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.
Under construction for the stellarator project Wendestein 7-X is a neutral beam heating system based on RF driven positive ion sources. It is planned to start operation with 2 sources capable of injecting 5 MW of heating power in deuterium. This paper gives the current status and future plans of the construction of the injector boxes and subsequent installation in the experimental hall. The fruitful collaboration with the National Centre for Nuclear Research in Swierk, Poland is also detailed. Lastly, results from an initial study on fast ions in Wendelstein 7-X will be given.
Articles you may be interested inThe dual role of nitrogen as alloying and confining element in GaAs-based dilute nitride semiconductors
The Ar-ion-beam mixing of Fe/Zr multilayers is studied in detail by conversion electron Miissbauer spectroscopy (CEMS) and x-ray diffraction (XRD). The dependence of the ion-beam induced amorphization and interfacial mixing on the sublayer thickness and ion dose (1X 1O'3-2X1O*6 Ar/cm') is studied systematically for samples with Fe to Zr thickness ratios d&,=1 and 0.5 and modulation wavelengths A=d,+d, of 5-80 nm and 7.5-90 nm, respectively. The CEMS results allowed the evaluation of the mixing efficiency from the thickness of the mixed layers. The experimentally determined mixing efficiency was compared with theoretical estimates based on the ballistic collision and thermal spike models, showing good agreement with the predictions of the modified ballistic collision model. For high degrees of amorphization the composition of the amorphous phase formed due to ion-beam mixing is close to the nominal composition of the sample, as revealed by CEMS measurements. These results were compared with those obtained for amorphous Fe-Zr alloys formed by vapor deposition. The XRD results fully agree with CEMS measurements and show that due to ion irradiation the amorphous Fe-Zr phase is formed. The XRD results show that a change of texture occurs from Zr(OO2) to Zr(100) in the samples with small A irradiated with high ion dose. XRD reveals in these samples the formation of the ZrOz phase. 5232
The experimental knowledge on interlayer potential of graphenites is summarized and compared with computational results based on phenomenological models. Besides Lennard-Jones approximation, the Mie potential is discussed, Kolmogorov-Crespy model and equation of Lebedeva et al. An agreement is found between a set of reported physical properties of graphite (compressibility along c-axis under broad pressure range, Raman frequencies for bulk shear and breathing modes under pressure, layer binding energies), when a proper choice of model parameters is made. It is argued that the Kolmogorov-Crespy potential is the preferable one for modelling. A simple method of fast numerical modelling, convenient for accurate estimation of all these discussed physical properties is proposed. It is useful in studies of other van der Waals homo/heterostructures.
A Chemical Vapor Deposition graphene monolayer grown on 6H–SiC (0001) substrates was used for implantation experiments. The graphene samples were irradiated by He+ and N+ ions. The Raman spectra and electrical transport parameters were measured as a function of increasing implantation fluence. The defect concentration was determined from intensity ratio of the Raman D and G peaks, while the carrier’s concentration was determined from the relations between G and 2D Raman modes energies. It was found that the number of defects generated by one ion is 0.0025 and 0.045 and the mean defect radius about 1.5 and 1.34 nm for He+ and N+, respectively. Hole concentration and mobility were determined from van der Pauw measurements. It was found that mobility decreases nearly by three orders of magnitude with increase of defect concentration. The inverse of mobility versus defect concentration is a linear function, which indicates that the main scattering mechanism is related to defects generated by ion implantation. The slope of inverse mobility versus defect concentration provides the value of defect radius responsible for scattering carriers at about 0.75 nm. This estimated defect radius indicates that the scattering centres most likely consist of reconstructed divacancies or larger vacancy complexes.
The aim of this paper is to give an experimental evidence that point defects (most probably gallium vacancies) induce decomposition of InGaN quantum wells (QWs) at high temperatures. In the experiment performed, we implanted GaN:Si/sapphire substrates with helium ions in order to introduce a high density of point defects. Then, we grew InGaN QWs on such substrates at temperature of 730 °C, what caused elimination of most (but not all) of the implantation-induced point defects expanding the crystal lattice. The InGaN QWs were almost identical to those grown on unimplanted GaN substrates. In the next step of the experiment, we annealed samples grown on unimplanted and implanted GaN at temperatures of 900 °C, 920 °C and 940 °C for half an hour. The samples were examined using Photoluminescence, X-ray Diffraction and Transmission Electron Microscopy. We found out that the decomposition of InGaN QWs started at lower temperatures for the samples grown on the implanted GaN substrates what provides a strong experimental support that point defects play important role in InGaN decomposition at high temperatures.
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