The international ITER fusion project now under assembly in France is designed to achieve an industrial scale 500 MW “burning plasma,” where fusion reaction dominates plasma heating. Creation and control of a burning plasma is considered an essential step for fusion energy development.

The Electron Cyclotron Heating and Current Drive (ECH&CD or ECH) system performs several functions in the ITER machine, including plasma heating, non-inductive current drive, neoclassical tearing modes instability suppression, plasma start-up, and vacuum vessel discharge cleaning. The ECH system consists of up to twenty-four 1.25 MW, 170 GHz gyrotrons, transmission lines (TLs), and launchers to inject the microwave beam power into the plasma.

The subject of this case study is the structural support of the TLs. The TLs are routed throughout the Tokamak complex supported by what most structural engineers would recognize as a typical pipe bridge. However, swapping out piping for ECH TLs makes this anything but your typical pipe bridge. The TLs require precision alignment so that at least 90% of the power injected into the TLs is delivered to the end of the TLs with the required mode purity. To achieve this precision alignment, the structural supports must provide uniform displacement regardless of load distribution. To achieve the required alignment, the TL supports must be aligned to within 0.25 mm. Tolerances such as this are unheard of in typical building construction. Thus, producing unique challenges when coupled with the industry standard tolerances associated with building construction and the fact that the system travels through multiple buildings. To meet these rigorous tolerances all areas of variability ignored in typical design were examined.

• Connection stiffness contributions – The stiffness of what would normally be considered “simple connections” (i.e. free to rotate) were analyzed in ANSYS to determine the true rotational stiffnesses. The stiffnesses obtained from the FEA (Finite Element Analysis) were in turn used in the global analysis of the ECH support bridge.
• To accommodate for construction tolerances the individual TL support beams utilized slotted connections to allow for vertical and horizontal alignment.
• Each TL mounting brackets will be shimmed in the field to achieve the final +/- 0.25 mm tolerance.

The ideal solution would have been to design adjustable TL mounting brackets. However, given the degree of accuracy required as well as the vast number of brackets this was deemed too costly. It is anticipated that in the future the system will need realignment as the building itself experiences settlement. However, assuming largely uniform settlement, the scope of the future realignment should be much less involved.

By Devin Malone, IDI Structural Engineer