Overview of first Wendelstein 7-X high-performance operation

Abstract

The optimized superconducting stellarator device Wendelstein 7-X (with major radius R = 5.5 m, minor radius a = 0.5 m, and 30 m3 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 × 1019 m-3 with central electron temperatures 5-10 keV were routinely achieved with hydrogen gas fueling, frequently terminated by a radiative collapse. In a first stage, plasma densities up to 1.4 × 1020 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 · 1019 m-3 a temperature of 3.4 keV with Te/Ti = 1 was transiently accomplished, which corresponds to nTi(0)TE = 6.4 × 1019 keV s m-3 with a peak diamagnetic energy of 1.1 MJ and volume-averaged normalized plasma pressure {B}= 1.2%. 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 1 × 1020 m-2 line integrated density and Te = Ti = 2 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.