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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 Italian Divertor Test Tokamak (IDTT) facility is a new machine proposed to study advanced exhaust solutions in conditions approaching those planned for DEMO. The capabilities and access characteristics of such machine open the possibility to test and validate relevant non-magnetic control diagnostics, in reactor like regimes, in support of a DEMO design implementation. O-mode reflectometry, due to its compatibility with a full grade reactor implementation, was proposed as a source of real-time (RT) plasma position and shape measurements for control purposes, in replacement or complement of standard magnetic measurements. On DEMO, the foreseen placement of magnetic sensors behind the blanket modules, and the inherent delay introduced in corresponding measurements, means that a fast position and shape feedback controller must make use of multiple, poloidally distributed, non-magnetic measurements such as the ones provided by O-mode reflectometry. This new control technique must be tested, in all of its aspects, before it can be fully implemented in future fusion reactors. IDTT presents itself as one of the best candidates to implement and build a knowledge database of nonstandard reflectometry (away from the equatorial plane) that will be needed on DEMO and is currently unavailable. The main goal of this work is to assess the performance of a set of 3 plasma position reflectometers (PPR) at the Lower Field Side (LFS) on IDTT. This assessment was carried out using the two-dimensional (2D) full-wave finite-difference time-domain (FDTD) code, REFMULF, in two of the foreseen IDTT plasma scenarios: 5MA Single Null (SN) and Double Null (DN) configurations, at the Start Of Flat (SOF), T=36s, that is at the start of the flat top of the plasma current. These scenarios are the most common ones and constitute a consistent starting point for the assessment of the possibility of having one or more PPR systems at IDTT.

This work analyzes the stability of the Yee scheme for non stationary Maxwell's equations coupled with a linear current model. We show that the usual procedure may yield unstable scheme for physical situations that correspond to strongly magnetized plasmas in X-mode (TE) polarization. We propose to use first order clustered discretization of the vectorial product that gives back a stable coupling. We validate the schemes on some test cases representative of direct numerical simulations of X-mode in a magnetic fusion plasma.

Nuclear fusion is a promising candidate to supply energy for future generations. At the high temperatures needed for the nuclei to fuse, ions and electrons are no longer bound into atoms. Magnetic fields confine the resulting plasma. One of the heating methods is the ion cyclotron resonant absorption of waves emitted by an external Ion Cyclotron Radio Frequency (ICRF) antenna. The efficiency of ICRF heating is strongly affected by rectified RF electric fields at antenna and other in-vessel components (so-called ‘sheath effects’). The chapter presents an overview of ICRF principles. Attention is given to characterising the detrimental sheath effects through experiments on a dedicated test facility (IShTAR: Ion cyclotron Sheath Test ARrangement). IShTAR has a linear magnetic configuration and is equipped with an independent helicon plasma source. The configuration and capabilities of the test-bed and its diagnostics are described, as well as an analysis of the plasmas.

12th International Congress on Plasma Physics, 25-29 October 2004, Nice (France); The effects of the non-zero average density perturbation on phase and amplitude measured by reflectometry are presented. The non-zero average density perturbation on the phase variation can be seen as an index effect as soon as the shape of the density perturbation does not introduce spectral effects. Amplitude modulation in time follows generally the properties of the cut-off layer seen as a mirror but some specific situations produce a time modulation two times higher than the input time variation of the density perturbation as observed in Tore Supra. The introduction of secondary cut-off can exhibit this effect as shown in 2D simulations.