Mitsushi Abe

Demonstration of ripple reduction by ferritic steel board insertion in JFT-2M

In the JFT-2M tokamak, the application of low activation ferritic steels to plasmas has been investigated (the so-called Advanced Material Tokamak Experiment (AMTEX) programme). In the first stage, toroidal field ripple reduction was examined by ferritic steel boards (FBs) inserted between the toroidal field coils and the vacuum vessel. It is demonstrated that FB insertion reduced toroidal field ripple and the losses of fast ions produced by tangential co-NBI. By optimizing the FB thickness, such that fundamental mode ripple is minimized to 0.07% at the shoulder of the inside wall, ripple trapped loss is reduced to an almost negligible level. It is determined that the reductions of the fundamental mode ripple and the ripple banana diffusion coefficients at the shoulder are most effective in reducing ripple ion losses. Ripple loss reduction by FBs is also confirmed with perpendicular beam injection. The insertion of FBs causes no undesirable effects on plasma production and control.

Evaluation of magnetic fields due to the ferromagnetic vacuum vessel and their influence on plasma discharge in tokamak devices

Abstract We studied characteristics of the magnetic fields due to a ferromagnetic vacuum vessel (F-VV) experimentally and computationally to clarify whether plasma discharge is possible with the F-VV in tokamak devices. We made three kinds of evaluations using the Hitachi tokamak HT-2. One was a discharge test with error field coil. The second was a numerical analysis of the magnetic field induced by a ferritic first wall. The third was a discharge test with the ferritic first wall. Consequently, we confirmed that a normal plasma discharge could be obtained with a ferritic first wall in the HT-2. The strength of the localized magnetic field induced by the F-VV in the plasma region was smaller in tokamak devices with the size of the JFT-2M and ITER than in the HT-2. Therefore, the F-VV should be applicable to tokamak devices.

Design and first experimental results of toroidal field ripple reduction using ferritic insertion in JFT-2M

In order to test the effect of the toroidal field ripple on the fast ion losses, ferritic steel boards were inserted between the vacuum vessel and the toroidal field coil in the JFT-2M tokamak. The experimental and computational results show that the ripple amplitude is reduced from 2.2 to 1.1%. The ion losses are monitored from the increase in the wall temperature measured by the infrared TV. The region of the ripple trapped ion losses moves to outer side by about 6 cm and the increment of the wall temperature due to the ion loss of ripple trapped and banana drift is reduced. The ripple loss of the fast ions is reduced by ferritic steel insertion for the first time in the world. No deleterious effect of the ferritic insertion on plasma production and plasma control has been observed so far.

A new technique to optimize the coil winding path for the arbitrarily distributed magnetic field and application to a helical confinement system

A new technique to calculate the optimum modular coil winding path for a helical plasma confinement system has been developed. The technique is based on current potential on an arbitrary surface with finite elements and singular value decomposition (SVD). No function is assumed for the current potential distribution, and then, little error occurs from modeling. The SVD is applied to the response matrix from the current potential to the magnetic field on the plasma surface. The current distribution is constructed from an eigen distribution obtained through SVD so that the normal field component is small. The coil winding path is determined along the flow lines or equi-contour lines of current potential. The current on the obtained coil winding path was confirmed able to generate good magnetic surfaces.

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.

Development of a Decision Method for Optimum Ferromagnetic Board Arrangements to Reduce Toroidal Magnetic Field Ripple

A new decision method for ferromagnetic board (FB) arrangements was developed to obtain the desired toroidal magnetic field (TF) distribution. An approximation, in which the magnetization vector is only pointed in the toroidal direction and its magnitude is magnetically saturated, was introduced to allow treatment as a linear problem. The desired thickness distribution of FBs could be obtained by calculating a generalized inverse matrix which transforms from the thickness distribution to the TF distribution due to FBs. We employed a singular value decomposition (SVD) method to calculate the generalized inverse matrix. The final TF distribution was confirmed by the general-purpose 3D non-linear magnetostatic field analysis code. This method was applied to the TF coils of JFT-2M. The maximum TF ripple in the whole plasma region could be reduced from 2.1% to 0.1%.

Low loop voltage startup and equilibrium control using multivariable poloidal field coils in the Hitachi Tokamak HT-2

Tokamak operation techniques to control the poloidal magnetic field using multivariable poloidal field coils (MPFCs) were applied to the Hitachi Tokamak HT-2. Two problems encountered in operating a tokamak with MPFCs were identified; low-voltage startup and equilibrium control without interference. The key to their solution was accurate control of the poloidal magnetic field. To obtain multipole fields, a singular value decomposition was applied to a response matrix from the coil current to the magnetic flux value at the plasma surface region. The multipole fields are orthogonal bases of the poloidal field, and the interference was cleared using these modes. A control technique using the multipole fields was applied to control the null point position of the poloidal magnetic fields during breakdown, which made it possible to get breakdown with a low loop voltage. During the flattop phase, good controllability without interference was obtained using the concept of a multipole magnetic field. 19 refs., 16 figs., 5 tabs.

The gap recession reduced MIG head

In order to reduce the decrease of head output level during running, the wear recession formed on a composite material and MIG heads with various ferrite crystallographic orientations was investigated. It was found that wear recess depends mainly on the wear rate ratio of materials, and that the decrease in output level was in proportion to the spacing loss due to the formation of a level difference at the interface between the metal and the ferrite core. However, the decrease measured using tape different from the running tape was also affected by the unfitness between head and tape. It was also found that the damage caused to metal evaporated (ME) tape depends on the ferrite orientation of the head. >

Experimental Study on Electromagnetic Interactions between Plasmas and a Vacuum Vessel during Disruptions in the Hitachi Tokamak HT-2

Electromagnetic interactions between plasmas and a vacuum vessel during disruptions are examined experimentally in the Hitachi tokamak HT-2. Eddy currents which flow in the toroidal direction and poloidal coil currents are determined from the measured magnetic data. The currents enable calculation of the electromagnetic force on the vacuum vessel and resistively dissipated magnetic energy. Eddy currents and electromagnetic forces are mainly due to the plasma displacement (shell effect), not decay of the plasma current. Large plasma current quench rate -dIP/dt is associated with scraping of the plasma by the inner limiter through the rapid plasma radial movement, and the decay rate in circular plasma is twice as large as that in elongated plasma. The magnetic energy dissipation is mainly due to the eddy current of the net toroidal current mode which is induced by large current quench rate.

Consideration on Current and Coil Block Placements With Good Homogeneity for MRI Magnets Using Truncated SVD

This paper describes a new method to determine the current and coil block placements with good homogeneity for magnetic resonance imaging (MRI) magnets, and discusses the relationships between the placements and homogeneity, using regularization of the truncated singular value decomposition (SVD). In the first step, the main coil (MC) is modeled as filament loop currents (FLCs) on a solenoid and the shield coil (SC) is modeled by coil blocks. The FLCs are determined so as to get a homogeneous magnetic field using a superposition of eigenmodes obtained by the SVD. In the second step, the FLCs are replaced by MC blocks. The findings are as follows. 1) The obtained FLC distribution in the axial direction has several peaks, which can be suitably replaced by MC blocks and the number of the peaks is equal to the number of MC blocks. 2) There is an optimum length to minimize the absolutely summed current and the 1542-mm length is the optimum for a 525-mm radius with six MC blocks. 3) A one-eigenmode (one MC block) increase or decrease changes the length by