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The ion velocity distribution parallel to magnetic field lines, f is extracted from retarding field analyzer (RFA) measurements in the ASDEX Upgrade scrape-off layer. The RFA ion current–voltage characteristic is transformed into a system of linear equations, from which f is unfolded using a standard regularization method. The algorithm is validated on numerically generated data. The experimentally measured f compares favorably with the ion velocity distributions calculated from a quasi-neutral kinetic simulation of the plasma pre-sheath. The technique can be used to measure f by RFAs in plasma processing, high-energy particle accelerators or by satellite-born RFAs.

A new method to measure power flux in strongly magnetized plasmas is described, while the flaws inherent in standard Langmuir probe techniques are demonstrated. Instead of small cylindrical pins which overestimate the ion current density by several 100%, a concave probe has been developed which is immune to sheath expansion, and which inherently provides accurate measurements. A retarding field analyzer directly measures the ion component of the power flux by means of an integral method that eliminates the need to calculate the heat transmission factor. Evidence shows that strong secondary electron emission from surfaces with non-oblique magnetic field incidence angles is ubiquitous in the scrape-off layer of the Tore Supra tokamak. This results in sheath collapse, causing the power flux to be dominated by the electrons. The radially integrated power flux measured by the probes agrees well with the power convected to the limiter.

Using high-resolution diagnostics, this paper investigates experimentally two ways of influencing the radiofrequency (RF) sheath patterns (magnitude and spatial distribution) developing around active Tore Supra ion cyclotron antennae. Firstly two types of Faraday screen electrical designs were compared. The new design aimed at reducing the parallel RF electric field integrated along “long field lines” passing in front of the antenna, by interrupting all parallel RF current paths on its front face. This proved inefficient for attenuating the RF-sheaths on the screen itself, and only weakly modified their distribution. Secondly the ratio of strap RF voltage amplitudes was varied, either during spontaneous bifurcations of the antenna electrical operational point or through the power balance between RF generators. This affected the left–right symmetry of screen heat loads: the local fluxes tended to evolve as the RF voltages on the nearest strap. Both experiments question the method used to optimize the new screen.

New measurements of ion transport in the scrape-off layer (SOL) during inter-ELM periods are obtained from a retarding field analyzer (RFA) in the ASDEX Upgrade tokamak. The ion current density measured in the far SOL features large-amplitude bursts associated with turbulent plasma filaments passing the RFA. These filaments convect plasma to the wall with larger density than the background plasma density. The ions in the filament also have higher impact energies than the background ion impact energies. These observations indicate that a filament-wall interaction can lead to erosion of the first wall in the burning plasma tokamaks such as ITER (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

15th International Conference on Plasma-Facing Materials and Components for Fusion Applications (PFMC), Aix en Provence, FRANCE, MAY, 2015; International audience; Infrared (IR) images of the ITER wide angle viewing system are modeled for the baseline plasma equilibrium and partially detached tungsten divertor, taking into account the three-dimensional structure of the first wall and the divertor. The modeling includes a comprehensive chain of calculations from the heat load specifications up to the synthetic, reflection-free IR images of the surface temperature, T-surf. The effect of the optical blur due to finite IR detector size and diffraction/aberrations-approximated by a Gaussian filter-on the measured T-surf is investigated. The optical blur characterized by sigma = 0.7 pixel (approximately twice the diffraction limit) leads to underestimation of T-surf,T-max on the inner vertical divertor target and near the upper X-point by < 6% and < 4%, respectively. This is within the required measurement accuracy of 10%. Larger underestimation of T-surf,T-max (< 12%) is observed on the outer vertical divertor target. The study demonstrates the importance of keeping the performance of the optical system as close as possible to the diffraction limit.

Measurements in the tokamak scrape-off layer show the existence of fast electrons as far as a few centimeters radially in front of a lower hybrid antenna grill, which is one order of magnitude further than earlier predictions of interaction zones not exceeding 5 mm in width (Rantamäki et al 2000 Nucl. Fusion 40 1477). The present contribution aims at understanding these experimental results on the basis of electron Landau damping in a turbulent scrape-off layer. A crucial guiding element from the experiments is the observation of temporal intermittency of the fast electron signal at positions radially away from the grill mouth at a rate comparable with the detachment rate of relatively hot and dense field-aligned plasma filaments, or ‘blobs,’ from the last closed flux surface. When the dynamic character of the scrape-off layer is taken into account, transient low density and low temperature states occur during which the Landau damping can be weak enough to allow the radiated spectrum to penetrate radially deep inward. When the lower hybrid wave intercepts a dense and hot ‘blob’ a few cm in front of the antenna, then strong damping of the high wave number components can occur. This is the scenario we propose for explaining the generation of fast electrons far away from the grill mouth.

Trabajo presentado a la 21st Topical Conference on High Temperature Plasma Diagnostics, celebrada en Madison, Wisconsin (US) del 5 al 9 de junio de 2016. ASDEX Upgrade Team: et al., A magnetically driven fast-ion loss detector system for the ASDEX Upgrade tokamak has been designed and will be presented here. The device is feedback controlled to adapt the detector head position to the heat load and physics requirements. Dynamic simulations have been performed taking into account effects such as friction, coil self-induction, and eddy currents. A real time positioning control algorithm to maximize the detector operational window has been developed. This algorithm considers dynamical behavior and mechanical resistance as well as measured and predicted thermal loads. The mechanical design and real time predictive algorithm presented here may be used for other reciprocating systems., This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 under Grant Agreement No. 633053.

International audience; The peak power flux densities from plasma ion bombardment on tokamak armor surfaces can deviate from geometric-optical projection when the ion thermal Larmor radius becomes comparable to the characteristic size of the surface relief (gaps, misalignments, shaping). We present a fast and reliable tool to estimate the effect of finite ion Larmor radius on heat load deposition for complex 3D elements. The two possible geometrical design options foreseen for WEST tungsten monoblocks are asymmetric chamfer shaping and no shaping on top of monoblocks. This paper presents an evaluation of peak power flux densities for a WEST ITER-like tungsten monoblock plasma facing component which is vertically misaligned with respect to the other components of a maximum expected value of +0.3 mm, for steady state convected plasma power with ion temperature ranging from 10 to 500 eV. With the +0.5 mm asymmetric chamfer, leading edges between plasma facing components are shadowed and there is no significant difference between the optical calculations and the full ion orbit model. For an unshaped and +0.3 mm misaligned monoblock, when considering the full ion orbit model, the heat load on the exposed edge is reduced by a factor of 3-5 compared to the geometric-optical projection. Energy is found to be deposited over several millimetres on top of the monoblock just after the leading edge which prevents the tungsten armor from melting. (C) 2016 Elsevier B.V. All rights reserved.

et al., A conceptual design of a reciprocating fast-ion loss detector for ITER has been developed and is presented here. Fast-ion orbit simulations in a 3D magnetic equilibrium and up-to-date first wall have been carried out to revise the measurement requirements for the lost alpha monitor in ITER. In agreement with recent observations, the simulations presented here suggest that a pitch-angle resolution of ∼5° might be necessary to identify the loss mechanisms. Synthetic measurements including realistic lost alpha-particle as well as neutron and gamma fluxes predict scintillator signal-to-noise levels measurable with standard light acquisition systems with the detector aperture at ∼11 cm outside of the diagnostic first wall. At measurement position, heat load on detector head is comparable to that in present devices., This research was supported in part by the Spanish Ministry of Economy and Competitiveness (Grant Nos. RYC2011-09152, FIS2015-69362-P, and ENE2012- 31087) and the Marie Curie FP7 Integration Grant (No. PCIG11-GA2012-321455).

The primary goal of the WEST project is to be a test bed to characterize the fatigue and lifetime of ITER-like W divertor components subjected to relevant thermal loads. During the first phase of exploitation (S2 2016), these components (W monoblock plasma facing unit—W-PFU) will be installed in conjunction with graphite components (G-PFU). Since the G-PFU will not be actively cooled, it is necessary to ensure the expected pulse duration allows the W-PFU to reach its steady state without overheating the G-PFU assembly structure or the embedded stainless-steel diagnostics. High heat flux tests were performed at the GLADIS facility to assess the thermal behavior of the G-PFU. Some operational limits based on plasma parameters were determined. It was found that it is possible to operate at an injected power such that the maximal incident heat flux on the lower divertor is 10 MW m−2 for the required pulse length.

We present first ion energy measurements obtained in the scrape-off layer (SOL) of the MAST tokamak using a Retarding Field Analyzer (RFA). Results from two sets of experiments are reported. First, ion temperature (Ti) profiles were measured in L-mode discharges. Ti is lower than 20 eV and larger than the electron temperature with a ratio in the range 1–2.6, even in beam-heated discharges. This is consistent with a relatively high degree of ion–electron thermal coupling. Second, the RFA was used to estimate the ion energy in ELMs. Two different scenarios were studied and gave very different results. In the first one, a type-I ELMy H-mode, ions with energies exceeding 500 eV were found as far as 20 cm away from the separatrix. More statistics would be needed to calculate Ti in ELM filaments. In the second one, featuring type-III ELMs, no ions with energy larger than 200 eV were detected 10 cm outside the separatrix.

DC currents flowing along magnetic field lines connected to powered ICRF antennas have been observed experimentally. Negative current is collected on the powered ICRF antenna structure, while positive current is collected by magnetically connected Langmuir probes. Models based upon a double probe configuration are tested with the 2D electrostatic PIC code OOPIC and the 2D fluid code SEM. These models can be excited in a symmetrical (two electrode in antiphase) or an asymmetrical way (one electrode). Transverse RF currents can be exchanged with surrounding flux tubes. Simulations show that only “asymmetric” drive can draw net DC currents. As in experiments, they are negative on the “driven” electrode and positive on the grounded one. An electrostatic and low frequency analytical model explains how DC current is produced. The perpendicular width of current structures L⊥ depends on the parallel length L//.

The scaling law for carbon erosion in Tore Supra previously established by Hogan et al. [1] (Φc(C/ s) = 5 × 1020 Pcond (MW), where Pcond is the conducted power) is revisited both from the experimental and the modelling point of view. New developments with the EIRENE code, that allow relating measured CII emission intensities to the total amount of carbon sputtered from the Toroidal Pumped Limiter, are presented. Recent measurements carried out at high input power show a good agreement with the database used to establish the scaling law.

Proceedings of the 19th International Conference on Plasma-Surface Interactions in Controlled Fusion; International audience; We are interested here in modelling the electron and ion temperature fields, Te and Ti respectively, in order to understand the main trends that govern the ratio Ti/Te that is being better documented in the SOL with RFA probes and . The experimental evidence gathered from several devices indicates that this temperature ratio significantly exceeds unity in most data sets that have been analysed, including measurements in the SOL of limiter devices like Tore Supra. Several issues of interest have been addressed with this version of the SOLEDGE-2D code. First, we have analysed the width of the SOL heat channels to the wall components and compared these values to analytical expressions. The key control mechanism of the width of the SOL heat channel is given by a balance between the sheath boundary conditions and the transverse transport. More advanced simulations address the interplay between the edge and SOL plasma allowing one to recover regimes with Ti/Te > 1.

In tokamaks, where possible in areas of high conducted heat flux flowing parallel to magnetic field lines (for example in the divertor strike point vicinity), surfaces are oriented with glancing angles to the magnetic field in order to spread the power over the widest possible area. As a consequence of grazing angles, any protruding edges on these surfaces (due for example to mechanical misalignment) can lead to serious power overload and therefore to rapid damage. Using particle-in-cell simulations, power fluxes impinging on the gaps between misaligned edges due to Edge Localised Mode transients have been evaluated and are presented in this paper. The results of the computations show that the power deposited to the protruding edges is lower than the one expected without taking into account particle orbits. Particle fluxes inside a gap are also investigated as a function of the field line orientation with respect to the gap direction.

Efforts to provide regular, automatic, and reliable reciprocating probe measurements in the scrape-off layer of the Tore Supra tokamak are described. Real time feedback on fast magnetic reconstruction is used to control the probe position relative to the last closed flux surface, while analysis of the current-voltage characteristics is used to adapt the voltage waveform applied to the probe, improving the dynamic range of the measurement. A large probe head is employed to ensure operation in the strongly magnetized plasma regime. Concave collectors embedded in the probe head, so-called “tunnel probes”, are shown to be immune to sheath expansion effects, providing absolutely calibrated measurements of parallel ion current density (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The retarding field analyzer (RFA) is one of the only widely accepted diagnostics for measuring the ion tem-perature Ti in the tokamak scrape-off layer. An overview of the outstanding RFA performance over ten years of operation in Tore Supra tokamak is given and the validation of Ti measurements is addressed. The RFA measurements in Tore Supra are found to be well reproducible. The ion-to-electron temperature ratio is higher than one at low-to-moderate ion-electron collisionality regime and converges to unity at high collisionality regime (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Betaines are well known as compatible solutes that exert protein- and membrane-stabilizing effects, including protective effects on photosynthesis in plants and free-living algae stressed by high irradiance or unusual temperatures. Betaines, however, have received minimal attention in reef-building corals. One goal of this research was to identify and quantify the betaines of reef-building corals with chemically definitive methods. Metabolite profiling was conducted on 10 species (6 genera) of Curacao corals by liquid chromatography/time-of-flight mass spectrometry calibrated using six stable-isotope-labeled internal standards. Glycine betaine (GlyB), proline betaine (ProB), alanine betaine (AlaB), β-alanine betaine, hydroxyproline betaine (HProB), taurine betaine (TauB), trigonelline (Trig) and the chemically related sulfonium compound dimethylsulfoniopropionate were found in all species. Relative levels of betaines varied across species, with GlyB and ProB being most prominent. Betaines were collectively abundant; estimated total concentrations were 12 to 204 (mean = 75) mmol per liter of tissue. A second goal was to examine ecological patterns in betaine concentrations in field populations of Curacao corals. Betaine concentrations exhibited intraspecific patterns that matched a priori predictions for molecules that defend photosynthesis against negative effects of high irradiance. In Madracis mirabilis—which occupies unshaded locations—GlyB, ProB, AlaB, HProB, and Trig were 37–94% more abundant in colonies at 5 m depth (high irradiance) than 20 m. In M. pharensis—which occupies exposed and shaded locations—GlyB, ProB, and AlaB were 30–44% more abundant in unshaded than shaded colonies at one depth. M. senaria exhibited 45–93% increases in concentrations of betaines—GlyB, AlaB, HProB, TauB, and Trig—between early and late in the day, although M. mirabilis and pharensis did not. The results indicate that multiple betaines occur commonly in reef-building coral species, and betaine concentrations are modulated in response to growth light conditions in ways consistent with betaines acting as agents of photoprotection of coral photosynthesis.

Thermal plasma loads to the ITER Fast Ion Loss Detector are studied for QDT = 10 burning plasma equilibrium using the 3D field line tracing. The simulations are performed for a FILD insertion 9–13 cm past the port plasma facing surface, optimized for fast ion measurements, and include the worst-case perturbation of the plasma boundary and the error in the magnetic reconstruction. The FILD head is exposed to superimposed time-averaged ELM heat load, static inter-ELM heat flux and plasma radiation. The study includes the estimate of the instantaneous temperature rise due to individual 0.6 MJ controlled ELMs. The maximum time-averaged surface heat load is 12 MW/m2 and will lead to increase of the FILD surface temperature well below the melting temperature of the materials considered here, for the FILD insertion time of 0.2 s. The worst-case instantaneous temperature rise during controlled 0.6 MJ ELMs is also significantly smaller than the melting temperature of e.g. Tungsten or Molybdenum, foreseen for the FILD housing.

Deposited layers in Tore Supra are graphitic disordered carbons and contain spherical nanoparticles, proving the existence of homogeneous growth within the plasma edge, in an unexpected state of advancement. Evidence of homogeneous catalysis comes from the observation of graphite-encapsulated metal particles. Here, carbon particle properties such as temperature and ablation rate in tokamak plasmas are modeled with the DUSTT code, showing that growth can occur in the far scrape-off-layer during normal operation of Tore Supra. These particles can be generated either through plasma chemistry, thanks to efficient trapping in the scrape-off-layer due to sheath repulsion, or through arcing, thanks to transient optimal conditions for catalysis. Their observation on the neutralizer surfaces shows that they are then transported, avoiding destruction.

In next step machines, tritium retention in the carbon walls is a major concern. In Tore Supra, long pulses with impurity seeding were developed to study deuterium retention in stationary conditions at low edge temperatures. A double feedback was implemented, with deuterium injection set on plasma density and impurity injection set on the radiated fraction. Long discharges (∼1 min) were obtained with radiated fractions in the range 60–80%. Neon and argon seeding were tested. In both cases, a small fraction of the injected impurity is transiently trapped in the wall, and released at the end of the shot. The deuterium retention rate decreases significantly with increasing radiated fraction, both for Ne and Ar seeding, in absolute and relative value (when related to D2 injection rate, or Dα recycling on the limiter). Interpretation is still an open question, as different retention mechanisms could be affected (implantation, codeposition, bulk diffusion).

A spatially resolved calculation of the charge exchange particle and energy fluxes on the Toroidal Pumped Limiter (TPL) of Tore Supra is presented, as a first step towards a better understanding and modelling of carbon erosion, migration, as well as deuterium codeposition and bulk diffusion of deuterium in Tore Supra. The results are obtained with the EIRENE code run in a 3D geometry. Physical and chemical erosion maps on the TPL are calculated, and the contribution of neutrals to erosion, especially in the self-shadowed area, is calculated.

Deuterium trapping in carbon fiber composite N11 and pyrolitic graphite PG99 irradiated with plasma ions and electrons was examined with thermal desorption spectrometry. It has been found that the deuterium trapping takes place even at ion and electron energies of about 10 eV. For equal ion fluences, the deuterium retention and probability of CD4 formation are higher for ion irradiation at lower ion flux. Peculiarities of the deuterium retention and CD4 formation are explained; driving forces and mechanisms of the D trapping are discussed.

The scrape-off layer (SOL) ion (Ti) and electron (Te) temperatures were measured in ohmic plasmas of the Tore Supra tokamak for a wide range of main plasma parameters. Ti increases strongly with the intensity of the toroidal magnetic field Bt, whereas Te is nearly unaffected. The ion-to-electron temperature ratio τ = T i / T e ∝ B t 2.5 and is a weak function of plasma density and plasma current. Close to the last closed flux surface τ = 2 → 6 so that equipartition is never reached. The SOL ion temperature e-folding length is about 30% greater than the electron temperature e-folding length meaning that τ increases with radius in the SOL.

The issue of parallel flows asymmetries in the edge plasma is tackled with a new first principle transport and turbulence code. TOKAM-3D is a 3D full-torus fluid code that can be used both in diffusive and turbulent regimes and covers either exclusively closed flux surfaces or both open and closed field lines in limiter geometry. Two independent mechanisms susceptible to lead to large amplitude asymmetric parallel flows are evidenced. Global ExB drifts coupled with the presence of the limiter break the poloidal symmetry and can generate large amplitude parallel flows even with poloidally uniform transport coefficients. On the other hand, turbulent transport in the edge exhibits a strong ballooning of the radial particle flux generating an up-down m = 1, n = 0 structure on the parallel velocity. The combination of both mechanisms in complete simulations leads to a poloidal and radial distribution of the parallel velocity comparable to experimental results.

In order to withstand strong thermo-mechanical stress in the ITER’s divertor, its plasma facing components will be castellated. Consequently, a larger area with complex geometry will be exposed to high fluxes coming from the plasma. In order to evaluate the possible damage caused by transient events, we present here calculations of the expected power loads in the ITER divertor tile gaps during ELMs. We use a self-consistent, two-dimensional particle-in-cell technique to model plasma deposition with realistic boundary conditions. The power loads on the tile surface and inside the gaps are investigated for various magnetic configurations of the strike point and strong plasma parameters.

Carbon fiber composite (CFC) Sepcarb® N11 is used in the tokamak Tore Supra as plasma-facing components. To investigate the fuel retention capability of this material, a mobile sample holder was used to expose CFC N11 samples to direct irradiation by the scrape-off layer plasma of Tore Supra at fluences up to 1 × 1025 m−2. Deuterium (D) elemental mapping using nuclear reaction analysis for the most-exposed CFC sample showed that D retention occurs at depths greater than 8 μm due to the presence of deep (>3.5 μm) local retention sites. In this work, combining transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS), we describe at a high spatial resolution where and how D atoms are trapped in these sites. TEM experiments performed on thin cross-sections of the plasma-modified surface show evidence of the presence of a 3.5 μm-thick deuterated amorphous carbon layer deposited on the CFC surface. We show that specific localized retention sites correspond to the filling of relatively large (∼3 μm.) and deep (at least 3 μm below the initial CFC surface) cracks between fibres and matrix by the deuterated amorphous carbon layer.

The possible influence of a small population of suprathermal electrons on the tunnel probe, a new kind of Langmuir probe for fast DC measurements of ion flux and electron temperature in the tokamak scrape-off layer, has been investigated by numerical simulations. It was found that suprathermal electrons can influence the ion current distribution over the inner plasma collecting surfaces of the tunnel probe, and thus influence tunnel probe Te measurements. The ion current distribution depends on the structure of the Debye and the magnetic sheath inside the tunnel probe. Therefore, we make here a first attempt at investigating the influence of suprathermal electrons on the scaling of those sheaths. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A novel diagnostic, the tunnel probe, is used to investigate the edge plasma of the CASTOR tokamak. Comparison with conventional small, cylindrical Langmuir probes (typical of those usually employed for turbulence measurements in magnetized plasmas) demonstrates the superiority of the tunnel probe. The collectors of the tunnel probe are concave, eliminating in theory all uncertainty of the effective collecting area, and thereby rendering the measurement of parallel ion current density, electron temperature, and floating potential more reliable. Two tunnel probes, mounted back-to-back in a Mach probe arrangement, are used to investigate directional asymmetries of the plasma parameters. The tunnel probes are used as standard Langmuir probes by applying voltage sweeps simultaneously to all their internal conductors. The measured electron temperature is higher on the electron side than on the ion side, and the floating potential is lower. The observed asymmetries, measured at low density and collisionality in CASTOR, could be consistent with a hot tail of non-thermal electrons flowing in the counter-current direction. The ratio of ion saturation currents to the internal conductors of the tunnel probe provides a second independent measurement of electron temperature whose directional asymmetry is less pronounced, in agreement with recent theoretical predictions that tunnel probes should be less sensitive to non-thermal electrons than Langmuir probes in certain conditions.

The 2D boundary layer code TECXY has been used to simulate edge plasma parameters in the Tore Supra tokamak. Experimental situations have been analysed when the plasma contact point is shifted to different poloidal positions (limiter at bottom, top, outboard and inboard midplane) as well as the edge plasma conditions during lower hybrid RF power scan experiments. With the results of the calculations it was possible to reproduce experimentally observed changes to the flow pattern in the scrape-off-layer (SOL) for different plasma positions. These changes can be associated with the dependence of the radial electric field and the corresponding E × B drift velocity on the limiter position as well as to the poloidal asymmetries of the input particle flux. However, the code results with 'standard radial transport' did not show the large changes to the SOL widths as observed in the experiment. In order to reproduce in the calculations the observed differences in the SOL width, it was necessary to assume strong convective (or diffusive) radial plasma flow localized at the outboard midplane.

The uniform layer with thickness at least 50–100 μm was found on the CFC tiles from the inboard midplane after more than four years of tokamak operation. The upper part of the uniform layer was amorphous, but at the depth of ∼5 μm a structure consisting of micro-size regions with aromatic chains located parallel to the surface was found. Gradual transition from uniform layer to underlying CFC structure was observed. The reciprocating material probe was used for installation of CFC samples in the Tore Supra deuterium plasma. The thermal desorptional spectra of these samples are compared with the spectra of the samples irradiated in the laboratory stand and with the spectra of hydrogenated carbon film. The peculiarities of hydrogen isotope trapping under plasma irradiation and at the atmosphere are presented and discussed.

The ion temperature is a particularly important but rarely measured plasma boundary quantity. In this work we introduce a new electric probe, the segmented tunnel probe (STP), which measures ion temperature, electron temperature, and parallel ion current density simultaneously and with high temporal resolution. The probe was built and tested in the CASTOR tokamak. The STP was operated in a Mach-probe arrangement in a DC mode, providing bi-directional measurements with the temporal resolution of 1 μs. Therefore, it has significant potential to be employed in the research of transient phenomena in tokamaks (e.g. turbulence, fluctuations and ELMs). We present radial profiles of the plasma parameters in the CASTOR tokamak measured by the STP, using calibrations from particle-in-cell simulations.

The measurements of ion saturation current fluctuations in the scrape-off layer of the Tore Supra tokamak are presented. As the radial distance from last closed flux surface increases more and more rare, but larger positive bursts become dominant in the signal and its probability density function becomes strongly asymmetric with pronounced positive tail. At the same time burst duration and inter-burst time increase dramatically. We explain this phenomenon by radial propagation and dynamics of the ensemble of different size large scale coherent structures.

Fuel retention inside plasma facing components will be a crucial issue not only in fusion reactors of the future, but also in ITER. The estimation of the fraction of the fuel which remains trapped inside the vessel is quite a difficult task. Particle balance analysis provides information for the whole vacuum chamber as a function of time and can be use to monitor the tritium in-vessel retention in real-time. On Tore Supra with a careful choice and position of pressure sensors, proper calibration procedures, the accuracy of the balance is around 10%. Particle balance analysis have been performed on many long pulse discharges and deuterium in-vessel retention has been found to be a constant around 5 × 1020 D/s after several minutes of plasma. The evolution of the retention rate with plasma parameters indicates that deuterium bulk implantation and diffusion could dominate codeposition with carbon atoms. Particle balance is a powerful tool that should be implemented in ITER.

This paper presents a model of gas injection treating coherently matter deposition and thermal effects on the plasma. It is demonstrated from an analytical model that an injection can trigger thermal bifurcations toward radiative or recombinant regimes. 1D simulations were performed to study the radial dynamics. They indicate that the injection can create a cold detached region in the outer plasma. The ionization front is located at the transition between this zone and the hot plasma, characterized by a radiative layer and strong temperature gradients. In spite of a better neutral penetration, strong sources are needed for efficient core fuelling and extrapolations for ITER remain pessimistic. Moreover a thermal instability linked to the bifurcation is evidenced for strong particle sources. However, a 1D parallel model shows that the perturbation linked to the injection can remain localized along the parallel direction, thus favoring neutrals penetration, but this effect strongly depends on the perpendicular–parallel transport interplay.

ICRF-induced SOL modifications are mapped for the first time in 2-D around Tore Supra ICRF antennas using reciprocating Langmuir probes. When probe heads are magnetically connected to powered antennas, radical modifications of floating potentials Vfloat, effective temperatures Teff and ion saturation currents are observed. Vfloat perturbations are located radially near antenna limiters, with a typical extension 2 cm. Poloidally they are locally minimal near the equatorial plane, and maximal near antenna box corners. Two possible interpretations for increased Teff are proposed: localised electron heating and RF loop voltage induced along probe circuit. Both interpretations rely on the generation of parallel RF fields by parallel RF currents on the antenna structure. The topology of such currents could explain the 2-D structure of Teff maps. Both interpretations also imply a positive DC biasing of the antenna environment. Differential biasing of nearby flux tubes drives DC E × B0 convection that could explain 2-D density patterns.

Plasma-facing divertors and limiters are armoured with castellated tiles to withstand intense heat fluxes. Recent experimental studies show that a non-negligible amount of deuterium is deposited in the gaps between tiles. We present here a numerical study of plasma deposition in this critical region. For this purpose we have developed a particle-in-cell code with realistic boundary conditions determined from kinetic calculations. We find a strong asymmetry of plasma deposition into the gaps. A significant fraction of the plasma influx is expelled from the gap to be deposited on the leading edge of the downstream tile.

The tunnel probe is a new kind of Langmuir probe for fast dc measurements of ion flux and electron temperature in the tokamak scrape-off layer. The probe is calibrated using two-dimensional kinetic analysis of the ion current distribution on the concave conductors. Though qualitative agreement with classical Langmuir probe measurements was found, the electron temperature given by the tunnel probe is several times lower. One possible explanation might be an overestimation of the electron temperature by the Langmuir probe, due to a non-Maxwellian electron velocity distribution which can be modelled in a first approach as a two-temperature distribution. Hence the possible influence on the tunnel probe of a small population of nonthermal electrons is investigated by means of the two-dimensional kinetic code XOOPIC. It is found that this influence can be seen as the result of two combined physical effects: nonthermal electrons will reach the back plate (BP) and the ion current distribution over tunnel and the BP will change. The resulting dependence on probe bias and parallel ion current density of the TP sensitivity to nonthermal electrons is not reflected in CASTOR measurement results. Thus nonthermal electrons on their own cannot fully explain the discrepancy between Langmuir and tunnel probe measurements.

This paper presents a detailed study of carbon production and transport from the ergodic divertor (ED) target plates to the plasma core in the Tore Supra tokamak. Adapted experimental and numerical modelling techniques have been used to describe each of the main phenomena in play. Edge electron density and temperature are measured with Langmuir probes. The C II, C III and Hα emission is measured with optical fibres and cameras. The background plasma flow is calculated consistently with the observed recycling pattern by the neutral transport code EDCOLL for the two magnetic connection schemes of interest (short or long connection lengths). 3D Monte-Carlo modelling of carbon near the neutralizer plate (BBQ code) shows that the transport of carbon ions is governed by the friction force in addition to the electric field. Finally, a simplified 3D test particle model is used to estimate the core penetration fraction of carbon. A high value is found for the carbon screening efficiency (fraction of particles that does not penetrate in the plasma core), in the range 95–97% depending on the edge plasma conditions. This value, combined with the calculated carbon influxes, yields the first quantitative estimate of the carbon core contamination during ED operation. The paper shows that the screening of carbon and core contamination are mainly dependent on the carbon source (partially controlled with the ED) and the plasma flow distribution in the laminar region (magnetic topology and particle drifts).

21st International Conference on Plasma-Surface Interactions in Controlled Fusion Devices (PSI), Natl Inst Fus Sci, Kanazawa, JAPAN, MAY 26-30, 2014; International audience; This paper aims at understanding the main processes responsible for the erosion/deposition pattern observed on the surface of the Toroidal Pumped Limiter of Tore Supra, using the 3D local impurity transport code ERO. The influence of the plasma impurity content, CX-flux and surface temperature on the global carbon balance and erosion/deposition pattern is discussed. Main results are (1) that considering medium-range transport of C ions is mandatory for reproducing the main characteristics of the global C balance and erosion/deposition pattern, (2) that impurities and CX-atoms increase the erosion by a factor 42 (without changing the net/gross erosion ratio), and (3) that chemical erosion is governed by the re-erosion of deposits, which depends strongly on the surface temperature. (C) 2014 Elsevier B.V. All rights reserved.

Higher than expected electron temperatures (Te) are often measured by divertor Langmuir probes (LP) in high recycling and detached regimes in JET and other tokamaks. As a possible mechanism to explain this discrepancy, we investigate the effect of penetration of fast, almost collisionless electrons connecting the hot upstream scrape-off layer (SOL) region to the divertor targets in JET. We simulate the electron velocity distribution function (EVDF) near the divertor targets using a simple 1D kinetic model using parallel SOL profiles from EDGE2D-EIRENE simulations. The resulting EVDF is used to construct synthetic LP current-voltage (IV) characteristics and evaluation of Te is performed in the same way as for experimental data. Results indicate that the process does not explain the anomalously high Te values estimated from the target probe measurements if the EDGE2D-EIRENE simulated parallel profiles are a good representation of reality.

International audience; The influence of the plasma facing components material on the scrape-off layer plasma is investigated. In particular, the energy recycling is found to be more pronounced for tungsten wall compared with carbon wall. Edge plasma simulations performed with the transport code SOLEDGE2D-EIRENE show that this enhanced energy recycling in the tungsten case leads to an increase of the scrape-off layer temperature. Moreover, the energy recycling depends on the ion angle of incidence with the wall. A PIC code has been used to model the ion acceleration in the magnetic pre-sheath and determine the later angle of incidence. These simulations show that ions mostly impact the wall with rather shallow incident angles leading to a further increase of the energy recycling.

International audience; The nuclear fusion research goal is to demonstrate the feasibility of fusion power for peaceful purposes. In order to achieve the conditions similar to those expected in an electricity-generating fusion power plant, plasmas with a temperature of several hundreds of millions of degrees must be generated and sustained for long periods. For this purpose, RF antennas delivering multi-megawatts of power to magnetized confined plasma are commonly used in experimental tokamaks. In the gigahertz range of frequencies, high power phased arrays known as " Lower Hybrid " (LH) antennas are used to extend the plasma duration. This paper reviews some of the technological aspects of the LH antennas used in the Tore Supra tokamak and presents the current design of a proposed 20 MW LH system for the international experiment ITER.

The WEST project (W-Environment in Steady-State Tokamak) aims to transform the Tore Supra limiter configuration to an x-point divertor, providing a test bed for ITER-like plasma-facing components (actively cooled W monoblocs) under high heat flux, steady-state plasma irradiation. The lower divertor includes an actively cooled, W-coated CuCrZr baffle to provide neutral compression and improve particle exhaust. As part of the new diagnostic equipment of Tore Supra within WEST project, a set of Langmuir probes will find place on the baffle in order to provide plasma flux and electron temperature measurements for physics studies and real-time machine protection functions during steady-state discharges. On the baffle top surface, irradiation coming from the plasma, energetic ripple-ions losses, photons and energetic neutrals from charge exchange reactions produce power fluxes up to 3 MW/m2, representing a challenge for the Langmuir probes operating conditions. In this paper Copper–Chrome–Zirconium (CuCrZr) cylindrical probe concept design is proposed. Finite element thermo-mechanical analysis (FEA) confirmed the consistency of this solution under the steady-state plasma condition in the worst case (highest thermal load).

International audience; In the perspective of operating tungsten monoblocks in WEST, the ongoing major upgrade of the Tore Supra tokamak, a dedicated modelling effort has been carried out to simulate the interaction between the edge plasma and the tungsten wall. A new transport code, SolEdge2D–EIRENE, has been developed with the ability to simulate the plasma up to the first wall. This is especially important for steady state operation, where thermal loads on all the plasma facing components, even remote from the plasma, are of interest. Moreover, main chamber tungsten sources are thought to dominate the contamination of the plasma core. We present here in particular new developments aimed at improving the description of the interface between the plasma and the wall, namely a way to treat sheath physics in a more faithful way using the output of 1D particle in cell simulations. Moreover, different models for prompt redeposition have been implemented and are compared. The latter is shown to play an important role in the balance between divertor and main chamber sources.

We present the measurements of plasma characteristics in the scrape-off layer (SOL) of the Tore Supra tokamak performed by means of reciprocating Langmuir probe. The probe is inserted into the machine from top. As the radial distance from last closed flux surface (LCFS) increases, ion saturation current exhibits stronger bursty character and its probability density function becomes increasingly skewed towards positive values. At the same time, burst duration and inter-burst time increase dramatically. We explain this phenomenon by radial propagation and dynamics of the ensemble of coherent turbulent structures of different size. The results of two-dimensional fluid modelling based of flux-driven interchange instability mechanism are in excellent agreement with experimental results. We obtained clear experimental evidence that most of the coherent structures are formed in poloidally localized region of the SOL around the outboard midplane. If the probe is magnetically connected to this region, the SOL is very wide and we detect bursty behaviour in the far SOL. On the other hand, if the probe is not magnetically connected to the outboard midplane region (magnetic field lines are intercepted by the outboard limiter), then the SOL is very thin and bursty behaviour is much less prominent. Detection of bursty behaviour in the far SOL is correlated with existence of wide SOL pointing on important role of bursty transport by means of coherent turbulent structures in establishing the width of the SOL in tokamaks. The measurements of parallel flow in the SOL shows that plasma particle radial flux coming from confinement region to the SOL is mostly poloidally localized around the outboard midplane. Our estimations show that more than 80% of plasma particle radial flux is coming from confinement region to the SOL in poloidally localized region — approximately ±15° — around the outboard midplane.

Équipe 107 : Physique des plasmas chauds; International audience; A series of experiments has been performed on JET to investigate the dynamics of transient melting due to edge localized modes (ELMs). The experiment employs a deliberately misaligned lamella in one module of the JET bulk tungsten outer divertor, allowing the combination of stationary power flux and ELMs to transiently melt the misaligned edge. During the design of the experiment a number of calculations were performed using 2D particle-in-cell simulations and a heat transfer code to investigate the influence on the deposited power flux of finite Larmor radius effects associated with the energetic ELM ions. This has been performed using parameter scans inside a range of pedestal temperatures and densities to scope different experimentally expected ELM energies. On the one hand, we observe optimistic results, with smoothing of the heat flux due to the Larmor gyration on the protruding side of the lamella which sees the direct parallel flux-the deposited power tends to be lower than the nominal value expected from geometric magnetic field line impact over a distance smaller than 2 Larmor radii, a finding which is always valid during ELMs for such a geometry. On the other hand, the fraction of the flux not reaching the directly wetted side is transferred and spread to the top surface of the lamella. The hottest point of the lamella (corner side/top) does not always benefit from the gain from the Larmor smoothing effect because of an enhanced power deposition from the second contribution.

A new pellet injector, able to inject continuously hydrogen or deuterium pellets, was installed on Tore Supra in 2003 and preliminary experiments aiming to fuel long discharges were performed. In combination with Lower Hybrid (LH) Current Drive, pure pellet fuelled discharges lasting up to 2 min were achieved. The LH power was switched off just before each pellet injection (LH notching) to maintain a relatively deep pellet penetration by reducing the energy of the super-thermal electrons driven by the LH wave. A comparison, based on a particle balance study, between two comparable pellet fuelled and gas fuelled discharges has been done. In the two cases, the volume average density is the same and the analysis shows that the particle source, the pumped flux and the wall retention are similar and appear to be independent of the fuelling method for the low plasma current and density conditions considered ( I p = 0.6 MA, 〈 n e 〉 = 1.5 × 10 19 m −3 ).

The implementation of actively cooled high heat flux plasma facing components (PFCs) is one of the major ingredients required for operating the Tore Supra tokamak with very long pulses. A pioneering activity has been developed in this field from the very beginning of the device operation that is today culminating with the routine operation of an actively cooled toroidal pumped limiter (TPL) capable to sustain up to 10 MW/m 2 of nominal convected heat flux. Technical information is drawn from the whole development up to the industrialisation and focuses on a number of critical issues, such as bonding technology analysis, manufacture processes, repair processes, destructive and non-destructive testing. The actual experience in Tore Supra allows to address the question of D retention on carbon walls. Redeposition on surfaces without plasma flux is suspected to cause the final ‘burial’ of about half of the injected gas during long discharges.

The first experiments with a mobile sample holder in Tore Supra are described. It exposes 10 CFC samples to direct irradiation by the scrape-off layer plasma. The plasma parameters are measured simultaneously by two Langmuir probes, and the temperature of the samples by embedded thermocouples. The cumulated irradiation dose during the first brief campaign was enough to exceed the classical saturation of the ion stopping zone, as verified by thermodesorption spectroscopy and nuclear reaction analysis. Scanning electron microscopy of some of the samples was performed before and after irradiation in order to investigate the evolution of the surface structure due to ion bombardment.