered higher order multipole stray fields. We present details of the calculation method and the results.

An optimal design of coaxial coils with constraints on inner and outer multipole magnetic fields

An optimal design method for magnets with double-layered coaxial coils to generate a uniform central magnetic field and a small stray magnetic field has been proposed. To find the optimal current density distribution, we used linear programming with constraints on the multipole field strengths, computed with analytical formulae, of the central and stray fields. The final coil positions were optimized with a quasi-Newton method, again taking into account the inner and outer multipole field strengths.

A shape optimization method based on complex expressions of 2-dimensional magnetic fields

A shape optimization method has been developed regarding magnetic materials in 2-dimensional geometry. The method combines (i) complex expressions of magnetic fields, (ii) computation of complex magnetizations based on integral equations and (iii) the quasi-Newton method. Details of the computational procedure and calculation results are reported.

Current distribution characteristics of superconducting parallel circuits

In superconducting parallel circuits of a insulated multi-strand cable or a parallel persistent current switch (PCS), the current distribution during the current sweep, the persistent mode, and the quench process were investigated. In order to measure the current distribution, two methods were used. (1) Each strand was surrounded with a pure iron core with the air gap. In the air gap, a Hall probe was located. The accuracy of this method was deteriorated by the magnetic hysteresis of iron. (2) The Rogowski coil without iron was used for the current measurement of each path in a 4-parallel PCS. As a result, it was shown that the current distribution characteristics of a parallel PCS is very similar to that of a insulated multi-strand cable for the quench process. >

Evaluation of efficiency and temperature rise of permanent magnet synchronous motor depending on magnetic properties

This paper reports a utility of the design method that conducts thermo-magnetic coupling analysis by using a heat equivalent circuit and describes evaluation results of relationship between losses and temperature rise based on magnetic prop- erties. We manufactured and tested prototype motors that were designed with thermo-magnetic coupling analysis and found that reduction of copper loss become of particular importance in order to reduce temperature rise, and total losses of motors.