U. Samm

Ghent University, Gand, Flanders, Belgium

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Publications (286)476.65 Total impact

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    ABSTRACT: The chemical kinetics of the reaction of thin films with reactive gases is investigated. The removal of thin films using thermally activated solid-gas to gas reactions is a method to in-situ control deposition inventory in vacuum and plasma vessels. Significant scatter of experimental deposit removal rates at apparently similar conditions was observed in the past, highlighting the need for understanding the underlying processes. A model based on the presence of reactive gas in the films bulk and chemical kinetics is presented. The model describes the diffusion of reactive gas into the film and its chemical interaction with film constituents in the bulk using a stationary reaction-diffusion equation. This yields the reactive gas concentration and reaction rates. Diffusion and reaction rate limitations are depicted in parameter studies. Comparison with literature data on tokamak co-deposit removal results in good agreement of removal rates as a function of pressure, film thickness and temperature.
    01/2015; 1:1-7. DOI:10.1016/j.nme.2014.11.002
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    ABSTRACT: An estimation of the contribution of gaps to beryllium deposition and resulting tritium retention in the divertor of ITER is presented. Deposition of beryllium layers in gaps of the full tungsten divertor is simulated with the 3D-GAPS code. For gaps aligned along the poloidal direction, non-shaped and shaped solutions are compared. Plasma and impurity ion fluxes from Schmid (2008 Nucl. Fusion 48 105004) are used as input. Ion penetration into gaps is considered to be geometrical along magnetic field lines. The effect of realistic ion penetration into gaps is discussed. In total, gaps in the divertor are estimated to contribute about 0.3 mgT s−1 to the overall tritium retention dominated by toroidal gaps, which are not shaped. This amount corresponds to about 7800 ITER discharges up to the safety limit of 1 kg in-vessel tritium; excluding, however, tritium release during wall baking and retention at plasma-wetted and remote areas.
    Physica Scripta 04/2014; 2014(T159):014063. DOI:10.1088/0031-8949/2014/T159/014063 · 1.03 Impact Factor
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    ABSTRACT: Thermo-chemical removal (TCR), or baking in reactive gases, is a candidate method to control the co-deposit related tritium inventory in fusion devices. TCR can be understood as reaction–diffusion processes in a porous material. O2-TCR was applied to 150–550 nm thick a-C:D layers with similar textures. A linear relation between the integral TCR rate and the layer thickness, as predicted by the understanding, was observed in the experiment, i.e. the time to remove the hydrogen inventory is independent of its initial amount. TCR with nitrogen dioxide (NO2) at temperatures of 200–350 °C was conducted with a set of a-C:D and W–C–H layers. At 350 °C NO2 removed ~ 15% porosity a-C:D within 3 min. The O retention in remaining a-C:D was ≈ 1017 O cm−2. An activation energy of ≈ 0.78 eV for reactions of NO2 with D and C was determined. The results were applied for predictions of the TCR effectivity in ITER. The treatment of W–C–H led to O uptake (O/W ≈ 2–3), while W and C contents remained unchanged.
    Physica Scripta 04/2014; 2014(T159):014065. DOI:10.1088/0031-8949/2014/T159/014065 · 1.03 Impact Factor
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    ABSTRACT: The ITER-like wall recently installed in JET comprises solid beryllium limiters and a combination of bulk tungsten and tungsten-coated carbon fibre composite divertor tiles without active cooling. During a beryllium power handling qualification experiment performed in limiter configuration with 5 MW neutral beam injection input power, accidental beryllium melt events, melt layer motion and splashing were observed locally on a few beryllium limiters in the plasma contact areas. The Lorentz force is responsible for the observed melt layer movement. To move liquid beryllium against the gravity force, the current flowing from the plasma perpendicularly to the limiter surface must be higher than 6 kA m−2. The thermo-emission current at the melting point of beryllium is much lower. The upward motion of the liquid beryllium against gravity can be due to a combination of the Lorentz force from the secondary electron emission and plasma pressure force.
    Physica Scripta 04/2014; 2014(T159):014041. DOI:10.1088/0031-8949/2014/T159/014041 · 1.03 Impact Factor
  • Physica Scripta 04/2014; T159:014054. DOI:10.1088/0031-8949/2014/T159/014054 · 1.30 Impact Factor
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    ABSTRACT: The control of the radioactive inventory in the vacuum vessel of ITER is a main safety issue. Erosion of activated plasma-facing components (PFC) and co-deposition of tritiated dust on PFC and in areas below the divertor constitute the main sources of in-vessel radioactive inventory mobilizable in the case of an accident and also during venting of the vessel. To trace the dust and tritium inventory in the machine, the use of collectors in the form of removable samples was evaluated, beside other techniques, since it provides a reliable way to follow the history of the deposits and check critical areas. Four types of removable probes and two optional active diagnostics were selected out of about 30 different options. For all four probes, a conceptual design was worked out and the feasibility was checked with preliminary estimations of thermal and electromagnetic loads, as well as remote handling paths. The highest temperature estimated for the front face of all probes lies in the range 300–500 °C, which is tolerable. Installed in representative places, such removable samples may provide information about the dust and tritium distribution inside the vacuum vessel.
    Physica Scripta 04/2014; 2014(T159):014004. DOI:10.1088/0031-8949/2014/T159/014004 · 1.03 Impact Factor
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    ABSTRACT: Cracking thresholds and crack patterns in tungsten targets after repetitive ITER-like edge localized mode (ELM) pulses have been studied in recent simulation experiments by laser irradiation. The tungsten specimens were tested under selected conditions to quantify the thermal shock response. A Nd:YAG laser capable of delivering up to 32 J of energy per pulse with a duration of 1 ms at the fundamental wavelength λ = 1064 nm has been used to irradiate ITER-grade tungsten samples with repetitive heat loads. The laser exposures were performed for targets at room temperature (RT) as well as for targets preheated to 400 °C to measure the effects of the ELM-like loading conditions on the formation and development of cracks. The magnitude of the heat loads was 0.19, 0.38, 0.76 and 0.90 MJ m−2 (below the melting threshold) with a pulse duration of 1 ms. The tungsten surface was analysed after 100 and 1000 laser pulses to investigate the influence of material modification by plasma exposures on the cracking threshold. The observed damage threshold for ITER-grade W lies between 0.38 and 0.76 GW m−2. Continued cycling up to 1000 pulses at RT results in enhanced erosion of crack edges and crack edge melting. At the base temperature of 400 °C, the formation of cracks is suppressed.
    Physica Scripta 04/2014; 2014(T159):014005. DOI:10.1088/0031-8949/2014/T159/014005 · 1.03 Impact Factor
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    ABSTRACT: The development of optical diagnostics, like endoscopes, compatible with the ITER environment (metallic plasma facing components, neutron proof optics, etc.) is a challenge, but current tokamaks such as JET provide opportunities to test fully working concepts. This paper describes the engineering aspects of a fully mirrored endoscope that has recently been designed, procured and installed on JET. The system must operate in a very strict environment with high temperature, high magnetic fields up to B = 4 T and rapid field variations (∂B/∂t ∼ 100 T/s) that induce high stresses due to eddy currents in the front mirror assembly. It must be designed to withstand high mechanical loads especially during disruptions, which lead to acceleration of about 7 g at 14 Hz. For the JET endoscope, when the plasma thermal loading, direct and indirect, was added to the assumed disruption loads, the reserve factor, defined as a ratio of yield strength over summed up von Mises stresses, was close to 1 for the mirror components. To ensure reliable operation, several analyses were performed to evaluate the thermo-mechanical performance of the endoscope and a final validation was obtained from mechanical and thermal tests, before the system's final installation in May 2011. During the tests, stability of the field of view angle variation was kept below 1° despite the high thermal gradient on endoscope head (∂T/∂x ∼ 500 K/m). In parallel, to ensure long time operation and to prevent undesirable performance degradation, a shutter system was also implemented in order to reduce impurity deposition on in-vessel mirrors but also to allow in situ transmission calibration. One of the main specifications of the shutter system was high reliability. This was considered as achieved when the prototype was successfully tested to 3000 cycles at a temperature of 300 °C.
    Fusion Engineering and Design 10/2013; 88(6-8):1400-1404. DOI:10.1016/j.fusengdes.2013.01.056 · 1.15 Impact Factor
  • Fusion Engineering and Design 10/2013; 88(6-8):1361-1365. DOI:10.1016/j.fusengdes.2013.02.053 · 1.15 Impact Factor
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    ABSTRACT: Development of methods to characterise the first wall in ITER and future fusion devices without removal of wall tiles is important to support safety assessments for tritium retention and dust production and to understand plasma wall processes in general. Laser based techniques are presently under investigation to provide these requirements, among which Laser Induced Desorption Spectroscopy (LIDS) is proposed to measure the deuterium and tritium load of the plasma facing surfaces by thermal desorption and spectroscopic detection of the desorbed fuel in the edge of the fusion plasma. The method relies on its capability to desorb the hydrogen isotopes in a laser heated spot. The application of LID on bulk tungsten targets exposed to a wide range of deuterium fluxes, fluences and impact energies under different surface temperatures is investigated in this paper. The results are compared with Thermal Desorption Spectrometry (TDS), Nuclear Reaction Analysis (NRA) and a diffusion model.
    Journal of Nuclear Materials 07/2013; 438, Supplement:S1155 - S1159. DOI:10.1016/j.jnucmat.2013.01.255 · 2.02 Impact Factor
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    ABSTRACT: First results from three-dimensional modeling of the divertor heat and particle flux pattern during application of resonant magnetic perturbation fields as ELM control scheme in ITER with the EMC3-Eirene fluid plasma and kinetic neutral transport code are discussed. The formation of a helical magnetic footprint breaks the toroidal symmetry of the heat and particle fluxes. Expansion of the flux pattern as far as 60 cm away from the unperturbed strike line is seen with vacuum RMP fields, resulting in a preferable heat flux spreading. Inclusion of plasma response reduces the radial extension of the heat and particle fluxes and results in a heat flux peaking closer to the unperturbed level. A strong reduction of the particle confinement is found. 3D flow channels are identified as a consistent reason due to direct parallel outflow from inside of the separatrix. Their radial inward expansion and hence the level of particle pump out is shown to be dependent on the perturbation level.
    Journal of Nuclear Materials 07/2013; 438:S194–S198. DOI:10.1016/j.jnucmat.2013.01.025 · 2.02 Impact Factor
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    ABSTRACT: On the TEXTOR tokamak various experiments aimed at investigation of tungsten erosion and transport are performed. In one experiment a spherical W/C twin limiter positioned close to the last-closed flux surface in the near scrape-off layer was exposed to a number of comparable plasma discharges with stepwise variations of edge plasma parameters. Spatial distribution of tungsten and carbon light emission was recorded with two dimensional CCD cameras and spectrometer systems with high spectral and spatial resolution. Penetration depths, tungsten sputtering fluxes and erosion yields were measured. Comparison between experimental data and the results of modelling with the 3D Monte-Carlo code ERO is performed. The main objective of this study was to test the adequacy of the existing atomic data for neutral tungsten. The modelled penetration depths of the light emission of tungsten are a factor of 2–3 smaller than in experiment, which may indicate the overestimation of ionization rates.
    Journal of Nuclear Materials 07/2013; 438:S351–S355. DOI:10.1016/j.jnucmat.2013.01.066 · 2.02 Impact Factor
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    ABSTRACT: Molecular spectroscopy was used to observe molecular deuterium at the outer strike point of the new bulk tungsten JET divertor. The rotational and vibrational populations of the deuterium molecules in the ground state were determined from the deuterium Q-branches of Fulcher-α band emission (d3Πu-→a3Σg+) in the 600-640 nm spectral range. For L-mode plasmas in the low recycling regime the molecular emission maximum is located in the vicinity of the strike point. The spatial profile of the emission was strongly modified during plasma detachment in both L- and H-mode plasmas. The rotational temperature of excited molecules reached 2760 K in L-mode. The vibrational population has a peculiarity: a remarkably high population of the d3Πu-(v = 0) vibrational level indicating a non-Boltzmann vibrational distribution of D2 in tungsten environment.
    Journal of Nuclear Materials 07/2013; DOI:10.1016/j.jnucmat.2013.01.242 · 2.02 Impact Factor
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    ABSTRACT: Tracer experiments have been carried out by injection 13C marked methane through test limiters exposed to the scrape-off-layer in TEXTOR. The influence of impact energy and flux on depositing 13C species has been studied. One experiment has been performed with biased test limiter (−300 V) in order to increase energy of positively charged ions and the other one with 10 times reduced 13CH4 injection rate compared to previously used injection rate. Biasing of the test limiter increases the resulting 13C deposition by a factor of ∼6 – post-mortem analysis yields a 13C deposition efficiency of ∼1.7% compared to ∼0.3% without biasing. Reducing the injection rate increases 13C deposition efficiency to ∼0.7%, which is more than two times larger compared to experiments with previously used injection rate. ERO modelling shows that enhanced re-erosion of redeposits is still necessary to reproduce measured 13C deposition efficiencies.
    Journal of Nuclear Materials 07/2013; DOI:10.1016/j.jnucmat.2013.01.153 · 2.02 Impact Factor
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    ABSTRACT: Dust will have severe impact on ITER performance since the accumulation of tritium in dust represents a safety issue, a possible reaction of dust with air and steam imposes an explosion hazard and the penetration of dust in core plasmas may degrade plasma performance by increasing radiative losses. Investigations were performed in TEXTOR where known amounts of pre-characterized carbon, diamond and tungsten dust were mobilized into plasmas using special dust holders.Mobilization of dust changed a balance between plasma–surface interactions processes, significantly increasing net deposition. Immediately after launch dust was dominating both core and edge plasma parameters. Remarkably, in about 100 ms after the launch, the effect of dust on edge and core plasma parameters was vanished: no increase of carbon and tungsten concentrations in the core plasmas was detected suggesting a prompt transport of dust to the nearby plasma-facing components without further residence in the plasma.
    Journal of Nuclear Materials 07/2013; 438:S126–S132. DOI:10.1016/j.jnucmat.2013.01.020 · 2.02 Impact Factor
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    ABSTRACT: L-mode and H-mode density limits with the ITER-like wall (ILW) have been investigated in the recent experimental campaign and compared with experiments in the JET carbon material configuration. The density limit is up to 40% higher in the JET-ILW than in the JET-CFC machine. This is linked to the formerly higher radiation fraction and, correspondingly, to earlier divertor detachment in the JET-CFC. In the ILW configuration, the discharge demonstrates a stable operation with a completely detached outer divertor in L- and H-mode. In contrary to the well-known "heating power independent" Greenwald limit, the L-mode densities limit increases moderately with rising heating power (˜Pheat0.4) independently of the wall material.The H-L transition constitutes an effective undisruptive density limit for an H-mode plasma. Detachment itself does not trigger the H-L back transition and does not present a limit on plasma density. In the range of neutral beam heating 8-10.5 MW, no dependence of the H-mode density limit on the heating power was observed.
    Journal of Nuclear Materials 07/2013; 438:S139–S147. DOI:10.1016/j.jnucmat.2013.01.022 · 2.02 Impact Factor
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    ABSTRACT: In the linear plasma simulator PSI-2, capable of producing electron temperatures and densities of 1-20 eV and 1017-1019 m‑3 in the target region, a deuterium plasma with a simultaneous ionising and recombining region has been produced. The latter appeared when a sufficiently high extra deuterium gas flow was applied at the target region. An imaging spectrometer was used to measure Te and ne - profiles via the analysis of the Paschen-series resulting in drops down to 0.1 eV and 2 × 1018 m‑3 in the recombining region whereas intensity ratios of Hα, Hβ and Hγ provided ne in the ionising part. In the latter case ne was also deduced from the rotational temperature of the D2 molecules. Comparison with probe data yielded a reasonable agreement in that case. A detailed analysis of the atomic level populations and their light emissions provided some insight into the contribution of possible recombination mechanisms such as EIR and MAR.
    Journal of Nuclear Materials 07/2013; DOI:10.1016/j.jnucmat.2013.01.277 · 2.02 Impact Factor
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    ABSTRACT: Resonant Magnetic Perturbations (RMPs) are applied with the Dynamic Ergodic Divertor (DED) at TEXTOR to control the plasma edge transport and the plasma surface interaction. This leads to the formation of a three-dimensional (3D) topology of the scrape-off layer (SOL). To quantify the erosion/deposition balance and the material migration in this 3D boundary, spherical test limiters were exposed to plasmas with and without RMP fields applied. Methane doped with 13C as tracer element was injected through a gas inlet in the test limiter. The local gas source was monitored by spatially resolving spectroscopy and the resulting deposition patterns on the limiters were analysed with colourimetry and nuclear reaction analysis. These measurements were compared to simulations of the magnetic field topology simulations. The data provide evidence of a particle migration dominated by an ExB drift within stochastic zones of the 3D plasma boundary.
    Journal of Nuclear Materials 07/2013; DOI:10.1016/j.jnucmat.2013.01.126 · 2.02 Impact Factor

Publication Stats

3k Citations
476.65 Total Impact Points

Institutions

  • 2014
    • Ghent University
      • Department of Applied Physics
      Gand, Flanders, Belgium
  • 1990–2014
    • Forschungszentrum Jülich
      • Zentralabteilung für Chemische Analysen (ZCH)
      Jülich, North Rhine-Westphalia, Germany
  • 2001
    • Max Planck Institute for Plasma Physics
      • Max Planck Institute for Plasma Physics, Greifswald
      Arching, Bavaria, Germany
  • 1993
    • University of Toronto
      • Institute for Aerospace Studies
      Toronto, Ontario, Canada