H. Niedermeyer

A trigger-time-event system for the W7-X experiment

Abstract All control and data acquisition systems of the WENDELSTEIN 7-X (W7-X) fusion experiment need to be perfectly synchronised with an accuracy of ⩽10 ns. Another essential requirement is to process and record events and trigger signals in such a way that they can be used for real-time control. This paper describes the architecture of a trigger–time–event (TTE) system for the W7-X experiment and gives an overview of the characteristics of the special board of the local TTE units. The main components of the board are a local oscillator synchronised by a central timing unit, and a field programmable gate array (FPGA). The FPGA comprises an event and trigger command receiver/decoder, a local 64-bit time counter, time capture registers, delay devices, a programmable state machine, trigger generators, and a programmable switch matrix, which allows output and input signals of the devices to be interconnected. The time, fast trigger, and event information are distributed from the central timing unit to the local TTE boards via a tree-type optical network. For hardware trigger signals an electrical network may be used. The board has configurable I/O ports for trigger signals and a 64-bit time port.

Control system of WENDELSTEIN 7-X experiment

Abstract The WENDELSTEIN 7-X stellarator will be capable of running pulses of up to 30 min duration. The control system of W7-X will support all discharge scenarios compatible with this capability, i.e. short pulses with arbitrary intervals, steady state discharges, and arbitrary sequences of phases with different characteristics (“segments”) in one discharge. The use of segments substantially reduces the time required for parameter scans and permits short tests of new settings without interfering with the main programme. The hierarchical layout of the control system will reflect the structure of the experimental device. Each technical component and each diagnostic system including its data acquisition will have its own control system permitting autonomous operation for commissioning and testing. The activity of these devices will be co-ordinated by a master control during the experimental sessions. System parameters where relevant to the experiment, will be exclusively controlled by complex software objects. By synchronous changing references to these objects in all computers the whole system behaviour can be modified from one cycle to the next. This allows to switch between segments or to the end of the discharge. The switching may be determined by fixed timing, by logical conditions, or by operator action.

Design and construction of WENDELSTEIN 7-X

Abstract Following the approval by EURATOM and the German government WENDELSTEIN 7-X (W7-X) is presently the largest fusion project under construction. W7-X is a helical advanced stellarator (HELIAS) which produces the confining magnet field only by magnet coils that enables steady-state operation. W7-X aims to demonstrate that the HELIAS configuration has the potential for a future power reactor. The successful application of new technologies for manufacturing prototypes and the positive results gained from tests allowed to design the machine in detail and to order major components. The geometry of the non-planar magnet coils has a considerable impact on the design of the machine in particular on the shape of the plasma vessel, positioning of the plasma-facing components and the size and orientation of the ports. The requirement for steady-state operation has consequences for many subsystems of W7-X. The magnet coils need to be superconducting and cooled to liquid helium temperature. Gyrotrons shall continuously provide 10 MW of ECR heating power. The divertor must be cooled to withstand heat loads of up to 10 MW/m 2 . The schedule of W7-X is determined by the delivery dates of the non-planar coils, the plasma vessel and the outer vessel. Start of commissioning and scientific operation is scheduled for spring 2006.

Synchronization of processes in a distributed real time system exemplified by the control system of the fusion experiment WENDELSTEIN 7-X

The fusion experiment Wendelstein 7-X demands synchronizing processes in many distributed technical components with an accuracy from milliseconds to nanoseconds. For this purpose a Trigger-Time-Event system (TTE system) was developed. Its main task is the synchronization of all control components and data acquisition systems. It allows time stamping, processing of trigger signals and event messages. The TTE system offers the possibility to program timing sequences and delays, logic combinations or conditions. After a description of the TTE system the task synchronization in the control system of WENDELSTEIN 7-X is discussed. A special software library (TimeLib) allows dealing with time related variables in a simple and comfortable manner. The efficiency of the synchronization system is exemplified

Divertor operation in stellarators: results from W7-AS and implications for future devices

The research on divertors for stellarators is at the beginning. Extensive studies are being prepared on large helical device (LHD) and W7-X. W7-AS is now being operated with an open island divertor (ID) which serves as a test bed for the W7-X diverter. The divertor enables access to a new NBI-heated, high-density operating regime with improved confinement properties. This regime-the high-density H-mode (HDH)-displays no evident mode activity, is extant above a threshold density and characterized by flat density profiles. high-energy- and low-impurity-confinement times and edge-localized radiation. Impurity accumulation, normally associated with ELM-free H-modes, is avoided. Quasi-steady-state discharges with n e up to 4 x 10 20 m -3 , edge radiation levels up to 90% and plasma partial detachment at the divertor targets can be simultaneously realized. The accessibility to other improved confinement modes in W7-AS (conventional H-mode anti OC-mode) is not restricted by the divertor. The results provide a promising basis for future experiments, in particular on W7-X, and recommend the ID as a serious candidate for solving the plasma exhaust problem in stellarators.

Wall conditioning for Wendelstein 7-X by glow discharge

Abstract The basic design of a reliable and robust glow discharge cleaning (GDC) system for Wendelstein 7-X is presented. Preliminary experiments at the DEMO cryostat, a full size prototype sector of W7-X, result in conclusions about the operating range, power consumption and discharge stability of a GDC in W7-X dimensions. Ten electrodes are planned to be integrated into the first wall of W7-X, thus avoiding complex manipulators. A spherical calotte anode made of graphite mounted in a shielding trough was found as the optimum electrode design assuring stable discharge operation without active cooling, even when exposed to steady state stellarator discharges.

Ethernet-based real-time control data bus

Wendelstein 7-X is designed as a steady-state experiment to demonstrate the fusion reactor relevance of the advanced stellarator concept. The experiments control and data acquisition will be performed by a distributed system of computers and programmable logic controllers (PLCs). Data of several systems have to be combined flexibly to control the machine, requiring a data exchange on a millisecond time scale between several connected units. A discharge can last up to half an hour, thus, the connections may vary during a discharge. Hence, it is desired to provide control relevant data, e.g., measurands, set points, interlock signals, cyclically via a bus system. The paper will analyze the special quality of control data streams and deduce the basic requirements for a real-time data bus. An Ethernet is a candidate for the data bus since it is a widely used broad-band bus with a foreseeable potential for development. Because of its nondeterministic arbitration algorithm Ethernet is generally considered not to be suited for hard real-time applications. It is possible to circumvent this disadvantage of Ethernet either by using switching techniques or a software token to obtain a reliable base for hard real-time data transport.

140 GHz electron cyclotron resonance heating experiments at the W7-AS stellarator

Abstract Electron cyclotron resonance heating (ECRH) at the W7-AS stellarator is performed two systems of comparable heating power at 70 GHz (0.8 MW, 3 s) and 140 GHz (0.9 MW, 0.4 s). Experiments with 140 GHz open a new parameter window with high density operation up to 1.1 × 1020 m−3. H mode transitions were observed in high density discharges at 2.5 T and at lower density at 1.25 T. The operational window for the stellarator H mode with emphasis on the density and power threshold and the influence of gas puffing on the H transitions are discussed. Perturbation experiments with modulated ECRH power were performed to determine the heat transport and the power deposition profile. Density control is achieved in combined heating with neutral beam injection (NBI) despite the beam particle fuelling, whereas with NBI alone a steady density rise is observed. The impact of combined heating on the impurity confinement was investigated. Electron cyclotron current drive (ECCD) was studied in different magnetic field configurations and the influence of trapped particles on the ECCD efficiency was examined experimentally and compared with theory.

Synchronization of Processes in a Distributed Real Time System Exemplified by the Control System of the Fusion Experiment WENDELSTEIN 7-X

The fusion experiment Wendelstein 7-X demands synchronizing processes in many distributed technical components with an accuracy from milliseconds to nanoseconds. For this purpose a Trigger-Time-Event system (TTE system) was developed. Its main task is the synchronization of all control components and data acquisition systems. It allows time stamping, processing of trigger signals and event messages. The TTE system offers the possibility to program timing sequences and delays, logic combinations or conditions. After a description of the TTE system the task synchronization in the control system of WENDELSTEIN 7-X is discussed. A special software library (TimeLib) allows dealing with time related variables in a simple and comfortable manner. The efficiency of the synchronization system is exemplified

Experiences with the ASDEX Neutralizer Plates and Construction of Water-Cooled Plates for Long-Pulse Heating

After dismantling of the titanium neutralizer plates inspection yielded satisfactory status of flat areas whereas edges and curved shapes were heavily melted. At the inner plates of the lower divertor strongly focused melting and cutting was found which is caused by fast electrons. These electrons are continuously produced. The production mechanism is not yet clear but runaway processes can be excluded.