A low-aspect-ratio quasi-axisymmetric stellarator CHS-qa was designed. An optimization code was used to design a magnetic field configuration with evaluations of physical quantities of quasi-axisymmetry, rotational transform, MHD stability and alpha particle collisionless confinement. It is shown that the electron neoclassical diffusion coefficient is similar to tokamaks for the low collisional regime. A self-consistent equilibrium with bootstrap current confirms the global mode stability up to 130 kA for R = 1.5 m and Bt = 1.5 T device. The evaluation of plasma rotation viscosity is greatly suppressed compared with conventional stellarators. Engineering design was completed with 20 main modular coils and auxiliary coils which provide flexibility of configuration study for confinement improvement and MHD stability.

ASDEX Upgrade has recently finished its transition towards an all-W divertor tokamak, by the exchange of the last remaining graphite tiles to W-coated ones. The plasma start-up was performed without prior boronization. It was found that the large He content in the plasma, resulting from DC glow discharges for conditioning, leads to a confinement reduction. After the change to D glow for inter-shot conditioning, the He content quickly dropped and, in parallel, the usual H-Mode confinement with H factors close to one was achieved. After the initial conditioning phase, oxygen concentrations similar to that in previous campaigns with boronizations could be achieved. Despite the removal of all macroscopic carbon sources, no strong change in C influxes and C content could be observed so far. The W concentrations are similar to the ones measured previously in discharges with old boronization and only partial coverage of the surfaces with W. Concomitantly it is found that although the W erosion flux in the divertor is larger than the W sources in the main chamber in most of the scenarios, it plays only a minor role for the W content in the main plasma. For large antenna distances and strong gas puffing, ICRH power coupling could be optimized to reduce the W influxes. This allowed a similar increase of stored energy as yielded with comparable beam power. However, a strong increase of radiated power and a loss of H-Mode was observed for conditions with high temperature edge plasma close to the antennas. The use of ECRH allowed keeping the central peaking of the W concentration low and even phases of improved H-modes have already been achieved.

Electron density and electron temperature profiles are reconstructed from Thomson scattering data on the stellarator Wendelstein 7-AS by means of systematic statistical modelling employing the Bayesian probability theory (BPT). The BPT allows for systematic combination of all information entering the measurement descriptive model considering all uncertainties of the measured data, calibration measurements, physical model parameters and measurement nuisance parameters. The BPT results are consistent with the ratio-evaluation method (REM) which is used to determine the electron temperature from the ratios of scattering signals. If compared to the sequential REM, the Bayesian error analysis is much more informative because it yields probability density functions of the quantities of interest. Moreover, systematic consideration of all the obtainable raw data, in particular those data suffering from low signal levels, results in an improved evaluation for weakly informative data. Sensitivity analysis of model parameters allows for finding crucial uncertainties which has impact on both diagnostic improvement and design.

Abstract. Understanding the influence of the microstructure of tungsten on hydrogen transport is crucial for the use of tungsten as first-wall material in fusion reactors. Here we report results from molecular dynamics and transition state studies on the influence of grain boundaries in tungsten on the transport of hydrogen. An exhaustive mapping of possible minimum activation energy migration trajectories for hydrogen as trace impurity reveals a strongly modified activation energy distribution in the neighborhood of grain boundaries together with an altered connectivity matrix. The results indicate that grain boundaries in polycrystalline tungsten may provide an important transport channel, especially for neutron damaged tungsten.