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International audience; The overall goal of the IShTAR (Ion cyclotron Sheath Test ARrangement) project is to study antenna near-fields in the presence of a plasma and magnetic field to assess theoretical predictions on radio frequency (RF) sheaths and to guide theoretical modelling [1, 2, 3]. For Ion Cyclotron Radio Frequency (ICRF) antenna applications it is favourable to operate at tokamak edge-like conditions for density and temperature. A helicon antenna at a frequency of 11.76 MHz and with power up to 3kW, is used to create argon or helium plasmas in IShTAR. IShTAR plasmas operations Figure 1: Radial plasma density profiles for various currents in the coils surrounding the main chamber (I big) and plasma source (I source). The radial position of the ICRF antenna is specified by a vertical green line. Technicalities and capabilities of the IShTAR device have been described in [1, 3]. Plasmas have now been characterised in a systematic way in order to obtain reproducible density profiles. An extensive exploration of the discharge parameters (such as the neutral gas pressure, the gas type, the helicon antenna power and the magnetic field topology) has been performed [4]. A selection of resulting radial density profiles is presented in figure 1. They have been obtained by

International audience; Significant progress in applying ICRF power to ASDEX Upgrade (AUG) through improved power coupling and reduced impurity production has been associated with progress in understanding and modelling. First, the coupling of the fast wave was improved using outer mid-plane gas injection. The local edge density increase in front of the antenna shifts the fast wave cutoff position closer to the antenna by ~2cm, a result confirmed with new density measurements in front of the antenna. The ICRF coupling increases by 120% (only 25% for top gas puffing). The increased density and resulting improved coupling. was also numerically modelled. Second, the new 3-strap antennas in AUG demonstrated clearly that a proper antenna design can successfully mitigate ICRF-specific tungsten (W) sputtering. The reduction of the W sputtering was achieved by minimizing the RF currents on the antenna surfaces exposed to the scrape-off-layer (SOL) plasma. The local RF currents, rectified DC currents and the W sputtering yield at the antenna side limiters experience a clear minimum close to a phasing between the central and the outer straps of 180° and a power balance ratio Pcen/Pout of ~ 2. At this optimum, the local source of sputtered W at the limiters is reduced by a factor between 1.5 and 6, depending on the location. The experiments and modelling confirm the hypothesis of sheath rectification as the source of the sputtered W. Furthermore, the new 3-ion ICRF heating scenario, which can produce very energetic particles, has been successfully reproduced in AUG. The progress in operation, in understanding and modelling is strongly supported by improved ICRF diagnostic coverage including density measurements directly in front of the antenna by reflectometry, advanced RF coupling characterization, measurements of antenna limiter currents, B-dot probes, Ion Cyclotron Emission (ICE) measurements and by dedicated tests on the experimental device IShTAR (Ion cyclotron Sheath Test ARrangement).

International audience; Understanding and exploiting cylindrical Langmuir probe measurements in magnetized plasma is a real challenge, although this technique is the most common one used to access plasma parameters such as density, temperature, potential, etc. Since the magnetic field confines the electrons in the plasma, the measurement is actually done on the flux tube connected to the cylindrical probe. In some conditions, the I(V) characteristics displays a bump between the exponential growth and the saturation of the electronic current. We propose a new interpretation of such distorted characteristics as a function of the angle between the probe and the magnetic field lines, the RF-power and the magnetic field magnitude.; Comprendre et exploiter les mesures de la sonde cylindrique de Langmuir dans le plasma magnétisé est un véritable défi, bien que cette technique soit la plus courante utilisée pour accéder aux paramètres du plasma tels que la densité, la température, le potentiel, etc. Le champ magnétique confinant les électrons dans le plasma, le la mesure se fait en fait sur le tube de flux connecté à la sonde cylindrique. Dans certaines conditions, les caractéristiques I (V) affichent une bosse entre la croissance exponentielle et la saturation du courant électronique. Nous proposons une nouvelle interprétation de ces caractéristiques déformées en fonction de l'angle entre la sonde et les lignes de champ magnétique, la puissance RF et la magnitude du champ magnétique.

International audience; Understanding and exploiting cylindrical Langmuir probe measurements in magnetized plasma is a real challenge, although this technique is the most common one used to access plasma parameters such as density, temperature, potential, etc. Since the magnetic field confines the electrons in the plasma, the measurement is actually done on the flux tube connected to the cylindrical probe. In some conditions, the I(V) characteristics displays a bump between the exponential growth and the saturation of the electronic current. We propose a new interpretation of such distorted characteristics as a function of the angle between the probe and the magnetic field lines, the RF-power and the magnetic field magnitude.; Comprendre et exploiter les mesures de la sonde cylindrique de Langmuir dans le plasma magnétisé est un véritable défi, bien que cette technique soit la plus courante utilisée pour accéder aux paramètres du plasma tels que la densité, la température, le potentiel, etc. Le champ magnétique confinant les électrons dans le plasma, le la mesure se fait en fait sur le tube de flux connecté à la sonde cylindrique. Dans certaines conditions, les caractéristiques I (V) affichent une bosse entre la croissance exponentielle et la saturation du courant électronique. Nous proposons une nouvelle interprétation de ces caractéristiques déformées en fonction de l'angle entre la sonde et les lignes de champ magnétique, la puissance RF et la magnitude du champ magnétique.

International audience; The diagnostics developed for the IShTAR (Ion Cyclotron Sheath Test ARrangement) facility are oriented towards measurements of plasma parameters and electric fields in the vicinity of a Radio Frequency (RF) antenna in order to provide input for sheath modelling codes [1, 2, 3]. The plasmas are created by a helicon antenna operated at a frequency of 11.76 MHz and with a power up to 3kW (the maximum power coupled to the plasma is around 2.7 kW). Plasma characterisation and optimisation A schematic view of IShTAR is presented in [1]. Improvements have been made for more complete density measurements in the helicon source and the main vessel. An additional array of RF compensated probes has been installed to allow for a better characterisation of the plasma parameters. In this way an optimised performance regime was found. By adjusting the magnetic topology the plasmas density can be increased by a factor 3 to 5, which is beneficial to the sheath studies. It is believed to be linked to the helicon wave propagation, but further investigation is needed to confirm the results. A current scan in helium plasmas is shown in figure 1. The current in the big magnetic field coils around the main chamber is varied between 200 and 800 A. The current in the small coils around the helicon source is kept constant at a level of 300 A. The corresponding magnetic field topology for the 200 A and 800 A cases are presented resp. in the figures 1 (b) and (c). It can be seen that for stronger magnetic fields the maximum densities are higher, but the radial profiles are more peaked. The RF antenna location is at radius r = −20 cm. The most homogenous and broad profile in this example is obtained for a coil current of ±650 A. In order to benchmark the electron density measurements from the Langmuir probes an interferometer has been designed. The schematic picture and hardware are shown in figures 2 (a) and (b). The probing frequency of 47 GHz, associated to an intermediate frequency of 144 MHz, is suitable for the typical plasma densities of IShTAR. The first results indicate a peak electron density in the helicon source of the order of 2 · 10 17 m −3 (figure 2(c)). The order of magnitude is in agreement with the results found by the Langmuir probes. Systematic scans for different plasma parameters and input power levels of the helicon antenna are planned.