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The W7-X experiment of the IPP, under construction in Greifswald Germany, is designed to operate in a quasi-steady-state scenario. The database structures and interfaces used for discharge description and execution have to reflect this continuous mode of operation. In close collaboration between the control group of W7-X and the data acquisition group a combined design of the data structures used for describing the configuration and the operation of the experiment was developed. To guarantee access to this information from all participating stations a TCP/IP portal and a proxy server were developed. This portal enables especially the VxWorks real-time operating systems of the control stations to access the information in the object-oriented database. The database schema includes now a more functional description of the experiment and gives the physicists a more simplified view of the necessary definitions of operational parameters. The scheduling of the long discharges of W7-X will be done by predefining operational parameters in segments and scenarios, where a scenario is a fixed sequence of segments with a common physical background. To hide the specialized information contained in the basic parameters from the experiment leader or physicist an abstraction layer was introduced that only shows physically interesting information. An executable segment will be generated after verifying the consistency of the high-level parameters by using a transformation function for every basic parameter needed. Since the database contains all configurations and discharge definitions necessary to operate the experiment, it is very important to give the user a tool to manipulate this information in an intuitive way. A special editor (ConfiX) was designed and implemented for this task. At the moment the basic functionality for dealing with all kind of objects in the database is available. Future releases will extend the functionality to defining and editing configurations, segments and scenarios with task-specific versions.

Hydride vapor phase epitaxy (HVPE) was used for the lateral overgrowth of thick GaN layers over masks of tungsten silicide nitride (WSiN). This mask material reduces sticking between the overgrown GaN and the mask and promotes the self-separation of the overgrown layers. Evaluation of the reflectance transient of the growing GaN layer indicates that the separation already occurs during growth and is caused by the intrinsic tensile strain of the growing GaN layer. This strain causes a strong concave bowing which is preserved even after cooling down. To reduce this residual bow of the freestanding GaN layers different approaches have been evaluated. It was found that the moment of separation is influenced by the mask geometry and that the temperature at this moment has an effect on the residual bow. The strain state of the template and the resulting bow at growth temperature are indicated to affect the residual bow as shown from the comparison of SiC- and sapphire-based GaN templates.

Freestanding c-plane GaN substrates are obtained using (100) γ-LiAlO2 substrates due to spontaneous separation during post-growth cool down. It is shown that by growth optimization the structural properties of 200 µm thick freestanding GaN layers can be improved resulting in FWHM values of ω-rocking curves below 400 arcsec for 002 and 302 reflection and threading-dislocation densities below 109 cm–2. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Self-separation of freestanding GaN layers of 50 mm diameter and thicknesses from around 50 µm to several hundred mm has been achieved using lateral overgrowth over silicon nitride masks with a high fraction of tungsten (WSiN). The moment of the separation was found to change with both the area ratio of the mask and the absolute structure size.

WENDELSTEIN 7-X is a long pulse experiment. One discharge may last up to 30 min and is segmented into many time intervals. A set of chained segments is called a scenario, and a few scenarios will make up the experiment program. For physical reasons segments of the experiment program must be available in a reasonable order. This means that physical programs need to be pre-planned and technical parameters need to be adjusted in the long run. All information necessary for this purpose is stored in the W7-X configuration database. To create, view and modify this information, an effective and user-friendly tool, the ConfiX editor, is needed. The ConfiX editor is a specialized editor for processing objects in the W7-X configuration database. It is used to prepare and describe the process of discharges. ConfiX is mainly used for three cases of application, namely to (1) define configurations, (2) prepare segment descriptions and (3) specify scenario descriptions. This contribution will give an overview of the purpose and functions of the ConfiX editor.

Intrinsic stress in GaN layers grown by HVPE on sapphire substrates results in wafer breakage for thicknesses exceeding 40 µm. Using ELOG 180 µm GaN can be grown on 2 inch. The bowing of the wafers caused by extrinsic stress due to the different thermal expansion is not affected by ELOG. Using WSiN as mask material a 600 µm thick free-standing 2 inch GaN layer has been obtained by exploiting the shear forces occurring during cooling after growth for separation from the ELOG template. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

(100) γ-lithium aluminium oxide substrates of two inch diameter were fabricated from crystals grown by the Czochralski technique. Onto these substrates c-plane GaN with a thickness of about 200 µm has been grown by hydride vapour phase epitaxy in a horizontal AIX HVPE reactor. The GaN layer spontaneously separates from the bottom lithium aluminium oxide substrate during the cool-down resulting in complete freestanding 2 inch GaN wafers. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

N-type doping of GaN in hydride vapour phase epitaxy (HVPE) has been studied. While silane was found to be not suitable, doping from solid silicon was found to be feasible but difficult to handle. Dichlorosilane was found to be a convenient Si doping source for HVPE growth of GaN. High electron mobilities as well as good optical and structural properties are obtained in the doping range of 6 × 10 17 cm -3 to 8 × 10 18 cm -3 using dichlorosilane.

A 230 μm thick free standing GaN layer has been grown on a LiAlO 2 substrate by hydride vapor phase epitaxy, with the separation of the layer from the substrate occuring spontaneously during cooling down after growth. A strong green luminescence band is observed from the top surface, while from the bottom surface, strong blue and red bands are seen in photoluminescence (PL). The evolution of these luminescence bands with layer thickness was monitored by cross-sectional cathodoluminescence (CL). PL and CL measurements indicate that the defect structure changes with growing layer thickness.

Conditions for optimized growth of thick GaN layers with crack-free surfaces by HVPE are reported. It was found that a 1:1 mixture of H2/N2 as carrier gas leads to the lowest density of cracks in the surface. Crack formation also depends on the properties of the GaN/sapphire templates used. Best results have been obtained for 5 µm thick GaN/sapphire templates grown by MOVPE with medium compressive strain ϵzz of about 0.05%. But there is no simple dependence of the crack formation on the strain status of the starting layer indicating that the HVPE growth of GaN can itself introduce strong tensile strain. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In total, 120 μm thick GaN layers without cracks have been grown on 2 in sapphire substrates by hydride vapor phase epitaxy. This has been achieved by optimization of the flow patterns in the reactor based on 3D process modelling, choice of the growth parameters especially the carrier gas composition and the usage of suitable GaN/sapphire templates. An important finding is that an H2 content of around 50% in the N2 carrier yields the lowest crack density.

Surveillance cameras and imaging diagnostics gain significance for plasma fusion experiments. They will become an integral part of the data acquisition system for the Wendelstein 7-X (W7-X) stellarator. In the scope of long duration discharges (∼30 min), as intended for W7-X, the data rate produced by numerous imaging systems will be huge. Data acquisition, processing, monitoring and archiving are to be carried out in continuous mode during the discharge. Upcoming large fusion experiments, such as W7-X and ITER, demand a paradigm shift regarding the future approach of scientific data analysis: The guideline should be the avoidance of permanent human inspection and interaction . Therefore, data acquisition for W7-X is designed to facilitate the implementation and configuration of data processing modules. The proper design of system components such as AnalysisModules (image processing) and TimeGroupModules (relevant for data acquisition performance) is essential to reduce image data before transmission to the archive. Pattern recognition will be offered as one approach to condense image data without loss of significant information. During the start-up phase of each experimental campaign that is operating in a new parameter regime, science images will have to be archived with high temporal, spatial, and dynamical resolution in order to derive significant patterns that will build up a pattern library.

Wendelstein7-X as well as the future ITER are new generation fusion devices, designed for long pulses or even steady-state operation. A large number of diagnostic systems will be implemented to investigate the behavior of the plasma. On the contrary to the previous shot oriented operation, long pulses require continuous sampling, archiving and displaying of collected data. The amount of data, that has to be handled, is some magnitudes higher and requires new techniques for compression, archiving and retrieval. In this paper, we give a status report on the progress of developing the data acquisition system of Wendelstein7-X. A prototype system was built to demonstrate all the required features for long pulse operation. This system includes three sample DAQ stations typical to the experiment, the timing system, the monitoring subsystem and the data archive.

An editor is to be developed to specify the system configuration of large and long-term physical experiments subject to permanent evolution. Based on this configuration, control programs are to be defined for long-duration experimental runs for the complete system as well as for subsystems if operated independently. The key model abstractions which have been developed for the description of such a system and serve as a basis for the data model used by the editor are presented.

In this work we present a comparative cross-sectional scanning tunneling microscopy study of different InAs and InGaAs quantum dot structures grown by metalorganic vapor phase epitaxy. The varying growth conditions lead to completely different spatial structures, as distinctive quantum dots, intermixed layers, and intermixed dots. Especially the overgrowth procedure and the arrangement of growth interruptions are responsible for segregation of the InAs material.

We present cross-sectional scanning tunneling microscopy results of self-organized In0.8Ga0.2As quantum dots covered by an In0.1Ga0.9As film inside a GaAs matrix prepared by metalorganic chemical vapor deposition. From images of quantum dots with atomic resolution, we determine a spatial distribution of the In composition within the dots with a shape of a reversed truncated cone. The wetting layer and the overgrown In0.1Ga0.9As layer show vertical intermixing.

To improve the plasma position and shape control system in ASDEX Upgrade, it is constantly expanded by the integration of data acquisition systems of important and interesting diagnostics. Usually the main responsibility of data acquisition systems is to collect data and subsequently put it into a data base archive system from where it is later analysed. The obligation now is also to simultaneously process the acquired data with an appropriate algorithm and send the pre-processed data in real-time to the plasma control system during a discharge. To achieve this requirements the used hardware must provide enough processing power and the operating system has to meet some real-time constraints. To avoid the burden of using proprietary real-time operating systems the trend is to use Open Source variants mainly based on Linux. Some of these solutions also allow us doing real-time capable communication using standard Ethernet hardware. By way of an example the implementation process of a prototype of a real-time data acquisition system based on a multi-core processor and Xenomai is demonstrated. Other possible solutions like Realtime Linux and their differences to Xenomai which we propose as the most sophisticated real-time framework for Linux are discussed in this paper.

The availability of GaN substrates is crucial for GaN-based laser diodes. Also high performance LEDs ask for high quality substrates. Methods for growth of bulk GaN are reviewed with a focus on hydride vapour phase epitaxy (HVPE), which currently is the method used for the production of GaN substrates. Also the ammonothermal approach is briefly discussed. Both approaches still have to overcome limitations before mass-production at affordable prices is feasible. These limitations are related to the maximum growth rate yielding high quality material as well as the boule length that can be achieved together with high material quality.

The coupling of continuous data acquisition and continuous online analysis keeping up with the acquisition is one of the features the W7-X data acquisition system is designed for. As proof of principle of this concept the magnetic diagnostics have been implemented. The magnetic diagnostics measure time derivatives of the magnetic fluxes and the signals have to be time integrated accordingly. Both measurement and analysis tasks are executed on a single PC based system. The configuration of both hard- and software of the system is presented. Special focuses are the task of software integration of a user supplied online analysis function into the data acquisition system and the incorporation of the complete system into the W7-X segment concept in use by the control system. The complete installation has been tested at the WEGA Stellarator experiment. Because WEGA is capable of continuous steady state plasma operation the practical suitability of the installation for W7-X could be demonstrated successfully.

The increasing number of data acquisition stations and setups in laboratory environments at Wendelstein 7-X does already produce a considerable amount of data by now. Diagnosticians want to view the acquired data in a comfortable way and use them for calculations in their own test and analysis algorithms. To prepare for full operation of W7-X, a reasonable approach is to provide data access methods that stay as close as possible to the final data access concept. This requires taking care of the special needs of W7-X regarding continuously acquired data, segment based parameter switching and synchronization of data from different diagnostics. Therefore, a data access interface has been designed and implemented considering continuous data acquisition as well as usability and performance issues. The interface and its usage in different software environments (e.g. high level scientific programming languages) will be explained. Also, the integration of user defined off line analysis algorithms will be described. Additionally, the DataBrowser will be presented. The DataBrowser is a Java application that can browse continuously acquired data of different kind, provides several types of plots including zooming functions to display the data and the corresponding parameters and offers export functions to save selected data locally.

Prismatic crystals of natural sphalerite are experimentally deformed (T = 450 to 600 °C) by axial compression approximately along [001] in order to favour mechanical twinning. Observation of prepolished surfaces reveals the heterogeneity of plastic deformation and information about activated slip and twinning systems. Texture goniometry indicates growth and deformation twin orientations. The deformed specimens exhibit broad slip bands along specific {111} planes. The width of the {111} deformation twins seems to be limited. TEM observations confirm these results. Moreover, a study of interactions between glide dislocations and twin boundaries shows that these interfaces have an hardening effect: perfect or partial dislocations either can'be locked on the twin boundary, or cross it leaving behind a Shockley partial or a stair rod dislocation. Appropriate models are discussed. Des monocristaux naturels de sphalerite ont ete deformes sous une orientation voisine de [001] de maniere a favoriser le maclage mecanique. Des observations par microscopie en reflexion, jointes a l'interpretation des figures de pooles permettent d'obtenir des informations sur les systemes de glissement et de maclage actives. Les echantillons deformes revelent de larges bandes de glissement suivant des plans {111} et des macles de deformation de largeur limitee. Les observations par microscopie electronique en transmission confirment bien les resultats precedents. De plus, une etude des interactions entre les dislocations apparteenant a un systeme de glissement et les joints de made permet de montrer que ces derniers jouent un role de durcisseur: les dislocations parfafites ou imparfaites peuvent etre bloquees sur les parois de made, ou traverser ces dernieres en laissant une dislocation de made ou une dislocation stair rod. Deux modeles theoriques sont discutes.

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