The main results obtained in the TJ-II stellarator in the last two years are reported. The most important topics investigated have been modelling and validation of impurity transport, validation of gyrokinetic simulations, turbulence characterisation, effect of magnetic configuration on transport, fuelling with pellet injection, fast particles and liquid metal plasma facing components. As regards impurity transport research, a number of working lines exploring several recently discovered effects have been developed: the effect of tangential drifts on stellarator neoclassical transport, the impurity flux driven by electric fields tangent to magnetic surfaces and attempts of experimental validation with Doppler reflectometry of the variation of the radial electric field on the flux surface. Concerning gyrokinetic simulations, two validation activities have been performed, the comparison with measurements of zonal flow relaxation in pellet-induced fast transients and the comparison with experimental poloidal variation of fluctuations amplitude. The impact of radial electric fields on turbulence spreading in the edge and scrape-off layer has been also experimentally characterized using a 2D Langmuir probe array. Another remarkable piece of work has been the investigation of the radial propagation of small temperature perturbations using transfer entropy. Research on the physics and modelling of plasma core fuelling with pellet and tracer-encapsulated solid-pellet injection has produced also relevant results. Neutral beam injection driven Alfvénic activity and its possible control by electron cyclotron current drive has been examined as well in TJ-II. Finally, recent results on alternative plasma facing components based on liquid metals are also presented.

Numerous observation exist of a population of high energetic ions with energies well above the corresponding thermal values in plasmas generated by electron cyclotron resonance (ECR) heating in TJ-II stellarator and in other magnetically confined plasmas devices. In this work we study the impact of ECR heating different conditions (positions and powers) on fast ions escaping from plasmas in the TJ-II stellarator. For this study, an ion luminescent probe operated in counting mode is used to measure the energy distribution of suprathermal ions, in the range from 1 to 30 keV. It is observed that some suprathermal ions characteristics (such as temperature, particle and energy fluxes) are related directly with the gyrotron power and focus position of the heating beam in the plasma. Moreover, it is found that suprathermal ion characteristics vary during a magnetic configuration scan (performed along a single discharge). By investigating the suprathermal ions escaping from plasmas generated using two gyrotrons, one with fixed power and the other modulated (on/off) at low frequency (10 Hz), the de-confinement time of the suprathermal ions can be measured, which is of the order of a few milliseconds (<4ms). A model that uses a zero-dimensional power balance is used to understand the de-confinement times in terms of the interaction of suprathermal ions and plasma components. This model also can be used to interpret experimental results of energy loss due to suprathermal ions. Finally, observations of increases (peaks) in the population of escaping suprathermal ions, which are well localized at discrete energies, is documented, these peaks being observed in the energy distributions along a discharge.

The existence of a population of ions, with energies well above the corresponding thermal values, in plasmas heated by neutral beam injection (NBI) in the stellarator TJ‐II and in other magnetically confined plasmas devices is well known. Moreover, during the NBI phase of the TJ‐II, edge‐localized mode‐like (ELM‐like) instabilities often appear. This work studies the relationship between fast ions escaping from TJ‐II plasmas and such ELM‐like events. For this, an ion luminescent probe (LP), operated with a fast scintillator (decay time = 27 ns) and high‐speed conditioning electronics (100 MHz), is used to measure, without pile‐up effects, the energy distribution of suprathermal ions across the energy range, from 1 to 30 keV. It is found that the LP detects suprathermal ion populations that can be related to ELM‐like events whose temperatures oscillate between 0.5 and 3 keV and that can be distinguished in a spectrogram of the LP response as an increase in frequencies from 15 to 150 kHz. Finally, observations of substructures within ELM‐likes events, consisting of bursts of ions, with times between 5 and 15 µs are also reported. Such substructures have been observed previously in both tokamaks and stellarators in ion saturation current, high‐speed camera images, etc. However, understanding the fast‐ion component can provide new and relevant information about the behaviour of ions.

•The ejection velocities for Li from both liquid metals are basically the same.•There is no significant evolution with temperature of the kinetic energy of the ejected atom over the temperature range recorded by the pyrometer (T > 150 ºC).•Hints of higher energies at T < 150 ºC were found, with values up to 0.5 eV.•The kinetic energy at high temperatures is a factor of 3 to 4 larger that the corresponding thermal energies, but also several times smaller than the expected sputtering energies.•There is no difference in the toroidal dispersal of Li and He ions around their sources, which can be ascribed to classical thermalization with the plasma ions.

The main results obtained in the TJ-II stellarator in the last two years are reported. The most important topics investigated have been: modelling and validation of impurity transport, validation of gyrokinetic simulations, turbulence characterisation, effect of magnetic configuration on transport, fuelling with pellet injection, fast particles and liquid metal PFCs. It must be noted that work done on TJ-II is relevant for W7-X.

In stellarators and tokamaks, long impurity confinement times, or impurity accumulation, is observed in some regimes [1, 2]. It is well known that avoidance of this deleterious effect is critical for present day devices and future fusion reactors. Electron Cyclotron Resonance Heating (ECRH) has been demonstrated as an effective tool to mitigate this problem, as its application has the capability to avoid impurity accumulation [1]. The goal of the present study is to investigate solid-rigid core rotation, in TJ-II plasma scenarios where Neutral Beam Injection (NBI) heating and ECRH are combined, and its role in particle and impurity confinement mitigation. For this purpose, the main plasma rotation diagnostic has been a high spectral resolution spectrometer [3]. It has been upgraded to permit collection of plasma light emissions without the need for a fibre bundle. Moreover, in order to overcome reduced impurity light emission, long exposure times and flexibility in the number of spatial channels employed (19 to 29) permit compatibility with the purpose of the experiment. The behaviour of core plasma poloidal rotation, as measured by passive Doppler spectroscopy of emission lines for the carbon ions C and C, versus line-averaged electron plasma density has been evaluated for reference NBI discharges in which ECRH is applied at different power levels and is focused at different plasma radii. Representative results, obtained for a range of ECRH overlapped with NBI heating scenarios, are shown in order to assess whether changes in radial electric field resulting from this operational method are a key parameter to mitigate impurity confinement in this hybrid heating regimes.

Two liquid metals (LM), Li and LiSn (20:80 at), presently considered as alternative materials for the divertor target of a fusion reactor, have been exposed to the plasma in a capillary porous system (CPS) arrangement in TJ-II. A negligible perturbation of the plasma has been recorded in both cases, even when stellarator plasmas are particularly sensitive to high Z elements due to the tendency to central impurity accumulation. The surface temperature of the LM CPS samples (made of a tungsten mesh impregnated in SnLi or Li) has been measured during the plasma pulse with ms resolution by pyrometry and the thermal balance during heating and cooling has been used to obtain the thermal parameters of the SnLi and Li CPS arrangements. Temperatures as high as 1150 K during TJ-II plasma exposure were observed for the LiSn solid case. Strong changes in the thermal conductivity of the alloy were recorded in the cooling phase at temperatures close to the nominal melting point. The deduced values for the thermal conductivity of the LiSn alloy/CPS sample were significantly lower than those predicted from their individual components.

The first experiments of exposure of a LiSn alloy (Li/Sn atomic ratio = 20/80) to a hydrogen plasma in TJ-II are here presented. Solid and liquid samples have been inserted at the edge and evidence of sample melting of a solid sample during plasma exposure has been observed. A negligible perturbation of the plasma has been recorded, even when stellarator plasmas are particularly sensitive to high Z elements due to the tendency to central impurity accumulation. Melting of the sample by the plasma thermal load did not lead to any deleterious effect on the plasma performance. Strong lithium emission was detected at the LiSn sample but no sign of Sn contamination and low values of Z(eff) and radiated power were deduced. Hydrogen recycling was studied at two different tem peratures and no change was detected in the range of 300-750 K. The retention of H-2 by the alloy was addressed in separate experiments at the laboratory. Values in the order of 0.01% H/(Sn + Li) were deduced in agreement with in situ TDS analysis of the plasma exposed samples and previous reports. (C) 2016 Elsevier Ltd.

The effects of 3D geometry are explored in TJ-II from two relevant points of view: neoclassical transport and modification of stability and dispersion relation of waves. Particle fuelling and impurity transport are studied considering the 3D transport properties, paying attention to both neoclassical transport and other possible mechanisms. The effects of the 3D magnetic topology on stability, confinement and Alfven Eigenmodes properties are also explored, showing the possibility of controlling Alfven modes by modifying the configuration; the onset of modes similar to geodesic acoustic modes are driven by fast electrons or fast ions; and the weak effect of magnetic well on confinement. Finally, we show innovative power exhaust scenarios using liquid metals.

A systematic study of scintillation materials was undertaken to improve the time resolution of the fast ion diagnostic currently installed at TJ-II stellarator. It was found that YAP:Ce (formula YAlO3:Ce, Yttrium Aluminum Perovskite doped with Cerium) ionoluminescence offers better sensitivity and time response compared to the standard detector material, SrGa2S4:Eu (TG-Green), currently used in TJ-II. A comparison between both materials was carried out by irradiating them with H+ ions of up to 40 keV using a dedicated laboratory setup. It is found that for the low energy ions of interest at TJ-II, YAP:Ce offers 20 times higher sensitivity than TG-Green and much faster decay time, 27 ns versus 540 ns. It is expected that the use of YAP:Ce in combination with a faster data acquisition and an ion counting software as part of the TJ-II ion luminescent probe will provide 20 times faster data on ion loss.

B. Zurro, J. L. Velasco, E.M. Hollmann, A. Baciero, M.A. Ochando, K.J. McCarthy, F. Medina, I. Pastor, R.J. Hajjar, J.M. Garcia-Regana, A. V. Melnikov 4, , L.G. Eliseev, HIBP Team 4, 6 and TJ-II Team 1 Laboratorio Nacional de Fusion, CIEMAT, Madrid, Spain 2 University of California-San Diego, La Jolla, CA, USA 3 Max Plank Institute for Plasma Physics, Greifswald, Germany 4 National Research Centre `Kurchatov Institute', 12382, Moscow, Russia 5 National Research Nuclear University MEPhI, 115409, Moscow, Russia 6 Institute of Plasma Physics, NSC KhIPT, 61108, Kharkov, Ukraine The confinement of impurities injected by laser blow-off into low-density electron cyclotron resonance heated plasmas [1] and into higher-density plasmas created during the neutral beam heating phase of the TJ-II stellarator is studied. These regimes represent general stellarator regimes. In order to compare these situations, a transport analysis of representative discharges is performed using the impurity transport code STRAHL, from which experimental impurity fluxes are obtained by matching the code results with the temporal behaviour of reconstructed local global radiation from bolometer arrays. When compared with neoclassical calculations, poor agreement is found for the low-density plasma while good agreement is found for the latter case. In the higher-density plasma, which is closer to the operational regimes relevant for the stellarator reactor program, impurity accumulation is predicted and observed.

Spectral emission lines continue to be a powerful tool for studying astrophysical, process, laser-produced, and magnetically confined plasmas, among others. Hence, numerous spectroscopy-based plasma diagnostics, from the x-ray to the infrared, make use of the relative intensity, width, displacement in wavelength, or temporal evolution of such emission lines emitted by the atoms and ions present in such plasmas. In this work, a spectral line survey, from 17.5-250 nm, is presented for electron cyclotron resonance heated (ECRH) and neutral beam injection (NBI) heated plasmas created and maintained in the TJ-II stellarator, a medium-sized magnetically confined plasma device. In these plasmas, for which hydrogen, deuterium or helium have been used as the working gas, central electron temperatures and densities up to 1 keV and 5 x 10(19) m(-3), respectively, are achieved. This work is a compilation of the identified spectral emission lines emitted by the working gas as well by the intrinsic and injected impurity ions in the above wavelength range. For this, spectra were recorded, over the past fifteen years of TJ-II operation, using a 1 m focal length normal incidence spectrometer equipped with a charge-coupled detector at its output focal plane. In total, almost 400 spectral emission lines from eighteen different elements have been identified using a number of atomic line emission databases.

The study of plasma-wall interactions and impurity transport in the plasma fusion devices is critical for the development of future fusion reactors. An experiment to perform laser induced breakdown spectroscopy, using minor modifications of our existing laser blow-off impurity injection system, has been set up thus making both experiments compatible. The radiation produced by the laser pulse focused at the TJ-II wall evaporates a surface layer of deposited impurities and the subsequent radiation produced by the laser-produced plasma is collected by two separate lens and fiber combinations into two spectrometers. The first spectrometer, with low spectral resolution, records a spectrum from 200 to 900 nm to give a survey of impurities present in the wall. The second one, with high resolution, is tuned to the wavelengths of the Hα and Dα lines in order to resolve them and quantify the hydrogen isotopic ratio present on the surface of the wall. The alignment, calibration, and spectral analysis method will be described in detail. First experimental results obtained with this setup will be shown and its relevance for the TJ-II experimental program discussed.

The main TJ-II results since 2012 are presented in this overview. Impurity confinement is studied showing an isotopic dependence of impurity confinement time, asymmetries in parallel impurity flows in TJ-II ion-root plasmas and impurity density asymmetries within a flux surface. In addition, first observations of electrostatic potential variations within the same magnetic flux surface are presented. Evidence of the impact of three-dimensional magnetic structures on plasma confinement and L–H transitions is also presented. The leading role of the plasma turbulence is emphasized by the observed temporal ordering of the limit cycle oscillations at the L–I–H transition. Comparative studies between tokamaks and stellarators have provided direct experimental evidence for the importance of multi-scale physics to unravel the impact of the isotope effect on transport. Novel solutions for plasma facing components based on the recently installed Li-liquid limiters (LLLs) have been developed on TJ-II, showing the self-screening effect of evaporating liquid lithium, protecting plasma-facing components against heat loads, and tritium inventory control. Regarding plasma stability, magnetic well scan experiments show that traditional stability criteria, on which the optimization of stellarator configurations is based, may miss some stabilization mechanisms. Further effects of ECRH on Alfvénic instabilities are investigated, showing that moderate off-axis ECH power deposition modifies the continuous nature of the Alfvén eigenmodes, and frequency chirping sets in. This result shows that ECH can be a tool for AE control that might be ITER and reactor-relevant.

The purpose of this work was to identify the Li III spectral line located at 4498 angstrom. First, the wavelength of an intense line lying close to 4498 angstrom was examined and compared with the information provided by different atomic databases. Some theoretical predictions were performed in order to simulate the behaviour of this line under different heating conditions. Afterwards, the spatial behaviour of this line was analysed and compared with other well-isolated spectral lines belonging to already-identified elements. Finally, an analysis of the temperature and intensity evolution from this line was performed. The results show a complex behaviour of the line, which is mainly dominated by Li.