Experimental verification of the axial and lateral stiffness of large W7-X rectangular bellows

The WENDELSTEIN 7-X (W7-X) is a super-conducting helical advanced stellarator. W7-X is provided with 299 ports, which connect the plasma vessel with the outer vessel. All the ports are equipped with bellows, which, during bake-out and final adjustment, compensate the relative movements of the vessels. The lateral stiffness which was calculated with the established standard EJMA code [EJMA code, Standards of the expansion joint manufactures association, INC, 25 North Broadway, Tarrytown, NY 10591. [1]] seemed to be relatively small. This appeared to be not correct in particular for non-circular bellows. The stiffness of rectangular, multilayer bellows have then been re-calculated with the finite element method (FEM) code ANSYS. The maximum difference between the FEM and EJMA code resulted to be up to 250% in particular with movements along the longer side of the bellows. In order to clarify the differences a test-campaign with the largest rectangular bellows was performed. The paper summarises the calculation, describes the test activities, the apparatus and reports the final results.