Engineering design study of quasi-axisymmetric stellarator with low aspect-ratio

The quasi-axisymmetric stellarator CHS-qa has been designed from physics point of view on the basis of the so-called optimization of helical magnetic field configurations. Once its engineering design was introduced briefly as a part of physics design, here the details are described. The toroidal period number N of 2 has been selected because it gives a small aspect ratio resulting in a large plasma volume favorable for physics experiments. CHS-qa has the following machine parameters: major radius R of 1.5 m, plasma minor radius a of 0.47 m (Ap(=R/a)=3.2), maximum toroidal magnetic field strength of 1.5 T, flat-top time of 1 sec at full field. Because of quasi-axisymmetry of the magnetic field structure with N=2 the whole structure of the machine is of highly non-axisymmetry and of deformation, which causes asymmetric centripetal forces on modular coils and complicated distributions of the electromagnetic force. Major engineering points on the modular coils and on the vacuum vessel are as follows: 1) modular coil design (curvature and twist of conductors), 2) supporting structures of modular coils, 3) manufacturing process of vacuum vessel and modular coils, 4) design of ports for heating and diagnostics. The following results have been obtained. 1) Shapes of modular coils are designed under the tolerable curvature and twist. 2) To cope with centripetal forces and overturning forces supporting structures for coils are designed by using elliptical plates on the inboard side and rods between modular coils. Stress analysis is also done under the condition where the ratio of modular coil currents is changed for controlling bumpiness of the magnetic field. 3) The vacuum vessel is inside the modular coils. How to make the whole assembly has been carefully examined. 4) Tangential injection of NB is available. (orig.)