Takeshi Nakayama

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.

Reduction of energetic particle loss by ferritic steel inserts in ITER

Alpha particle loss due to the toroidal field ripple is significantly enhanced in reversed shear plasmas, especially in a high-q regime. To ensure a tolerable heat load on the wall due to the loss of alpha particle in reversed shear discharges with various q profiles, ferritic steel (FS) inserts are considered as a countermeasure for reducing the ripple amplitude at a low cost. With this being the situation, an assessment of ripple loss for various conditions in ITER is an important study that should to be conducted in the International Tokamak Physics Activities. This paper describes an assessment result, indicating that FS inserts lead to a dramatic reduction of ripple loss. When the arrangement of the FS inserts is optimized, the resulting alpha particle loss is anticipated to be reduced by an order of magnitude or more, and the power loss fraction is as low as 1% even for a reversed shear plasma with a relatively high qmin of 3.

Investigation on ripple loss reduction by ferritic steel plate insertion in JFT-2M: comparison between experimental and computational data

Ripple loss reduction resulting in reduced losses of fast ions was demonstrated successfully at the JFT-2M tokamak by installing ferritic plates between the vacuum vessel and toroidal field coils. This paper describes the comparison between the experimental results and computation based on the orbit following a Monte Carlo code. They are in qualitatively good agreement in the cases where the toroidal distribution of the toroidal field ripple is almost periodic (e.g. ferritic plate installed resulting in a complex periodic structure of the magnetic ripple), but differ in the case of toroidally small average ripple exhibiting non-periodic large local ones (e.g. ferritic plate installed resulting in a more complex non-periodic structure of the magnetic ripple).

Measuring magnetisation reversal in micron-sized Nd2Fe14B single crystals by microbeam x-ray magnetic circular dichroism

Magnetisation reversal of micron-sized Nd2Fe14B single crystals with magnetisation as weak as 10−9 emu (1 µm size) was studied. Single-crystal specimens (cylinders with diameter and height of 1 to 6 µm) were prepared by focused-ion beam so that both the magnetic easy and hard axes were included in the basal plane. Their magnetic hysteresis loops were measured when they were rotated with respect to the cylindrical axis by using microbeam hard-x-ray magnetic circular dichroism (XMCD) under transmission geometry. It was found that coercivity is inversely proportional to the cosine of the angle between the magnetocrystalline easy axis and magnetic-field direction and that the magnetisation reversal is dominated by domain-wall pinning in two different modes. One is related to penetration of the reversed domain nucleated in a subsurface soft layer into the bulk hard phase, of which the hysteresis loops exhibit a single-stage abrupt jump in magnetization. The other mode is pinning of the walls within the bulk grain, of which the hysteresis loops exhibit a plateau. The multi-domain structure associated with the pinning was confirmed by XMCD mapping. The proposed method fills the gap between conventional bulk magnetic measurement and submicron-scale electrical-transport measurement for nanofabricated thin films and/or fine particles. It is expected to provide new insights into elemental magnetisation processes in micron-scale regions.

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.

Articulatory characteristics of speakers with apraxia of speech during sentence production at different speaking rates

The present study developed quantitative descriptions of articulatory movements in three speakers with apraxia of speech during sentences produced at different speaking rates. Point‐parameterized articulatory data were obtained using the x‐ray microbeam (XRMB) technique, and were compared with similar materials from 24 normal young‐adult speakers of American English drawn from an existing XRMB database. Speed histories of markers attached to the lips and jaw during production of the test sentence ‘‘The other one is too big.’’ spoken at slow, habitual and fast rates were analyzed to determine mean peak speed, the number of speed peaks, and the timing of particular speed peaks for each sentence replicate by each talker. An attempt was made to derive speed histories for the lip marker expressed relative to concurrent movements of the jaw, to evaluate the relative contribution of jaw speed to lip speed in both talker groups. A description and discussion of results from the analysis will emphasize speaking rat...