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TJ-II stellarator results on modelling and validation of plasma flow asymmetries due to on-surface potential variations, plasma fuelling physics, Alfvén eigenmodes (AEs) control and stability, the interplay between turbulence and neoclassical (NC) mechanisms and liquid metals are reported. Regarding the validation of the neoclassically predicted potential asymmetries, its impact on the radial electric field along the flux surface has been successfully validated against Doppler reflectometry measurements. Research on the physics and modelling of plasma core fuelling with pellets and tracer encapsulated solid pellet injection has shown that, although post-injection particle radial redistributions can be understood qualitatively from NC mechanisms, turbulence and fluctuations are strongly affected during the ablation process. Advanced analysis tools based on transfer entropy have shown that radial electric fields do not only affect the radial turbulence correlation length but are also capable of reducing the propagation of turbulence from the edge into the scrape-off layer. Direct experimental observation of long range correlated structures show that zonal flow structures are ubiquitous in the whole plasma cross-section in the TJ-II stellarator. Alfvénic activity control strategies using ECRH and ECCD as well as the relation between zonal structures and AEs are reported. Finally, the behaviour of liquid metals exposed to hot and cold plasmas in a capillary porous system container was investigated.

In a future fusion reactor like DEMOnstration reactor (DEMO) one of the main concerns is the handling of the power exhaust from the plasma, especially at the divertor. The expected power loads cannot easily be handled by traditional armor solutions based on solid materials like tungsten, especially when the effect of intense neutron bombardment is also considered. Interest in armor concepts based on liquid metals has been subsequently on the rise, as they prove to be more resilient against high, fast power loads and neutron bombardment. However, engineering solutions for those concepts are very complex, and need to be tested. For this purpose, Optimization of Liquid Metal Advanced Targets project (OLMAT) has been envisaged. The project will use the Neutral Beam Injection of the TJ-II stellarator to irradiate liquid metal targets with power densities (neutrals plus occasionally ions) relevant to DEMO steady state operation, in the range of 20 MW/m2. OLMAT design will allow a series of experiments that other divertor simulator devices cannot easily perform: in-situ measurements of hydrogen retention, redeposition, vapor shielding, material fatigue, dust and precipitates effects, etc. Moreover, a high-power fiber laser will be used to simulate Edge Localized Modes in a small area, or to simulate the strike point power deposition profile.

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 Alfvén Eigenmodes properties are also explored, showing the possibility of controlling Alfvén 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.

The use of liquid metals (LMs) as plasma facing components in fusion devices was proposed as early as 1970 for a field reversed concept and inertial fusion reactors. The idea was extensively developed during the APEX Project, at the turn of the century, and it is the subject at present of the biennial International Symposium on Lithium Applications (ISLA), whose fourth meeting took place in Granada, Spain at the end of September 2015. While liquid metal flowing concepts were specially addressed in USA research projects, the idea of embedding the metal in a capillary porous system (CPS) was put forwards by Russian teams in the 1990s, thus opening the possibility of static concepts. Since then, many ideas and accompanying experimental tests in fusion devices and laboratories have been produced, involving a large fraction of countries within the international fusion community. Within the EUROFusion Roadmap, these activities are encompassed into the working programs of the plasma facing components (PFC) and divertor tokamak test (DTT) packages. In this paper, a review of the state of the art in concepts based on the CPS set-up for a fusion reactor divertor target, aimed at preventing the ejection of the liquid metal by electro-magnetic (EM) forces generated under plasma operation, is described and required R+D activities on the topic, including ongoing work at CIEMAT specifically oriented to filling the remaining gaps, are stressed.

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 Zeff and radiated power were deduced. Hydrogen recycling was studied at two different temperatures and no change was detected in the range of 300–750K. The retention of H2 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. Keywords: Plasma facing Materials, Liquid metals, LiSn alloys, Hydrogen retention, Reactor materials

Lithium sputtering is studied at TJ-II with lithiated, boron coated walls. As reported previously, the sputtering yield measured in H plasmas was found to be significantly lower than expected, based on laboratory experiments and TRIM code calculations. The edge temperature dependence of the yield could be consistent with either an energy threshold far exceeding the corresponding pure lithium threshold, or with a strong perturbation of the impinging ion energies. The material mixing effect, one of the candidates for explaining the observed behavior, was studied by depositing boron on the Li layer. Also, particle retention and release was investigated in H/He plasmas in this very low recycling scenario. The release of either species in the opposite plasma was studied in different plasma conditions as well as in glow discharge (GD) plasmas. In He GD plasmas, the I/V characteristics of lithiated stainless steel electrodes were found to be anomalous, in spite of the fact that the dependence of sputtering on the incident particle energy agreed reasonably with expectations. The possible implications of these phenomena for the interaction of reactor plasmas with lithium elements are addressed.

This paper summarizes the different wall conditioning strategies applied in the TJ-II stellarator since the operational starting in 1997. In a first stage (1997–2001) the all-metal machine (stainless steel) was conditioned by He glow discharge. This procedure allowed an acceptable density control in low power ECRH plasmas. The boronization of TJ-II (starting in 2001) led to obtain low Z plasmas with high ECRH injected power, but the density control under NBI injection was not possible. Finally, the coating of the boronized walls with a lithium thin film has allowed to obtain high reproducible NBI plasmas with good density control.

In the TJ-II stellarator, L- to H-mode transitions are achieved under NBI heating conditions when operating with lithium coated walls. Several magnetic configurations have been explored in order to study the influence of the rotational transform in the transition. The shear of the negative radial electric field, Er, increases at the L-H transition by an amount that depends on the magnetic configuration and heating power. Magnetic configurations with and without a low order rational surface close to the plasma edge show differences that may be interpreted in terms of local changes in the radial electric field induced by the rational surface that could facilitate the L-H transition. Fluctuation measurements show a reduction in the turbulence level that is strongest at the position of maximum Er shear. High temporal and spatial resolution measurements indicate that turbulence reduction precedes the increase in the mean sheared flow, but is simultaneous with the increase in the low frequency oscillating sheared flow. These observations may be interpreted in terms of turbulence suppression by oscillating flows, the so-called zonal flows (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The Spanish stellarator TJ-II has been operated under lithium wall conditions for two years so far. Important changes in plasma parameters and, in particular, on particle recycling have been recorded with respect to the normal, boronized wall conditions previously prevailing. The specific effects that the new recycling scenario could have on the improved plasma parameters, and in particular to the global energy balance of electrons and protons are addressed in the present work. In addition, the possible increase in ion energies impinging on the walls should be mirrored by the incoming flux of sputtered lithium atoms. However, a strong decrease of the corresponding sputtering yield is recorded. This effect is analyzed in terms of possible material mixing effects among others (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Low order rational values of the rotational transform -magnetic resonances, for brevity- can be present inside ECR-heated plasmas of the TJ-II Heliac, in low magnetic shear conditions, without causing damage to confine- ment. Moreover, in agreement with previous experience in the TJ-II, the resonances seem to benefit confinement and are associated to changes in the radial electric field, at least in the density gradient region. These results en- courage considering magnetic resonances as possible external actuators on confinement in stellarator/heliotron devices. c

In the present work, a new diagnostic for the simultaneous measurement of ne, Te and Ti profiles at the edge of a fusion plasma is proposed and tested. First, the propagation of a supersonic He beam is self-consistently simulated from the electron parameter profiles reconstructed from the line ratio method. The radial profile of excited HeII ions, measured simultaneously with the HeI lines, is then compared with the simulated profile and the total effective ion loss rate from the observation volume is deduced, which is function of Ti and the local plasma parameters. The values so obtained are normalized to those from passive CX measurements (NPA). Steeper gradients and lower values are found in the Ti profile compared to those in the Te profiles. The implication of these findings in ion transport at the edge of the TJ-II stellarator is addressed.

Lithiumization in the TJ-II heliac has been recently performed by vacuum evaporation of metallic Li. Compared to previous wall coating scenarios, an enhanced density control by external puffing and discharge reproducibility, leading to the drastic enlargement of the operational window, has been obtained for hydrogen and helium plasmas. Strong changes in recycling, impurity composition, edge radiation profiles and global energy confinement have been recorded. The long-lasting effect of the new coating extends to H-wall inventories beyond the expected saturation of the Li layer by particle implantation. Sputtering yields for Li much lower than bibliographic values were measured. These changes had also impact on the plasma profiles under NBI heating scenarios, with increased peaking of the electron density profiles and fuelling rates corresponding to the nominal beam current. Strong ELM-type activity was seen, associated to significant changes in edge fluctuation levels and the presence of steep gradients in edge parameters.

Under a carbon-based divertor operation of the International Tokamak Experimental Reactor, tritium retention by codeposits in hard-to-reach areas represents an important concern, and an urgent need exists for cleaning techniques suitable for the in situ removal of these codeposits. The present design of divertor modules includes castellated structures, with narrow gaps prone to accumulate material. On the other hand, glow-discharge cleaning in He∕O2 mixtures is today one of the main candidates for suppressing tritiated deposits in plasma facing components. In the present work, the effectiveness of this technique for removing a‐C:H films in gaps is addressed. The authors show that a higher-than-expected efficiency, as compared to the penetration ability of plasma ions, can be obtained. Optimization of the plasma parameters for complete oxidation of the films has also been performed. Erosion rates above 3nm∕min at room temperature can easily be achieved in fully exposed films deposited in laboratory plasmas.

Hydrogen recycling and puffing through a hole in a poloidal limiter of TJ-II is studied by spatially resolved Hα-emission spectroscopy with a CCD camera. Very different emission profiles are obtained depending on whether the hydrogen enters the plasma when recycling at the carbon limiter or whether it is puffed through a hole in the limiter directly inside the last closed flux surface of the plasma. From the emission profiles, the neutral density profile of the Hα-emission precursor is deduced and compared to that calculated with the help of a simple one-dimensional model. The analysis shows, that in front of a limiter with Te > 30 eV, the Hα-emission has two main sources: dissociative excitation of thermally desorbed molecular hydrogen and excitation of atomic hydrogen with kinetic energy of typically 0.3 eV. The relative importance of this last term decreases in front of a strong gas-puffing source. If the puffing rate is very strong, the plasma is measured to be locally perturbed and this can be the origin of the different emission profiles measured with and without puffing.

A laser-based technique for the direct testing of rate constants used in the collisional–radiative model, for application in the analysis of the supersonic He beam diagnostic, is presented. The time evolution of populations of some relevant excited levels upon the laser pumping of one of them is simulated by solving the corresponding system of coupled differential equations. Transitions have been identified which show good sensitivity in the changes induced by the laser interaction to the particular values of the relevant collisional rate constants for both the singlet and triplet systems of the He atom. The available experimental set-up and some previous results on the implementation of the technique in TJ-II are also shown.

The influence of limiter biasing on plasma confinement, turbulence and plasma flows has been investigated in the TJ-II stellarator. Experimental results show that it is possible to modify global particle confinement and edge plasma parameters with both positive and negative biasing. Significant and minor modifications in the structure of plasma fluctuations have been observed during the transition to improved confinement regimes induced by limiter biasing. These results show evidence of electric field induced improved confinement via multiple mechanisms. The investigation of the relaxation of plasma potential and electric fields shows evidence of two different characteristic decay times.

This paper describes a series of experiments performed in TJ-II stellarator with the aim of studying the influence of magnetic configuration on stability and transport properties of TJ-II plasmas. Plasma potential profiles have been measured in several configurations up to the plasma core with the heavy ion beam probe diagnostic. Low-order rational surfaces have been positioned at different plasma radii observing the effect on transport features. Plasma edge turbulent transport has been studied in configurations that are marginally stable due to decreased magnetic well. Results show a dynamical coupling between gradients and turbulent transport. Experiments on the influence of magnetic shear on confinement are reported. Global confinement issues as well as enhanced confinement regimes found in TJ-II are discussed as well.

The boronization of TJ-II stellarator was carried out by injecting o-carborane (C2B10H12) during He glow discharge (GD). The optimum parameters for the deposition were previously tested in a small plasma reactor and the results extrapolated to the TJ-II conditions. Low radiated power, different recycling behavior and high plasma edge temperature were found in H2 and He plasmas after boronization. He GD after H2 plasmas desorbed about 1015 H atoms/cm2 from the B/C film (a factor 4 times higher than in the case of metal walls). Desorption cross section of He by Ar ions σ=5×10−15 cm2 were measured on B/C film. Finally, GD in Ar+H2 admixtures showed a clear removal of the B/C film.

The TJ-II is a four period, low magnetic shear stellarator, with high degree of configuration flexibility (rotational transform from 0.9 to 2.5) which has been operating in Madrid since 1998 (R=1.5 m, a

TJ-II is a four period, low magnetic shear stellarator (R = 1.5 m, a < 0.22 m, B0 ≤ 1.2 T) which was designed to have a high degree of magnetic configuration flexibility. In the most recent experimental campaign, coupling of the full ECRH power (PECRH ≤ 600 kW) to the plasma has been possible using two ECRH transmission lines which have different power densities. Both helium and hydrogen fuelled plasmas have been investigated. The article reviews the latest physics results in particle control, configuration effects, and transport and fluctuation studies.

The edge parameters of electron cyclotron resonance heated plasmas in the TJ-II stellarator are reported. Data from atomic beam diagnostics and electrical probes have been used for edge and scrape-off layer characterization. Scans in heating power and plasma density for H and He plasmas have been performed, for a given magnetic configuration. A linear increase of the diffusion coefficient at the last-closed magnetic surface with the ratio of injected power to plasma density and a similar value of that parameter for the two atomic species investigated were obtained. Global particle confinement times between 3 and 15 ms have been deduced, and transition to an enhanced confinement mode in H plasmas has been observed under some conditions. The role of high-energy particle losses, due to trapping into the relatively high magnetic ripple, in the global energy balance of TJ-II plasmas is addressed.

A kinetic model for second harmonic electron cyclotron breakdown in stellarators, based on the results obtained from non-linear wave-particle interaction theory, is presented. A set of particle balance equations that describes the H2 ionization chain by means of an electron momentum space discretization is introduced and solved by numerical integration. The time evolution of the various species densities is determined and breakdown related predictions are obtained for different prefill pressure conditions and different values of injected power. Analysis of the TJ-II stellarator experimental data shows good agreement with the theoretical results. This is the first time, to the authors' knowledge, that precise predictions are directly compared with experimental behaviour.

TJ-II is a medium size (major radius R= 1.5 m, average minor plasma radius a < 0.2 m, on axis magnetic field B= I T) helical axis stellarator operating in its first phase with up to 600 kW of ECH power. Two mobile poloidal limiters can control the last closed magnetic surface (LCMS) and diagnose the plasma boundary with a set of Langmuir probes and with CCD-cameras equipped with interference filters. In the described experiments, plasmas with different minor radii interact either with the toroidal limiter or with the mobile poloidal limiters. The electron density and temperature profiles are characterised in the plasma centre, boundary and scape-off layer (SOL) for the different configurations. The global energy for the different configurations is compared and the ratio of the energy confinement time to that obtained by the LHD-scaling-law, seem to be improved when inserting the poloidal limiters. Finally, it is discussed whether the large connection lengths obtained in certain poloidal limiter SOL regions effectively reduce the plasma size, making the definition of the plasma minor radius ambiguous.

Helium glow discharge cleaning (GDC) at room temperature, applied during the overnight periods between experimental days, has been used as standard method of wall conditioning in the TJ-II stellarator during the initial operation with ECRH plasmas in an all stainless steel scenario. However, the density control at medium and high power injection (more than 300 kW) becomes difficult due to the contribution of He implanted on the wall during GDC and desorbed by plasma discharges. He desorption rates of ∼ 4 × 10 10 cm 3 ms 1 have been estimated from residual gas analyser (RGA) postpulse measurements. In order to overcome this effect, an Ar GDC of a duration of 30 min prior to the TJ-II operation, and after the overnight He GDC have been applied in the last campaign. The Ar GDC removes of the order of 10 21 He atoms (about one monolayer) from the walls allowing a good control of the plasma density by means of external gas puffing. Values of Z eff ∼ 2.0-2.5 in H 2 plasmas and mass spectra measurements of thermal desorption show that the contribution of Ar implanted to the plasma fuelling is not important.

He and Li beams have been installed and set into operation at the TJ-II stellarator with the aim of measuring electron temperature and density profiles in the edge and scrape-off layer regions of the plasma. The effusive thermal Li beam penetrates up to a normalized effective radius of ρ=0.6–0.7 in typical electron cyclotron heated plasmas. Density profiles deduced from the Li beam are wider than those obtained from extrapolation of the profile measured by Thomson scattering. The He beam is a pulsed supersonic one. Simultaneous detection of three emission lines (667.2, 706.5, and 728.1 nm) is made with a set of three photomultipliers. First He beam measurements of ne and Te in TJ-II are presented and the agreement among results of different plasma edge diagnostics is discussed.

Electron cyclotron emission (ECE) measurements are used to investigate the changes in the electron temperature profile during on axis and off axis EC-heating in TJ-II stellarator. The modifications of the electron temperature profiles are discussed and compared with the predictions of ray tracing calculations. Recently, the two gyrotrons (PECRH ≤600 kW) were simultaneously applied for the plasma heating in TJ-II. ELM-like events are clearly observed in these plasmas. A first description of these phenomena is also presented.

ELM-like activity has been recently observed in TJ-II, in plasmas with stored energy above 1 kJ. The plasma is observed to develop bursts of magnetic activity (seen in Mirnov coil signals) which are followed by a large and distinct spike in the Hα signal. An increase in electrostatic and magnetic fluctuations at the plasma edge and a cold pulse towards the plasma centre are also characteristics of these events. In addition, the electron temperature profile locally flattens at the plasma radius where the temperature is in the range 100-200 eV. This flattening can be explained in terms of enhanced electron heat conductivity. Between ELM-like events the electromagnetic turbulence at the edge decreases and the Te profiles recover their former shapes. This activity is probably triggered by a resonant m = 2, n = 3 mode.

First plasmas have been produced in the Spanish stellarator TJ-II. They have been generated by 250 kW ECR heating under all metal wall conditions. Two different magnetic configurations, corresponding to similar iota values but different plasma volumes, have been used. Helium glow discharge conditioning has been systematically applied previous to and between discharges. Good plasma start-up performance has been obtained, characterised by a short delay respect to the gyrotron pulse and controlled peak value of the electron density. The present results stress the important role that plasma–wall interaction phenomena are expected to play in the generation and optimisation of TJ-II plasmas in future campaigns.

TJ-II is a medium size (major radius R=1.5 m, average plasma radius a

The well known He beam diagnostic, able to provide profiles of the electronic plasma edge parameters, has undergone a significant improvement by the application of supersonic expansion techniques in the last years. The use of pulsed fast piezoelectric valves was initiated in the TJ-IU and implemented in the TJ-II stellarators. In its former configuration the repetition rate of the beam was ultimately limited in the expansion chamber by the pressure buildup. In the actual upgraded version, however, a repetition rate up to 200 Hz can be achieved, and the recording of the electron temperature and density profiles with millisecond resolution is possible. In the present work, the technical development of the diagnostic required for optimized performance is described. In addition, the validation of the collisional–radiative model needed for the reconstruction of the edge profiles and the analysis of the complementary information given by the beam attenuation is also addressed.

This work deals with the results on flow dynamics in TJ-II plasmas under Li-coated wall conditions, which produces low recycling and facilitates the density control and access to improved confinement transitions. The low-density transition, characterized by the emergence of the shear flow layer, is described from first principles and within the framework of neoclassical theory. The vanishing of the neoclassical viscosity when approaching the transition from below explains the observation of a number of turbulent phenomena reported in TJ-II in recent years; a unifying picture is provided in which zonal, i.e. large scale, radially structured, perturbations are observable when the neoclassical damping is sufficiently small. Preliminary linear, collisionless gyrokinetic simulations are carried out to assess that the measured time scale of relaxation of such perturbations is reasonably understood theoretically. In higher density regimes, the physical mechanisms behind the L–H transition have been experimentally studied. The spatial, temporal and spectral structure of the interaction between turbulence and flows has been studied close to the L–H transition threshold conditions. The temporal dynamics of the turbulence-flow interaction displays a predator–prey relationship and both radial outward and inward propagation velocities of the turbulence-flow front have been measured. Finally, a non-linear relation between turbulent fluxes and gradients is observed.

In April 1994 first ECRH plasmas (28 GHz, 2nd harmonic) have been produced in the TJ-I U torsatron (R=0.6m, [a]=0.1m, B(0)=0.52 T, E(0)=0.21). In November 1994 a second experimental campaign using a 37.5 GHz gyrotron (B(o)=0.69 T) has started. The main parameters of the device, the basic characteristics of the heating system and some results concerning magnetic surface mapping and first plasma discharges, are reported. 4 refs., 4 figs.

An integrated data analysis system based on Bayesian inference has been developed for the TJ-II stellarator. It reconstructs the electron density profile at a single time point, using data from interferometry, reflectometry, Thomson scattering, and the Helium beam, while providing a detailed error analysis. In this work, we present a novel analysis of the ambiguity inherent in profile reconstruction from reflectometry and show how the integrated data analysis approach elegantly resolves it. Several examples of the application of the technique are provided, in both low-density discharges with and without electrode biasing, and in high-density discharges with an (L-H) confinement transition.

In this paper, measurements in TJ-I of the outer edge scrape-off layer (SOL) parameters and in-out asymmetries by means of Langmuir probe and local Hαemission at several values of plasma density and toroidal field (n¯e=(0.8−3.3)×1019m−3, Bt = 1.0 and 1.4 T) are reported. Good agreement was found between the data obtained by the two methods. The evolution of the global particle confinement time, τp, with plasma density at two values of the toroidal field was also determined. The ratio of τp at the the two values of Bt studied and the degree of asymmetry are density dependent. The results indicate that the local value of the magnetic field could be the relevant parameter determining the degree of asymmetry for the entire range of plasma densities studied, suggesting that local transport is responsible for global confinement, but that the dominant mechanism is different at low and high densities.

A summary of the second harmonic ECRH breakdown experiments performed in the TJ-II stellarator is given here. The goal of these experiments is to determine the dependence of second harmonic X-mode plasma breakdown efficiency on several relevant parameters as the EC power, wave polarization, beam injection angle, tokamak-like induced electric field, neutral gas pre-fill pressure and discharge creation time with respect to the ramp-up of the currents in the coils. For each different scenario, the breakdown properties are determined, as well as the initial pressure conditions and the level of initial radiation before the injection of the ECRH power, which plays an essential role in the breakdown process.

A laser‐based technique for the direct testing of the predictions of the Collisional‐Radiative Model, of application to the analysis of the supersonic He beam diagnostic, is presented. The time evolution of the populations of some relevant excited levels upon the laser pumping of one of them is simulated by solving the corresponding system of coupled differential equations. Good sensitivity of the changes induced by the laser interaction to the particular values of the relevant collisional rate constants is observed for both, the singlet and triplet systems of the He atom. The available experimental set‐up and some previous results are also shown, and the required improvements for its application to the TJ‐II plasma conditions are addressed.

The response of transport in low density (line average ) plasmas to electron cyclotron resonance heating (ECH) power, 100 kW QECH 400 kW, is documented for the TJ-II Heliac-type stellarator. Radially resolved electron heat balance shows no significant differences between boron or lithium coating of the vacuum chamber walls. The main trends in electron heat transport are found to be similar to other stellarator/heliotron devices and are compatible with neoclassical calculations in the bulk of the plasma. According to our calculations the heat fluxes are anomalous near the edge, ρ 0.8 where ρ is the normalized minor radius, but the uncertainties there are large. Particle transport in the density gradient region, ρ ≈ 0.8, has little sensitivity to the variation of heating power and is compatible with neoclassical predictions. Neoclassical transport of particles and electron heat is found to be dominant in the gradient regions of typical ECH plasmas of the TJ-II Heliac.

A new, compact atomic beam source based on the supersonic expansion of He has been developed for application as a plasma edge diagnostic. The beam is produced from a pulsed valve with a duration between 0.2 to 2 ms and a nominal repetition rate < 500 Hz. A terminal speed ratio > 10 and a divergence of ± 1° have been achieved at stagnation pressures below 2 bar. The diagnostic has been tested in ECRH plasmas on the TJ-IU torsatron, representing the first application of a supersonic beam to plasma characterization, to our knowledge. Operational conditions which minimized the total amount of He injected into the plasma were chosen. Non-perturbative injection conditions in the low density plasmas could be obtained at local He densities of ⋍ 1 × 1011 cm−3 and a beam diameter < 1 cm. Due to the relatively low electron density of the ECRH plasmas, and to the good penetration characteristics of the supersonic He beam, the diagnostic could be used up to fairly low values of the normalized plasma minor radius, View the MathML source. Details of the optimization of the atomic beam diagnostics and typical results for steady state conditions in the TJ-IU plasmas are presented. © 1997 Elsevier Science B.V.

A description and transport analysis (energy and particles) of neutral beam heated plasmas is shown for two almost identical magnetic configurations except for a ?1% difference in their rotational transform, . Due to the low magnetic shear of TJ-II magnetic fields, such difference takes the low order rational to the plasma edge in one of them, while in the other the 8/5 value is near the edge but inside the plasma. A transport transition happens at lower density and plasma energy for the configuration that includes the , which exhibits better particle confinement already before the transition.

TJ-II is a medium-sized Heliac-type stellarator operating at low magnetic shear. Low-order rational values of the rotational transform (magnetic resonances, for brevity) can be introduced anywhere in its plasmas causing modifications in the electric potential and, consequently, radial structures in the radial electric fields that can be used to alter transport and stability in an externally controllable way. The ability of the Heliac to perform dynamic configuration scans has been used to illustrate these aspects and find practical realizations, such as exerting control on the L?H transition.

This paper presents an overview of experimental results and progress made in investigating the link between magnetic topology, electric fields and transport in the TJ-II stellarator. The smooth change from positive to negative electric field observed in the core region as the density is raised is correlated with global and local transport data. A statistical description of transport is emerging as a new way to describe the coupling between profiles, plasma flows and turbulence. TJ-II experiments show that the location of rational surfaces inside the plasma can, in some circumstances, provide a trigger for the development of core transitions, providing a critical test for the various models that have been proposed to explain the appearance of transport barriers in relation to magnetic topology. In the plasma core, perpendicular rotation is strongly coupled to plasma density, showing a reversal consistent with neoclassical expectations. In contrast, spontaneous sheared flows in the plasma edge appear to be coupled strongly to plasma turbulence, consistent with the expectation for turbulent driven flows. The local injection of hydrocarbons through a mobile limiter and the erosion produced by plasmas with well-known edge parameters opens the possibility of performing carbon transport studies, relevant for understanding co-deposit formation in fusion devices.

Sheared flows have been experimentally studied in TJ-II plasmas. In lowdensity ECH plasmas, sheared flows can be easily controlled by changing the plasma density, thereby allowing the radial origin and evolution of the edge velocity shear layer to be studied. In high density NBI heated plasmas a negative radial electric field is observed that is dominated by the diamagnetic component. The shear of the negative radial electric field increases at the L‐H transition by an amount that depends on the magnetic configuration and heating power. Magnetic configurations with and without a low order rational surface close to the plasma edge show differences that may be interpreted in terms of local changes in the radial electric field induced by the rational surface that could facilitate the L‐H transition. Fluctuation measurements show a reduction in the turbulence level that is strongest at the position of maximum Er shear. High temporal and spatial resolution measurements indicate that turbulence reduction precedes the increase in the mean sheared flow, but is simultaneous with the increase in the low frequency oscillating sheared flow. These observations may be interpreted in terms of turbulence suppression by oscillating flows, the so-called zonal flows. (Some figures in this article are in colour only in the electronic version)

In the last campaign, the TJ-II heliac has been operated under lithium-coated walls, representing the first stellarator ever working under these boundary conditions. Enhanced density control and discharge reproducibility, leading to the drastic enlargement of the operational window, have been obtained. A strong decrease in recycling together with changes in the shot by shot fuelling characteristics and in the wall particle inventory have been recorded. These changes, associated with the new wall scenario, had led to a long-lasting good density control. The new conditions were also mirrored in the plasma profiles under NBI heating scenarios with increased peaking of the electron density profiles. Fuelling rates corresponding just to the nominal beam current were obtained for the first time, and transitions from bell to dome-type plasma profiles, with different collapsing limits, were observed and tentatively ascribed to changes in the local edge power balance. ELM-type activity was observed in concomitance to reduced fluctuation levels and confinement improvement. Record values of plasma energy content were measured at central densities up to 8 × 1019 m−3 under Li-coated walls.

This paper is a continuation of previous studies (de la Cal et al 2007 J. Nucl. Mater. 363–365 764, de la Cal et al 2007 Proc. 34th EPS Conf. (Poland, Warsaw) P 2.029), where hydrogen recycling under ionizing plasma conditions was analysed by spatially resolved Hα-emission spectroscopy with a tangentially viewing camera looking at a poloidal limiter of the TJ-II stellarator operated with a low density, high temperature plasma edge (n e = (1–10) × 1012 cm−3 and T e = 40–400 eV). In this study, the first objective is to validate for hydrogen the recently implemented EIRENE codea www.eirene.de by comparing simulated Hα-emission chords with that obtained experimentally. The second objective is to analyse the atom and molecular neutral distributions in front of the limiter and at other plasma and chamber locations. The third one is to study the atomic and molecular reactions involved in the dissociation, ionization and excitation reactions, as calculated from the EIRENE code, in order to study hydrogen atomic and molecular physics in a low density, high temperature fusion plasma edge, with special focus on interpretation of Hα-emission. The contribution of the different reactions to the emission is analysed as a function of plasma radius. A relevant result obtained from EIRENE under this item is that the main precursor of the molecular Hα-emission is not H2 as proposed in many previous studies, but .

Experiments aimed at understanding the effects of material properties on plasma edge parameters in limiter biasing scenarios have been addressed in TJ-II under full lithium wall coverage. Two different materials, graphite and liquid lithium were used as limiting surfaces. Positive and negative biasing respect to the plasma, as well as inter-limiter biasing experiments were carried out. A very different response of the plasma to the applied voltage was found depending on limiter material. While a week effect was seen in C limiter biasing for both polarities, enhancement of particle and energy confinement was observed when the liquid lithium limiter (LLL) was biased positively respect to the plasmas. Furthermore, spontaneous currents at the SOL were produced when both limiters were simultaneously inserted due to very different floating voltages achieved by both material. The implications of secondary electron emission properties of the plasma-exposed surfaces are addressed.

In the 1999 experimental campaign, the Spanish stellarator, TJ-II (R = 1.5 m, a < 0.22 m, B 0 < 1 T) has been operated under a broad range of parameters, including changes in the magnetic configuration, working gas (H 2 vs He), microwave heating power (100-600 kW, two independent lines at 53.2 GHz, second harmonic X-mode) and plasma-wall interaction conditions (wall conditioning, poloidal vs toroidal limiter). Although a close coupling between the plasmas and the TJ-II vacuum vessel is naturally present in most conditions, a good control of central plasma values has been achieved for both atomic species even under conditions close to the highest power density (n e0 < 1.7 x 10 13 cm 3 , T e0 < 1.3 keV). For this purpose, a careful control of wall conditions has been required. In addition, the low electron density and temperature of TJ-II edge plasmas have led to a significant reduction of the expected plasma-wall interaction. In this work, the issues of plasma density control and edge characteristics for the different plasma species and heating power are addressed. Results of new edge diagnostics, as a thermal lithium beam and a supersonic helium beam, among others are presented.

The first experimental campaign of the TJ-II stellarator has been conducted using electron cyclotron resonance heating (f = 53.2 GHz, P-ECRH approximate to 250 kW) with a pulse length of Delta t approximate to (80-200) ms. The flexibility of the device has been used to study five different configurations varying plasma volume and rotational transform. In this paper, the main results of this campaign are presented and, in particular, the influence of plasma-wall interaction phenomena on TJ-II confinement is briefly discussed.

ECR (electron cyclotron resonance) heated plasmas have been studied in the low magnetic shear TJ-II stellarator (R = 1.5 m, a < 0.22 m, B = 1 T, f = 53.2 GHz, P-ECRH = 300 kW, power density = 1-25 W cm(-3)). Recent experiments have explored the flexibility of the TJ-II across a wide range of plasma volumes with different rotational transforms and rational surface densities. In this paper, the main results of this campaign are presented and, in particular, the influence of iota and rational surfaces on plasma profiles is discussed.

This paper presents the latest results on confinement studies in the TJ-II stellarator. The inherently strong plasma–wall interaction of TJ-II has been successfully reduced after lithium coating by vacuum evaporation. Besides H retention and low Z, Li was chosen because there exists a reactor-oriented interest in this element, thus giving special relevance to the investigation of its properties. The Li-coating has led to important changes in plasma performance. Particularly, the effective density limit in NBI plasmas has been extended reaching central values of 8 × 1019 m−3 and T e ≈ 250–300 eV, with peaked density, rather flat T e profiles and higher ion temperatures. Due to the achieved density control, a second type of transition has been added to the low density ones previously observed in ECRH plasmas: higher density transitions characterized by the fall in Hα emission, the onset of steep density gradient and the reduction in the turbulence; which are characteristics of transition to the H mode. Confinement studies in ECH plasmas indicate that lowest order magnetic resonances, even in a low shear environment, locally reduce the effective electron heat diffusivities, while Alfven eigenmodes destabilized in NBI plasmas can influence fast ion confinement.