Successful physical experiment campaigns have been performed on the most advanced modular stellarator Wendelstein 7-X (W7-X) during the second operation phase (OP1.2). The completion phase (CP2) lasting until 2021 is devoted to the installation of cryo-vacuum pumps (CVPs), new diagnostics, and actively cooled in-vessel components instead of the inertially cooled ones used in OP1.2. This update allows us to move forward to achieve the steady state operation during the next operation phase (OP2). Several first wall components which are exposed to heat flux in the range from 250 to 500 kW/m 2 are covered by graphite tiles facing the plasma. Due to construction constraints, the backside area of the graphite tiles is not fully covered by the actively cooled CuCrZr heat sinks. This results in the presence of high-temperature graphite rims, which become an additional heat radiation source for the components behind the first wall. This article presents stepwise development of backside protections (BSPs) to mitigate the problem mentioned above. The process is supported by: 1) the study of BSP shielding performances through thermal analysis taking into account plasma radiation and electron-cyclotron resonance heating (ECRH) loads; 2) BSP sizing restricted by electromagnetic forces during main coil/plasma current decay; and 3) mechanical analysis to confirm the structural fixation of BSP. In addition, several analysis iterations for CVP have been repeated to minimize BSP cost by identification of first wall components with minor heat radiation to the CVP and marginal influence on its cooling system capacity.

The superconducting bus bar system of the Wendelstein 7-X (W7-X) experiment consists of 121 bus bars and 184 bus bar joints, which provide the electrical and hydraulical connections between bus bars and coils and between different bus bar sections. The bus bars are made of the same superconductor as the coils. The W7-X superconductor is made of 243 NbTi copper stabilized strands enclosed in an aluminum jacket. The bus bars were cut, bent, and partially insulated before they were delivered to the assembly area of W7-X. The assembly of the bus bars was a logistical and technical challenge because of numerous interfaces and geometrical boundary conditions. The complexity of installation required extensive quality assurance and assembly plans, where each work step had to be precisely defined. The assembly of all bus bars and bus bar joints has been finished. The installation of the current leads that connect the bus bar system to the power supplies is underway.

The stellarator fusion experiment Wendelstein 7-X (W7-X) is presently under assembly at the Greifswald branch of the Max Planck Institute for Plasma Physics (IPP), Germany. The W7-X superconducting magnet system basically consists of 50 non planar and 20 planar coils including the interconnecting bus bars, and the support structure. The seven electrical circuits with ten coils each in series, are supplied by 14 current leads (CL) operating between the cryogenic and ambient temperature environments. A special feature of these feed troughs is their upside-down orientation to save space in the vicinity of the machine. Basic electrical CL requirements are maximal steady state currents of 18.2 kA and voltage strengths of 13 kV to ground. A W7-X current lead consists of a copper conductor which also acts as a heat exchanger at the warm end side, in its continuation of a high temperature superconductor part, and at the cold end side of a copper bar with integrated Nb3Sn rods. All components are fully contained within a CL vacuum chamber which is separated from the main W7-X cryostat vacuum. The high voltage electrical insulation is built up of a glass tape epoxy resin system. Mechanical support of the current leads is achieved by a warm and cold glass fiber reinforced plastic flange. There are three He cooling circuits: one for the bus bar and contact cooling, one for the cold end of the CL (both at about 5 K), and one for the CL's heat exchanger (entrance temperature about 50 K). After intensive tests of two prototypes the series production has been established and completed. The current leads were tested at room temperature, and with electrical currents up to the maximum current at cryogenic temperatures.

The superconducting magnet system of the WENDELSTEIN 7-X (W7-X) experiment consists of 50 nonplanar and 20 planar coils which are connected by 121 bus bars in series in seven groups of ten coils each. The terminal ends of the coils and bus bars are not insulated by the manufacturers and will be insulated during assembly. The insulation has to withstand a voltage of 13 kV dc for 1 min at Paschen conditions. Furthermore it has to be considered that the quench detection wires which were fed inside the insulation along the coil and bus bar ends have to be fed through the insulation near the joints