Numerical analysis of electromagnetic stress induced in bellows for a magnetic fusion reactor

The use of bellows in a magnetic fusion reactor gives, in many cases, more efficient penetration of the poloidal electric field and very efficient plasma heating in the first phase of operation. This is because bellows sections lead to higher electrical resistance of the vacuum vessel. There are many examples of its application to tokamak type experimental devices such as TFTR, JT-60 and so on. In addition to its higher electrical resistance, the bellows has an ability to absorb large deformation due to thermal expansion and radiation creep, and thus has been applied to the design of a first wall configuration in some conceptual designs of a commercial power reactor. Its application to a vacuum vessel of the Reacting Plasma machine, which was proposed by the Institute of Plasma Physics of Nagoya University, was examined for circular plasmas. When applying the numerical analysis to the bellows, a computational difficulty occurs. Too many unknows appear in a mesh division with a general shell element and these are not easily handled. Therefore in the present paper a finite element code is made to consider an axisymmetric structure loaded by nonsymmetric forces. In the code various types of electromagnetic forces are taken into account. These include the diamagnetic electromagnetic force, the toroidal-current-induced-electromagnetic forces, the electromagnetic forces due to toroidal coil current change and saddle shaped electromagnetic forces. Aluminum is selected as bellows material because of its low activation nature but there are large induced currents because of its higher electrical conductivity. Results of stress analysis show that a leak of the saddle shaped current into the thick aluminum vacuum vessel generates maximum bending stress near a joint between the bellows and the vessel. Stress due to other types of electromagnetic force are significantly smaller.