After completing the main construction phase of Wendelstein 7-X (W7-X) and successfully commissioning the device, first plasma operation started at the end of 2015. Integral commissioning of plasma start-up and operation using electron cyclotron resonance heating (ECRH) and an extensive set of plasma diagnostics have been completed, allowing initial physics studies during the first operational campaign. Both in helium and hydrogen, plasma breakdown was easily achieved. Gaining experience with plasma vessel conditioning, discharge lengths could be extended gradually. Eventually, discharges lasted up to 6 s, reaching an injected energy of 4 MJ, which is twice the limit originally agreed for the limiter configuration employed during the first operational campaign. At power levels of 4 MW central electron densities reached 3  ×  1019 m−3, central electron temperatures reached values of 7 keV and ion temperatures reached just above 2 keV. Important physics studies during this first operational phase include a first assessment of power balance and energy confinement, ECRH power deposition experiments, 2nd harmonic O-mode ECRH using multi-pass absorption, and current drive experiments using electron cyclotron current drive. As in many plasma discharges the electron temperature exceeds the ion temperature significantly, these plasmas are governed by core electron root confinement showing a strong positive electric field in the plasma centre.

The pyrochlore oxides (A2B2O7) exhibit a remarkable range of structural, physical, and magnetic properties related to their various chemical compositions. This article reports the phase transformations induced by high electronic excitation in pyrochlores of the Gd2(ZrxTi1−x)2O7 family irradiated with swift ions. The structural changes, investigated by using several analytical techniques (x-ray diffraction, Raman spectroscopy, and transmission electron microscopy), strongly depend on the chemical composition. The high electronic excitation along the ion trajectory results in the amorphization of ion tracks for Gd2Ti2O7 and Gd2TiZrO7, whereas a defective fluorite structure is formed in Gd2Zr2O7. Moreover, the results underline the existence of an electronic stopping power threshold of 6 keV/nm for amorphizable compounds and 10 keV/nm for Gd2Zr2O7, below which phase transformations do not occur. Finally, the study of the thermal recovery of irradiated pyrochlores provides the recrystallization temperature for amorphized samples and reveals differences in the recovery process which are related to the chemical composition.

The next step in the Wendelstein stellarator line is the large superconducting device Wendelstein 7-X, currently under construction in Greifswald, Germany. Steady-state operation is an intrinsic feature of stellarators, and one key element of the Wendelstein 7-X mission is to demonstrate steady-state operation under plasma conditions relevant for a fusion power plant. Steady-state operation of a fusion device, on the one hand, requires the implementation of special technologies, giving rise to technical challenges during the design, fabrication and assembly of such a device. On the other hand, also the physics development of steady-state operation at high plasma performance poses a challenge and careful preparation. The electron cyclotron resonance heating system, diagnostics, experiment control and data acquisition are prepared for plasma operation lasting 30 min. This requires many new technological approaches for plasma heating and diagnostics as well as new concepts for experiment control and data acquisition.

6H-SiC and 3C-SiC single crystals were simultaneously irradiated at room temperature with 100-keV Fe ions at fluences up to 4x10 13 cm-2 (~0.7 dpa), i.e. up to amorphization. The disordering behaviour of both polytypes has been investigated by means of Rutherford backscattering spectrometry in the channelling mode and synchrotron X-ray diffraction. For the first time, it is experimentally demonstrated that the general damage build-up is similar in both polytypes. At low dose, irradiation induces the formation of small interstitial-type defects. With increasing dose, amorphous domains start to form at the expense of the defective crystalline regions. Full amorphization of the irradiated layer is achieved at the same dose (~0.45 dpa) for both polytypes. It is also shown that the interstitial-type defects formed during the first irradiation stage induce a tensile elastic strain (up to ~3.8%) to which is associated an elastic energy. It is conjectured that this stored energy destabilizes the current defective microstructure observed at low dose and stimulates the formation of the amorphous nanostructures at higher dose. Finally, the disorder accumulation has been successfully reproduced with two models (namely MSDA and DI/DS). Results obtained from this modelling are compared and discussed in the light of experimental data.

This work reports the study, via the combination of Rutherford backscattering spectrometry and channeling, x-ray diffraction, and transmission electron microscopy experiments, of the damage formation in cubic yttria-stabilized zirconia single crystals irradiated with medium-energy (4 MeV) heavy (Au) ions. The damage buildup, which is accounted for in the framework of the multistep damage accumulation model, occurs in three steps. The first step at low fluences (up to 10(15) cm(-2)), characterized by a regular increase in both the damage yield and the elastic strain, is related to the formation of small defect clusters. The second step in the intermediate fluence range (from 10(15) to 5 X 10(15) cm(-2)) leads to a sharp increase in the damage yield and to a large drop of the strain due to the formation of dislocation loops which collapse into a network of tangled dislocations. The third step at high fluences (above 5 X 10(15) cm(-2)) exhibits a surprising decrease in the damage yield, which may be attributed to the reorganization of the dislocation network that leads to the formation of weakly damaged regions with a size of the order of 100 nm. (C) 2009 American Institute of Physics. [doi:10.1063/1.3236567