[Show abstract][Hide abstract]ABSTRACT: The tunnel probe is a concave Langmuir probe designed to operate in strongly magnetized plasma. Due to its shape, the tunnel probe is immune to sheath expansion effects and thus provides absolutely calibrated measurements of the parallel ion current density. A two-dimensional, self-consistent kinetic model is employed to model the flow of charges within the cavity of the tunnel probe. The calculation predicts that the distribution of the ion flux onto the inner conductors depends on the electric field inside the tunnel, which in turn depends on the electron temperature. Therefore, if the tunnel is divided into two negatively biased collectors, it is possible to use the simulation results to determine the electron temperature from the measured ion current ratio. This means that a DC-biased tunnel probe can be used to provide fast, simultaneous measurements of the parallel ion current density and the electron temperature without collecting a single electron. Measurements in the CASTOR and Tore Supra tokamaks agree well with the numerical simulations.
Full-text · Article · Mar 2016 · Journal of Physics Conference Series
[Show abstract][Hide abstract]ABSTRACT: Particles and heat transport in the scrape-off layer (SOL) of tokamaks is not yet fully understood. COMPASS is a small-size tokamakp where the edge plasma is well diagnosed in view of studying the competition between the parallel and the cross-field transport in the SOL. In order to better characterize SOL dynamics, in particular the poloidal asymmetry of the main parameters' radial profiles, two new in-situ magnetically driven reciprocating manipulators have been recently installed in COMPASS. These manipulators, the so-called pecker probes, are two additional poloidal measurement points to the existing two (vertical and horizontal) reciprocating manipulators. The pecker probes are located at the low field side of COMPASS at ±47.5o with respect to the outer mid-plane and are equipped with identical tunnel probe heads, providing simultaneous measurements of the ion saturation current density Jsat, the electron temperature Teand the parallel Mach number M//with high temporal resolution. In this paper, a detailed description of the pecker probe system in COMPASS is described and first measurements are presented.
Preview · Article · Mar 2016 · Journal of Physics Conference Series
[Show abstract][Hide abstract]ABSTRACT: 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.
[Show abstract][Hide abstract]ABSTRACT: 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.
[Show abstract][Hide abstract]ABSTRACT: 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.
No preview · Article · Jan 2015 · Journal of Nuclear Materials
[Show abstract][Hide abstract]ABSTRACT: 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.
[Show abstract][Hide abstract]ABSTRACT: The interaction of ELM filaments with the ITER beryllium first wall panels (FWPs) is studied using a simple ad-hoc fluid model of the filament parallel transport, taking into account the full, three-dimensional structure of the FWPs, including magnetic shadowing effects. The calculated ELM surface heat loads are used as input to the RACLETTE heat transfer code to estimate the FWP surface temperature rise. The results indicate that controlled ELMs in ITER during burning plasma operation (ΔWELM ≈ 0.6 M J) will not lead to melting or significant evaporation of the beryllium surfaces, even in the case of high ELM broadening and the minimum allowable distance between the primary and secondary separatrices. The ELM-averaged steady-state heat load also stays below the maximum power handling capability of the FWPs.
No preview · Article · Dec 2014 · Journal of Nuclear Materials
[Show abstract][Hide abstract]ABSTRACT: Recently, the surface of carbon fibre composite tiles of the toroidal pump limiter of Tore Supra has been analysed by scanning electron and atomic force microscopies. In regions where fibres are perpendicular to the surface, a specific erosion pattern has been observed. It is constituted of a striation oriented with an angle oblique with respect to the magnetic field. The characteristic wavelength of this structure is micrometric, and similar to the fibre size. Modelling has been undertaken to reproduce this micrometric pattern. It is shown to originate from the carbon composite structure, for which it has been found by measurement using laboratory plasma that the erosion rate of the fibres is different from that of the surrounding matrix. Modelling emphasizes the effect of the impinging flux angle distributions of deuterium ions and carbon impurities that are preliminarily determined from computation of the magnetic sheath. In the case of deuterium the sheath is shown to have little effect on the particle trajectories for the simulation parameters considered here, although when impurities are included the sheath deflection is significant. Furthermore this study shows how the fibre organization in the composite influences the striation direction and points out the importance of the angular dependence of the sputtering yield.
[Show abstract][Hide abstract]ABSTRACT: The EDGE2D-EIRENE code is applied for simulation of divertor detachment during matched density ramp experiments in high triangularity, L-mode plasmas in both JET-Carbon (JET-C) and JET-ITER-Like Wall (JET-ILW). The code runs without drifts and includes either C or Be as impurity, but not W, assuming that the W targets have been coated with Be via main chamber migration. The simulations reproduce reasonably well the observed particle flux detachment as density is raised in both JET-C and JET-ILW experiments and can better match the experimental in-out divertor target power asymmetry if the heat flux entering the outer divertor is artificially set at around 2–3 times that entering the inner divertor. A careful comparison between different sets of atomic physics processes used in EIRENE shows that the detachment modelled by EDGE2D-EIRENE relies only on an increase of the particle sinks and not on a decrease of the ionization source. For the rollover and the beginning of the partially detached phase, the particle losses by perpendicular transport and the molecular activated recombination processes are mainly involved. For a deeper detachment with significant target ion flux reduction, volume recombination appears to be the main contributor. The elastic molecule-ion collisions are also important to provide good neutral confinement in the divertor and thus stabilize the simulations at low electron temperature (T e), when the sink terms are strong. Comparison between EDGE2D-EIRENE and SOLPS4.3 simulations of the density ramp in C shows similar detachment trends, but the importance of the elastic ion-molecule collisions is reduced in SOLPS4.3. Both codes suggest that any process capable of increasing the neutral confinement in the divertor should help to improve the modelling of the detachment. A further outcome of this work has been to demonstrate that key JET divertor diagnostic signals—Langmuir probe T e and bolometric tomographic reconstructions—are running beyond the limit of validity in high recycling and detached conditions and cannot be reliably used for code validation. The simulations do, however, reproduce the trend of the evolution of the line integrated bolometer chord measurements. The comparison between the code results and high-n Balmer line radiation intensity profiles confirms that a strong volume recombination is present during the experimental detachment and may play a role in this process, as suggested by the code.
[Show abstract][Hide abstract]ABSTRACT: The upgrade of its ion cyclotron resonance (ICRH) and lower hybrid current drive (LHCD) heating systems makes the Tore Supra (TS) tokamak particularly well suited to address the physics and technology of high-power and steady-state plasma–surface interactions. High radio frequency (RF) heating powers have been successfully applied up to 12.2MW coupled to the plasma, in which about 7.85MW flows through the scrape-off layer. Thermal calculation based on thermography measurements gives the heat flux density distribution on the TS toroidal limiter located at the bottom of the machine. The target heat flux densities are divided by the incidence angle of the field lines with the surface and mapped to the magnetic flux surface to evaluate the power flowing in the scrape-off layer (SOL). The power profile shows a narrow component near the last closed flux surface and a wide component in the rest of the SOL. The narrow component is attributed to significant cross-field heat flux density around the plasma contact point, about 0.8% of the parallel heat flux density in the SOL, when incident angles are nearly
tangential to the surface. The wide component is used to derive the experimental heat flux decay length (λq ) and parallel heat flux in the SOL. The power widths are measured for a series of 1 MA/3.8 T discharges involving a scan of RF injected power 3.5 � Ptot � 12.2MW. Independently of the heating power, we measured λq,OMP = 14.5±1.5mm at the outer mid-plane and parallel heat flux in the SOL in the range 130 � QLCFS 490MWm−2. TS values obtained with L-mode limiter plasmas are broader than those derived from L-mode divertor plasmas, confirming earlier results obtained with an ohmically heated plasma leaning on the inboard wall of TS.
[Show abstract][Hide abstract]ABSTRACT: A theory-based scaling for the characteristic length of a circular,
limited tokamak scrape-off layer (SOL) is obtained by considering the
balance between parallel losses and non-linearly saturated resistive
ballooning mode turbulence driving anomalous perpendicular transport.
The SOL size increases with plasma size, resistivity, and safety factor
q. The scaling is verified against flux-driven non-linear turbulence
simulations, which reveal good agreement within a wide range of
dimensionless parameters, including parameters closely matching the TCV
tokamak. An initial comparison of the theory against experimental data
from several tokamaks also yields good agreement.
[Show abstract][Hide abstract]ABSTRACT: In parallel to the direct contribution to the procurement phase of ITER and Broader Approach, CEA has initiated research & development programmes, accompanied by experiments together with a significant modelling effort, aimed at ensuring robust operation, plasma performance, as well as mitigating the risks of the procurement phase. This overview reports the latest progress in both fusion science and technology including many areas, namely the mitigation of superconducting magnet quenches, disruption-generated runaway electrons, edge-localized modes (ELMs), the development of imaging surveillance, and heating and current drive systems for steady-state operation. The WEST (W Environment for Steady-state Tokamaks) project, turning Tore Supra into an actively cooled W-divertor platform open to the ITER partners and industries, is presented.
[Show abstract][Hide abstract]ABSTRACT: To support the design of an ITER ion-cyclotron range of frequency heating (ICRH) system and to mitigate risks of operation in ITER, CEA has initiated an ambitious Research & Development program accompanied by experiments on Tore Supra or test-bed facility together with a significant modelling effort. The paper summarizes the recent results in the following areas:
Comprehensive characterization (experiments and modelling) of a new Faraday screen concept tested on the Tore Supra antenna. A new model is developed for calculating the ICRH sheath rectification at the antenna vicinity. The model is applied to calculate the local heat flux on Tore Supra and ITER ICRH antennas.Full-wave modelling of ITER ICRH heating and current drive scenarios with the EVE code. With 20 MW of power, a current of ±400 kA could be driven on axis in the DT scenario. Comparison between DT and DT(3He) scenario is given for heating and current drive efficiencies.First operation of CW test-bed facility, TITAN, designed for ITER ICRH components testing and could host up to a quarter of an ITER antenna.R&D of high permittivity materials to improve load of test facilities to better simulate ITER plasma antenna loading conditions.
[Show abstract][Hide abstract]ABSTRACT: This paper presents an analysis of the carbon–deuterium circulation and the resulting balance in Tore Supra over the period 2002–2007. Carbon balance combines the estimation of carbon gross erosion from spectroscopy, net erosion and deposition using confocal microscopy, lock-in thermography and SEM, and a measure of the amount of deposits collected in the vacuum chamber. Fuel retention is determined from post-mortem (PM) analyses and gas balance (GB) measurements. Special attention was paid to the deuterium outgassed during the nights and weekends of the experimental campaign (vessel under vacuum, Plasma Facing Components at 120 °C) and during vents (vessel at atmospheric pressure, PFCs at room temperature). It is shown that this outgassing is the main process reconciling the PM and GB estimations of fuel retention, closing the coupled carbon–deuterium balance. In particular, it explains why the deuterium concentration in deposits decreases with increasing depth.
[Show abstract][Hide abstract]ABSTRACT: 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.
No preview · Article · Jul 2013 · Journal of Nuclear Materials
[Show abstract][Hide abstract]ABSTRACT: The influence of RF electric fields on retarding field analyzer (RFA)
measurements of sheath potential, Vsh is investigated.
One-dimensional particle-in-cell simulations show that the RFA is able
to measure reliably the rectified dc sheath potential only for ion
plasma frequencies ωpi similar to the rf wave frequency
ωrf, while for real SOL conditions
(ωpi > ωrf), when the RFA is
magnetically connected to an RF antenna, it is strongly underestimated.
An alternative method to investigate RF sheaths effects is proposed that
uses broadening of the ion distribution function as evidence of the rf
electric fields in the sheath. RFA measurements in Tore Supra indicate
that the average effects of rf potentials do indeed propagate from the
antenna 12 m along magnetic field lines.
No preview · Article · Jul 2013 · Journal of Nuclear Materials
[Show abstract][Hide abstract]ABSTRACT: 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.
No preview · Article · Jul 2013 · Journal of Nuclear Materials