Engineering Lessons Learned in the Assembly, Commissioning, Initial Operation and in the Further Upgrading of Wendelstein 7-X

Wendelstein 7-X (W7-X) was constructed from 2005 until 2015 at the Max-Planck-Institute for Plasma Physics, Greifswald, Germany. W7-X is one of the world's largest stellarator; it is five-fold symmetric; has a superconducting magnet system; and weighs about 1000 tons. W7-X started its first operation in 2015 with up to 4.3-MW electron cyclotron resonance heating (ECRH) and a heating duration of up to 6 s, and injected energy per pulse up to 4 MJ. The injected energy per pulse increased up to 200 MJ, for example, for 100 s at 2 MW and also 30 s at 5 MW in the third operation campaign in 2019. The machine was upgraded twice since 2016 and a third upgrading phase is being carried out now. The main aim of upgradation is the increase of the plasma energy as well as the increase of the heating energy flow through the machine. That requires, among others, the installation of a high heat flux (HHF) divertor and the strengthening of the associated cooling system. This contribution provides a general overview of the construction of W7-X. It summarizes some engineering lessons learned in the construction phases and in the first operation campaigns (OP1.1, OP1.2a, and OP1.2b). Furthermore, this article explains the technology challenges as well as modifications due to the first experiences in commissioning and operation. Technical, organizational, and schedule aspects of the assembly are discussed as well.