A study on the buckling behaviour of a bellows-type vacuum vessel with torus shape in a reversed field pinch device

Buckling strength of a thin bellows vacuum vessel with torus shape is studied for design work on a reversed field pinch (RFP) fusion experimental device. In the RFP both toroidal and poloidal magnetic fields must be rapidly changed in the initial setting-up phase of the magnetic field configuration and high one-turn resistances of the vessel in both directions are required. Thus a very thin bellows with shallow convolution has to be used and the mechanical strength of the bellows-type vessel is a crucial design point. Here the buckling strength of the vessel is analysed numerically by applying the finite element method to a half-pitch of the bellows in torus shape. Results show that the buckling strength of the bellows torus is proportional to the third power of the equivalent bending thickness and approximately equal to that for a straight cylindrical tube having the same plate thickness as the equivalent bending thickness. Thus the torus effect seems to be unimportant in a bellows torus. This result is different from the previous study for simple toroidal vessels, where the buckling strength is increased by the torus effect.