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International audience; Within the European development of a future tokamak demonstration fusion power plant (DEMO) [1] the pre-conceptual studies on the plasma diagnostic and control (D&C) system are progressing to prepare the basis for reliable plasma operation at high overall performance [2]. A variety of plasma diagnostics will be employed on DEMO together with advanced control techniques in order to provide an accurate knowledge of the plasma state, which is needed to maintain plasma operation within the allowed physical and technical limits. The integration of diagnostic front-end components has to cope with strong adverse effects arising from neutron and gamma irradiation, heat loads, impinging particles and forces. In this environment, the quality of measurements can only be ensured for longer periods by using robust diagnostic components, mounting them in sufficiently protected (retracted) locations, and any maintenance can only be performed via remote handling. Major open issues are the durability of magnetic measurements in the presence of irradiation induced effects and the feasibility of detachment control under DEMO conditions. In parallel to diagnostic developments, the details of the main control issues are being formulated and investigated by quantitative plasma control simulations. To obtain the envisaged performance DEMO operates close to some physics limits where even small disturbances, if not properly controlled, can trigger major variations of the plasma parameters. Equilibrium control requires high control power and can drive the poloidal field coil system to its operational limits. Within this paper, we will provide an overview on the current status of the ongoing D&C developments for the European DEMO concept.

The development of the plasma diagnostic and control (D&C) system for a future tokamak demonstration fusion reactor (DEMO) [1] faces significant challenges [2]. These comprise the required reliability of operation, the high accuracy to which the plasma parameters are to be controlled, and the robustness of components and methods against any adverse effects or disturbances. The ongoing developments for the ITER D&C system represent an important starting point for progressing towards DEMO. ITER diagnostic development is guided by a measurement requirements table, in which different categories of diagnostics for machine protection, basic and advanced control, as well as for evaluation and physics studies are being distinguished. Since ITER is an experiment aiming to explore and optimize in detail the physics of a burning plasma, ambitious targets for space and time resolution as well as for the measurement accuracies have been defined. These are pushing the ITER diagnostic design towards using sophisticated methods and aiming for large coverage and high signal intensities, forcing to mount many front-end components in forward positions. This results in many cases in a rapid aging of diagnostic components, so that additional measures like protection shutters, plasma based mirror cleaning or modular approaches for frequent maintenance and exchange have to be developed. Under the even stronger fluences of plasma particles, neutron/gamma and radiation loads on DEMO, high reliability and long lifetime of diagnostics can only be achieved by selecting the methods with regard to their robustness, and retracting vulnerable front-end components into protected locations in the machine. Based on this approach, an initial DEMO D&C concept has been elaborated, which is covering all major control issues by main control parameters to be derived using at least two different diagnostic methods. Within this paper, on overview on the current status of D&C development for DEMO will be provided.

Das Bundesministerium für Bildung und und Forschung (BMBF) stellte in 2007/2008 Mittel imRahmen der Projektförderung zur Verfügung, mit der Zielsetzung eine stärker sichtbareBeteiligung der deutschen Fusionsinstitute am Aufbau von ITER zu erreichen, sowie dieChancen deutscher Unternehmen auf die Übernahme von Aufträgen für den Aufbau vonITER zu stärken.Eine wichtige Zielsetzung des hier beschriebenen Forschungsvorhabens (Projektnummer03FUS0007) war es demnach, kritische Prototyp-Komponenten für ITER zu entwickeln undzu testen, sowie entsprechende Mess- und Prüfeinrichtungen aufzubauen. Gleichzeitigwurde in der Projektbearbeitung sehr eng mit einer ganzen Reihe von Unternehmenzusammengearbeitet, um so einen intensiven Know-How Transfer in beiden Richtungen imHinblick auf die Entwicklung von Komponenten für Fusionsanlagen zu erreichen

In a nuclear fusion experiment, the concept of an Integrated Data Analysis (IDA) allows the consistent estimation of plasma parameters from heterogeneous data sets [1]. Here, the determination of the ion effective charge (Zeff ), a critical local measure of impurity concentration, is considered. Several diagnostic methods exist for the determination of Zeff , but the results are in general not in agreement. Moreover, so far none of the available methods has provided a Zeff estimate that is reliable over the entire plasma cross-section, which is at present a real challenge. In this work, the problem of Zeff estimation is approached from the perspective of IDA, in the framework of Bayesian probability theory. The ultimate goal is the estimation of a full Zeff profile that is consistent both with measured bremsstrahlung emissivities, as well as individual impurity spectral line intensities obtained from Charge Exchange Spectroscopy (CXS). We present an overview of the various uncertainties that enter the calculation of a Zeff profile from bremsstrahlung data on the one hand, and line intensity data on the other hand. These appear at several levels, including the measurement process itself (together with independent electron density and temperature measurements), the inversion procedure (including knowledge of the magnetic equilibrium), the atomic data, the diagnostic calibrations, etc. We discuss a simple Bayesian model permitting the estimation of a central value for Zeff and the electron density ne on TEXTOR from bremsstrahlung emissivity measurements in the visible, and carbon densities derived from CXS. Both the central Zeff and ne are sampled using an MCMC algorithm. Extensions of the model to a full Bayesian analysis, incorporating all critical measurement and model uncertainties, are examined. Relevance to ITER through the pilot active beam experiment on TEXTOR is discussed. References: [1] R. Fischer and A. Dinklage, Rev. Sci. Instrum. 75, 4237 (2004).

An essential element for stationary stellarator operation is the understanding of the impurity transport behavior. Neoclassical theory predicts an impurity transport towards the plasma core for the standard ion root regime in stellarators [1,2]. The performance of a quasi-stationary device like Wendelstein 7-X stellarator (W7-X, presently in the commissioning phase in Greifswald, Germany) could be limited in case of strong impurity accumulation. Therefore, a set of plasma diagnostics is foreseen to obtain key experimental quantities for the neoclassical transport modeling as ion temperature profile, density gradients and impurity concentration [1]. The core impurity content is monitored by the High Efficiency eXtreme ultraviolet Overview Spectrometer system (HEXOS) [2], covering the wavelength range 2.5–160 nm (intermediate ionization states of all relevant heavy intrinsic impurity species) with high spectral resolution and a time resolution of 1 ms, adequate for transport analysis. Impurity radiation at shorter wave lengths (4 nm–0.06 nm) will be monitored with the SX pulse height analysis system (PHA) [3]. The ion temperature profile can be deduced from inversion of data from the High Resolution X-ray Imaging Spectrometer (HR-XIS), which measures the concentration and temperature of argon tracer gas in helium-like ionization stages [6,7,8]. A second X-ray Imaging Crystal Spectrometer (XICS), which will additionally provide the poloidal ion rotation velocity, is under preparation [8,9]. The total radiation will be measured by two bolometer cameras [10,11]. The status of the impurity diagnostics for the first operational phase in W7-X is summarized in this paper and an outlook for the next experimental campaign is given.