The optimized stellarator as a candidate for a fusion power plant

The stellarator is a promising concept for the magnetic confinement of Deuterium-Tritium plasmas for energy production by means of nuclear fusion. However, the magnetic field geometry of the stellarator needs to be carefully optimized to overcome intrinsic performance limitations. The superconducting stellarator device Wendelstein 7-X, currently under construction in Greifswald, Germany, is the key device for the verification of stellarator optimization principles. To establish the optimized stellarator as a serious candidate for a fusion power plant, reactor-relevant plasma parameters must be achieved in fully integrated steady-state plasma discharge scenarios. It is the goal of the project Wendelstein 7-X to demonstrate this for the first time. After more than 10 years of construction, the completion of the device is now ahead. In the first part of the present paper, we briefly introduce into the principles of nuclear fusion and magnetic confinement. In the second part we discuss lessons learned during the device assembly and first experiences with the remaining major work packages. In the third part of the paper, we report on the planning for the first operation phase (5-10s discharge duration at 8 MW heating power), the completion and hardening of the device for full power steady-state operation, and the second operation phase (up to 30 min discharge duration at 10 MW heating power). Finally, a preview of a possible design concept for a stellarator-based fusion power plant is presented.