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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.

A full campaign of comparative Li/LiSn/Sn testing has been initiated in TJ-II stellarator plasmas. Solid and liquid samples of the three candidates, in a Capillary Porous System (CPS) arrangement, have been exposed to the edge plasma and the associated perturbation on the plasma has been recorded.In this work, only studies on Li and LiSn alloys are reported. The surface temperature of the liquid metal/CPS samples (a tungsten mesh impregnated with SnLi or Li) has been measured during plasma pulses with milliseconds resolution by pyrometry and radial profiles of Li, Li+, and Balmer Hα were recorded together with the electron edge parameters. A simple 1D model, based on the density decay, was applied to the data, allowing for the evaluation of the kinetic energy (Ek) of ejected atomic species while their residence time at the edge was determined by monitoring the ratio of first ion/neutral emission light intensities. Some evolution of Ek with sample temperature was seen for Li atoms, this being likely associated to the different relative contributions of sputtered/evaporated atoms, although values below 0.25 eV were found for sample temperatures above 500 K. The recorded Li+/Li ratios were analyzed through a simple model accounting for several mechanisms for the dispersal of the injected impurity away from the injection location. The same model was applied to injected He atoms (high recycling impurity) in order to check for prompt redeposition of Li.A transition from sputtering driven to evaporation of Li is observed with increasing temperature. Furthermore, no evidence of prompt redeposition is observed in our conditions. Keywords: Plasma facing materials, Liquid metals, LiSn alloys, Capillary porous system, Impurity screening, Reactor materials

A new method for studying the thermalization and transport of injected impurities at the edge of hot plasma, (considering the last closed magnetic surface, the free path is between 1 and 2 cm approx.) under no perturbative conditions, is presented. In the proposed technique, a Nd:YAG laser is used to ablate Li from the lithiated wall of the stellarator TJ-II. While the laser pulse allows for the analysis of the released species through laser induced breakdown spectroscopy (LIBS), its laser pulse also provides a time reference for the time-of-flight (TOF) measurements of the Li+ ions performed. This is done by positioning light detection systems sensitive to an intense Li II spectral line at different toroidal locations away from such a source. TOF times of tens to hundreds of microseconds are recorded. Then, by de-convolving the shape of the recorded light pulse, the velocity distribution of the lithium-ion during its thermalization with the background plasma can be extracted. From this velocity distribution, the ion temperature of the background ions and the toroidal rotation at the plasma periphery can be deduced. In contrast with conventional Doppler spectroscopy, this technique uses filter-scope detectors rather than high-spectral-resolution spectrometers thus a tradeoff between spectral and time or space resolution is required. Finally, preliminary results of the application of this novel technique are shown to validate this proof of principle.

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.

Secondary electron emission (SEE) yields, γ , of Li, stainless steel (SS) and W surfaces immersed in a He Direct Current Glow Discharge (dc-GD) Plasma have been calculated from the experimental I – V curves as a function of electron mean energy. The data obtained showed that γ Li > γ SS > γ W . Line emission ratios 728/706 of excited He and Langmuir probe measurements provide a clear evidence of the presence of a suprathermal electron tail responsible for the observed SEE. The results show that SEE is well correlated with the anomalous extra current component found in the I – V curves. The resulting value of γ Li is significantly higher than its theoretical value suggesting a possible synergetic effect of the ion bombardment in the SEE of lithium. The effect of Li surface oxidation has also been addressed, leading to a substantial decrease of both, sputtering yield and SEE yield of Li with higher oxygen content.

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.

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.

Lithium sputtering has been studied in TJ-II under lithiated walls. The measured sputtering yield in H plasmas was found to be significantly lower than that expected from laboratory experiments and TRIM code calculations. Moreover, the dependence of that yield on edge temperature is consistent with an energy threshold much larger than that of pure lithium. The effect of material mixing, candidate for the observed behaviours was studied by means of boron coating of the Li layer. Also the particle retention and release under H/He plasmas in this very low recycling scenario was investigated. Recycling coefficients R

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.

This paper is an extension of global plasma confinement in electron-cyclotron operation to neutral beam heated plasmas in TJ-II. The reported NBI plasmas have been obtained under lithium-coated wall conditions. The plasma operation exhibits a substantially better density control and improved confinement as compared to similar boron-coated wall discharges heated by NBI. Regression analyses of energy confinement time yield a scaling law indicating degradation with power with an exponent of αP = 0.8. The scaling shows an improvement with density with exponent αn = 0.5. The dependencies on plasma radius and rotational transform have not been resolved so far because the –still incomplete– NBI database suffers from a high correlation between these two parameters (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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.

Hydrocarbons (CH 4 and C 2 H 4 ) have been injected in TJ-II He plasmas by means of the supersonic helium beam set-up typically used for the edge characterization in hydrogen plasmas. The radial profiles of the emission of the CH(A-X) band and Hα were recorded along the hydrocarbon penetration. For all cases, the emission profiles of the recorder species can be fitted to a simple model with one single parameter, related to the velocity along the observation line of the CH and H radicals resulting from the initial cracking of the hydrocarbon. A H 2 beam was also used for reference of velocity distribution. The penetration of H atoms, which emit Hα, is in the case of hydrocarbon cracking twice as large as in H 2 dissociation, depending on hydrocarbon. A similar radial profile of the CH radicals was found for both hydrocarbons, thus suggesting the existence of a common precursor. The ratio of Hα/CH(A-X) photon emissions for the two hydrocarbons investigated is compatible with a reaction stoichiometry yielding 3 H atoms per CH.

The continuous control of tritium inventory in ITER calls for the development of new conditioning techniques [G. Federici et al., Nucl. Fus. 41 (2001) 1967]. For carbon plasma-facing components, this implies the removal of the T-rich carbon co-deposits. In the presence of strong oxygen getters, such Be, the use of oxygen-based techniques will be discouraged. In addition, tritiated water generated by these techniques poses extra problems in terms of safety issues [G. Saji, Fus. Eng. Des. 69 (2003) 631; G. Bellanger, J.J. Rameau, Fus. Technol. 32 (1997) 196; T. Hayashi, et al., Fus. Eng. Des. 81 (2006) 1365]. In the present work, oxygen-free (nitrogen and ammonia) glow discharge plasmas for carbon film removal were investigated. The following gas mixtures were fed into a DC glow discharge running in a ∼200 nm carbon film coated chamber. Erosion rate was measured in situ by laser interferometry, RGA (Residual Gas Analysis) and CTAMS (Cryotrapping Assisted Mass Spectrometry) [J.A. Ferreira, F.L. Tabares, J. Vac. Sci. Technol. A25(2) (2007) 246] were used for the characterization of the reaction products. Very high erosion rates (similar to those obtained in helium–oxygen glow discharge [J.A. Ferreira et al., J. Nucl. Mater. 363–365 (2007) 252]) were recorded for ammonia glow discharge.

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.

The well-documented effect of film inhibition/perturbation that nitrogen molecules have in the plasma-assisted (PA)CVD of a-C/N:H films in a DC glow discharge reactor under low pressure is investigated. Plasma parameters, gas-phase composition, and film characteristics are recorded using several diagnostic techniques, including the newly developed cryo-trapping assisted mass spectrometry (CTAMS). Five gas mixtures are investigated; CH4/H2, CH4/N2/H2, and CH4/Ar/H2 as precursors for the films, at low relative concentration of the hydrocarbon molecule (methane), and H2/N2 and He/N2 as film etchers. There are found to be strong differences in the composition of reaction product between systems in which plasma/surface interactions are dominated by ion-induced processes (sputtering), and those where chemical reactions with nitrogenated species would behave as scavengers of the film precursor. The implications of these findings for the proposed mechanisms of a-C/N:H film growth are addressed. The application of the scavenger technique for the inhibition of tritium-containing carbon co-deposits in the divertor region of fusion reactors, as an international thermonuclear experimental reactor (ITER) in the carbon-dominated scenarios, is analyzed in the light of these findings.

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.

Injection of methane and ethylene at the plasma edge of the TJ-II stellarator has been performed by using an instrumented, mobile limiter. The insertion of the limiter into the plasma leads to enhanced carbon contamination due to the local deposition of hydrocarbon films. The deposition characteristics of these films, their removal by plasma erosion and the spectroscopic signatures of such erosion have been recorded. The relative erosion efficiency by locally injected hydrogen and plasma particles has been compared. The relative yields of photon emission during the deposition (CH, Hα) and erosion (CH) for the different species used is analysed and compared to pure H injection. It is found that ethylene yields only a 15% of Hα photons per injected molecular H atom compared to H2 fuelling. Also, the ratio of CH/Hα photons delivered upon molecular cracking at the edge is a factor of three higher for ethylene than for methane.

High efficiency for the removal of carbon coatings in narrow (1 mm) gaps and carbon films directly exposed has been found in He/O 2 discharges, corresponding to >70% of the yield of directly exposed surfaces. Moreover, the absolute removal rate for the naked films was found to be 12 nm min [J.A. Ferreira, F.L. Tabares, D. Tafalla, in: 12th ICFRM Conference, Santa Barbara, USA]. This has been associated to the different wall scenarios in the present work, leading to enhanced atomic recycling, of positive effect in the cleaning rate of the carbon films. Also, an alternative cleaning technique was studied, based on glow discharges in H 2 /N 2 . This is compared with He/O 2 . A high cleaning efficiency is also seen, based on the formation of high stable, N–C containing products. Finally, changes in carbon film composition by the addition of B have been identified as responsible for the much higher resistance of doped films against both types of cleaning plasmas.

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 injection of nitrogen, as well as other species generally considered as scavengers of the film precursors, in carbon deposition plasmas can completely inhibit the film formation under given plasma conditions, as it has been extensively shown in technical plasmas. Experiments aimed at using this technique to the suppression of tritium/carbon co-deposits in fusion devices have been initiated in several devices. To date, a moderate inhibition (∼40%) of carbon deposition efficiency has been detected in nitrogen-seeded Elmy plasmas at JET. In addition to this, the divertor plasma simulator, Pisces-B, has been used to benchmark the concept under divertor conditions, while maintaining a better control of the experimental parameters. In the present work a review of the state of the art is given. The results from the available experiments are critically analysed in view of the present understanding of the re-deposited film formation in divertors.

Carbon fuelling experiments have been performed in ECRH TJ-II plasmas by injecting C 2 H 4 (ethylene) at several radial positions through gas injection ports located on poloidal diagnosted limiters. Simultaneous plasma edge characterization has been performed with atomic beams (He, Li) and by emission spectroscopy. From comparisons of plasmas having He and H as the main species it has been determined that physical sputtering in the interaction region between the plasma and the vacuum vessel is the source of intrinsic carbon contamination. Target plasmas with a range of magnetic configurations and having different edge topologies have been also used. In particular, configurations with rational numbers that result in island chains at the edge have been explored for possible application as ‘divertor-type’ plasmas. Fuelling efficiencies of a few percent for injected carbon have been deduced and clear enhanced screening for chemically bounded carbon is observed to occur in the presence of peripheral magnetic islands.

Complete suppression of amorphous C:H film deposition has been reported for N2/CH4 ratios ∼1, in low-pressure technical plasmas. This finding has been recently used by the authors for the proposal of a possible technique for the inhibition of re-deposited T-containing carbon films at the divertor region of fusion devices. Although several works aiming to the understanding of the underlying physicochemical processes have been published, the complexity of the system is far from being properly described by the proposed models. In the present work, experiments in DC glow discharges at low pressure of H2/CH4/N2 admixtures (90:0–5:0–5) are described. Mass spectrometry of neutral species as well as plasma mass spectrometry for ion detection have been used as the main diagnostics. Several plasma conditions (plasma current, gas composition, etc.) as well as isotopic exchange (H/D) have been investigated. Also, the effect of progressive film growing on the metal walls of the reactor in the composition of gas-phase species has been investigated. It is concluded that wall carbonisation is required to trigger the inhibition process. Ethylene and acetylene are found as the main reaction products. © 2004 Elsevier Ltd, supported by Dirección General de Investigación Científica y Técnica (DGICYT) under Project FTN2000-0915-C03-01, by CAM (Project 07N/0058/2002) and by M.E.C.D. of Spain (Project SB2000-0051)

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 assesses the three-dimensional (3D) effects of the edge magnetic field structure on divertor/scrape-off layer transport, based on an inter-machine comparison of experimental data and on the recent progress of 3D edge transport simulation. The 3D effects are elucidated as a consequence of competition between transports parallel ($\parallel $ ) and perpendicular ($\bot $ ) to the magnetic field, in open field lines cut by divertor plates, or in magnetic islands. The competition has strong impacts on divertor functions, such as determination of the divertor density regime, impurity screening and detachment control. The effects of magnetic perturbation on the edge electric field and turbulent transport are also discussed. Parameterization to measure the 3D effects on the edge transport is attempted for the individual divertor functions. Based on the suggested key parameters, an operation domain of the 3D divertor configuration is discussed for future devices.

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.