Engineering Challenges in W7-X: Lessons Learned and Status for the Second Operation Phase

In 2015, the optimized stellarator Wendelstein 7-X stellarator (W7-X) started with operation. The main objective of W7-X is the demonstration of the integrated reactor potential of the optimized stellarator line. An important element of this mission is the achievement of high heating power and high confinement in the steady-state operation. The approach to this mission is the following three steps. First, plasmas were produced in a limiter configuration [operation phase (OP 1.1)], then a test divertor unit is being installed (temporary divertor unit) for the next campaign, OP 1.2, before the full steady-state capability will be achieved implementing active cooling of all in-vessel components and a steady-state high heat-flux divertor. In December 2015, the first helium plasma was generated using electron cyclotron resonance heating (ECRH), in February 2016, the working gas was switched to hydrogen. The first OP (OP 1.1) was successfully finished in March 2016. At the end of OP 1.1, the discharge duration was close to 6 s, the limit for the integrated heating power was increased to 4 MJ and electron temperatures of ~10 keV were achieved. Due to the low densities in the range of 10
¹⁹
cm
⁻³
and the pure electron heating by ECRH, the ion temperatures reached only 2 keV. At present, W7-X is undergoing the next completion phase, including the installation of the test divertor unit, the installation of the carbon tiles on the inner plasma vessel wall, an upgrade of existing diagnostics, and the installation of new diagnostics. This paper discusses the first operational phase, lessons learned and implemented, and the status before the start of the second OP (OP 1.2).