Overview of first Wendelstein 7-X high-performance operation

The optimized superconducting stellarator device Wendelstein 7-X (with major radius $R=5.5\,\mathrm{m}$, minor radius $a=0.5\,\mathrm{m}$, and $30\,\mathrm{m}^3$ 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 $1-4.5\cdot 10^{19}\,\mathrm{m}^{-3}$ with central electron temperatures $5-10\,\mathrm{keV}$ were routinely achieved with hydrogen gas fueling, frequently terminated by a radiative collapse. Plasma densities up to $1.4\cdot 10^{20}\,\mathrm{m}^{-3}$were reached with hydrogen pellet injection and helium gas fueling. Here, the ions are indirectly heated, and at a central density of $8\cdot 10^{19}\,\mathrm{m}^{-3}$ a temperature of $3.4\,\mathrm{keV}$ with $T_e/T_i=1$ was accomplished, which corresponds to $nT_i(0)\tau_E=6.4\cdot 10^{19}\,\mathrm{keVs}/\mathrm{m}^3$ with a peak diamagnetic energy of $1.1\,\mathrm{MJ}$. The discharge behaviour has further improved with boronization of the 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 $1\cdot 10^{20}\,\mathrm{m}^{-2}$ line integrated density and $T_e=T_i=2\,\mathrm{keV}$ 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.