Overview of first Wendelstein 7-X high-performance operation

The optimized superconducting stellarator device Wendelstein 7-X (with major radius [ image ], minor radius [ image ], and [ image ] plasma volume) restarted operation after the assembly of a graphite heat shield and 10 inertially cooled island divertor modules. This paper reports on the results from the first high-performance plasma operation. Glow discharge conditioning and ECRH conditioning discharges in helium turned out to be important for density and edge radiation control. Plasma densities of [ image ] with central electron temperatures [ image ] were routinely achieved with hydrogen gas fueling, frequently terminated by a radiative collapse. In a first stage, plasma densities up to [ image ] were reached with hydrogen pellet injection and helium gas fueling. Here, the ions are indirectly heated, and at a central density of [ image ] a temperature of [ image ] with [ image ] was transiently accomplished, which corresponds to [ image ] with a peak diamagnetic energy of [ image ] and volume-averaged normalized plasma pressure [ image ]. The routine access to high plasma densities was opened with boronization of the first wall. After boronization, the oxygen impurity content was reduced by a factor of 10, the carbon impurity content by a factor of 5. The reduced (edge) plasma radiation level gives routinely access to higher densities without radiation collapse, e.g. well above [ image ] line integrated density and [ image ] central temperatures at moderate ECRH power. Both X2 and O2 mode ECRH schemes were successfully applied. Core turbulence was measured with a phase contrast imaging diagnostic and suppression of turbulence during pellet injection was observed.