| 531. |
- Tegnered, Daniel, 1987, et al.
(author)
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Comparative gyrokinetic analysis of JET baseline H-mode core plasmas with carbon wall and ITER-like wall
- 2016
-
In: Plasma Physics and Controlled Fusion. - : IOP Publishing. - 0741-3335 .- 1361-6587. ; 58:4
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Journal article (peer-reviewed)abstract
- Following the change of plasma facing components at JET from a carbon wall (CW) to a metal ITER-like wall (ILW) a deterioration of global confinement has been observed for H-mode baseline experiments. The deterioration has been correlated with a degradation of pedestal confinement with lower electron temperatures at the top of the edge barrier region. In order to investigate the change in core confinement, heat transport due to Ion Temperature Gradient (ITG)/Trapped Electron Mode (TEM) turbulence is investigated using the gyrokinetic code GENE. Two pairs of CW and ILW discharges that are matched according to several global parameters are simulated at mid radius. The simulations included effects of collisions, finite beta, realistic geometries, and impurities. A sensitivity study is performed with respect to the key dimensionless parameters in the matched pairs. The combined effect of the relative change in these parameters is that the ITG mode is destabilized in the ILW discharges compared to the CW discharges. This is also reflected in nonlinear simulations where the ILW discharges show higher normalized ion and electron heat fluxes and larger stiffness. The ion energy confinement time within rho = 0.5 is found to be comparable while the electron confinement time is shorter for the ILW discharges. The core confinement in the ILW discharges is expected to improve if the edge pedestal is recovered since that would favourably change the key plasma parameters that now serve to destabilize them.
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| 532. |
- Tegnered, Daniel, 1987, et al.
(author)
-
Gyrokinetic simulations of particle transport in pellet fuelled JET discharges
- 2017
-
In: Plasma Physics and Controlled Fusion. - : IOP PUBLISHING LTD. - 0741-3335 .- 1361-6587. ; 59:10
-
Journal article (peer-reviewed)abstract
- Pellet injection is a likely fuelling method of reactor grade plasmas. When the pellet ablates, it will transiently perturb the density and temperature profiles of the plasma. This will in turn change dimensionless parameters such as a/L-n, a/L-T and plasma beta. The microstability properties of the plasma then changes which influences the transport of heat and particles. In this paper, gyrokinetic simulations of a JET L-mode pellet fuelled discharge are performed. The ion temperature gradient/trapped electron mode turbulence is compared at the time point when the effect from the pellet is the most pronounced with a hollow density profile and when the profiles have relaxed again. Linear and nonlinear simulations are performed using the gyrokinetic code GENE including electromagnetic effects and collisions in a realistic geometry in local mode. Furthermore, global nonlinear simulations are performed in order to assess any nonlocal effects. It is found that the positive density gradient has a stabilizing effect that is partly counteracted by the increased temperature gradient in the this region. The effective diffusion coefficients are reduced in the positive density region region compared to the intra pellet time point. No major effect on the turbulent transport due to nonlocal effects are observed.
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| 533. |
- Telesca, G., et al.
(author)
-
High power neon seeded JET discharges : Experiments and simulations
- 2017
-
In: Nuclear Materials and Energy. - : Elsevier BV. - 2352-1791. ; 12, s. 882-886
-
Journal article (peer-reviewed)abstract
- A series of neon seeded JET ELMy H-mode pulses is considered from the modeling as well as from the experimental point of view. For two different Ne seeding rates and two different D puffing gas levels the heating power, P-heat, is in the range 22-29.5 MW. The main focus is on the numerical reconstruction of the total radiated power (which mostly depends on the W concentration) and its distribution between core and divertor and of Z(eff)(which mostly depends on the Ne concentration). To model self-consistently the core and the SOL two input parameters had to be adjusted case by case: the SOL diffusivity, D SOL, and the core impurity inward pinch, v(pinch). D-SOL had to be increased with increasing Gamma(Ne) and the level of v(pinch) had to be changed, for any given Gamma(Ne), according to the level of P-heat : it decreases with increasing P-heat. Since the ELM frequency, f(ELM), is experimentally correlated with P-heat, (it increases with P-heat) the impurity inward pinch can be seen as to depend on f(ELM). Therefore, to maintain a low v(pinch) level (i.e. high f(ELM)) Gamma(Ne)/P-heat should not exceed a certain threshold, which slightly increases with the Gamma(D) puffing rate. This might lead to a limitation in the viability of reducing the target heat load by Ne seeding at moderate Gamma(D), while keeping Z(eff) at acceptably low level.
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| 534. |
- Telesca, G., et al.
(author)
-
Simulation of JET ITER-Like Wall pulses at high neon seeding rate
- 2017
-
In: Nuclear Fusion. - : IOP Publishing. - 0029-5515 .- 1741-4326. ; 57:12
-
Journal article (peer-reviewed)abstract
- A series of neon seeded JET ELMy H-mode pulses is considered from the modeling as well as from the experimental point of view. For two different Ne seeding rates and two different D puffing gas levels the heating power, P-heat, is in the range 22-29.5 MW. The main focus is on the numerical reconstruction of the total radiated power (which mostly depends on the W concentration) and its distribution between core and divertor and of Z(eff) (which mostly depends on the Ne concentration). To model with the self-consistent code COREDIV the core and the SOL two input parameters had to be adjusted case by case: the SOL diffusivity, D-SOL, and the core impurity inward pinch, v(pinch). D-SOL had to be increased with increasing Gamma(Ne) and the level of v(pinch) had to be increased with decreasing the ELM frequency, f(ELM). Since for any given gas puffing level f(ELM) decreases with the Ne seeding rate, this might lead to a limitation in the viability of reducing the target heat load by Ne seeding at moderate Gamma(D), while keeping acceptably low Z(eff). In the considered range of temperatures and densities of the SOL, the numerical results suggest the balance between friction and thermal forces to be in favor of the frictional drag for tungsten, thus providing a W screening effect.
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| 535. |
- Testa, D., et al.
(author)
-
Sparse representation of signals : from astrophysics to real-time data analysis for fusion plasmas and system optimization analysis for ITER and TCV
- 2016
-
In: Plasma Physics and Controlled Fusion. - : Institute of Physics Publishing (IOPP). - 0741-3335 .- 1361-6587. ; 58:12
-
Research review (peer-reviewed)abstract
- Efficient, real-time and automated data analysis is one of the key elements for achieving scientific success in complex engineering and physical systems, two examples of which include the JET and ITER tokamaks. One problem which is common to these fields is the determination of the pulsation modes from an irregularly sampled time series. To this end, there are a wealth of signal processing techniques that are being applied to post-pulse and real-time data analysis in such complex systems. Here, we wish to present a review of the applications of a method based on the sparse representation of signals, using examples of the synergies that can be exploited when combining ideas and methods from very different fields, such as astronomy, astrophysics and thermonuclear fusion plasmas. Examples of this work in astronomy and astrophysics are the analysis of pulsation modes in various classes of stars and the orbit determination software of the Pioneer spacecraft. Two examples of this work in thermonuclear fusion plasmas include the detection of magneto-hydrodynamic instabilities, which is now performed routinely in JET in real-time on a sub-millisecond time scale, and the studies leading to the optimization of the magnetic diagnostic system in ITER and TCV. These questions have been solved by formulating them as inverse problems, despite the fact that these applicative frameworks are extremely different from the classical use of sparse representations, from both the theoretical and computational point of view. The requirements, prospects and ideas for the signal processing and real-time data analysis applications of this method to the routine operation of ITER will also be discussed. Finally, a very recent development has been the attempt to apply this method to the deconvolution of the measurement of electric potential performed during a ground-based survey of a proto-Villanovian necropolis in central Italy.
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| 536. |
- Tippawan, Udomrat, et al.
(author)
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Light-ion Production in 175 MeV Neutron-induced Reactions on Oxygen
- 2011
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In: Journal of the Korean Physical Society. - : Korean Physical Society. - 0374-4884 .- 1976-8524. ; 59:2, s. 1979-1982
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Journal article (peer-reviewed)abstract
- Data on light-ion production in light nuclei such as carbon, nitrogen and oxygen are particularly important in calculations of dose distributions in human tissue for radiation therapy at neutron beams, and for dosimetry of high energy neutrons produced by high-energy cosmic radiation interacting with nuclei (nitrogen and oxygen) in the atmosphere. When studying neutron dose effects, special consideration on carbon and oxygen is needed since they are, by weight, the most abundant elements in human tissue. The MEDLEY setup at The Svedberg Laboratory (TSL), Uppsala, Sweden has been used to measure such data with double-differential cross sections (DDX) for the (n, xp), (n, xd), xt), (n,(3)He), and (n,alpha) reactions from C, 0, Si, Ca, Fe, Pb, and U around 96 MeV. At the new Uppsala neutron beam facility the available energy range of quasi mono-energetic neutron beams is extended up to 175 MeV. The detector setup used in MEDLEY consists of eight so-called telescopes mounted at different angles inside all evacuated reaction chamber. Each of the telescopes consists of two fully depleted Delta E silicon surface barrier detectors (SSBD) and a CsI(Tl) crystal. In order to make measurements at this higher neutron energy possible some changes in the detector setup compared to the campaign at 96 MeV were applied Accordingly, the second Delta E detectors have been replaced by 1000 mu m thick SSBDs as well as the size of the crystals used as E detectors was increased to a total length of 100 mm and a diameter of 50 mm. The Delta E - E technique is used to identify the light ions, and cutoff energies as low as 2.5 MeV for protons and 4.0 MeV for alpha particles are achieved. The data are normalised relative to elastic up scattering measured in one of the telescopes at 20 degrees. Preliminary DDXs for oxygen are presented and compared with theoretical calculations.
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| 537. |
- Tiseanu, I., et al.
(author)
-
X-ray micro-laminography for the ex situ analysis of W-CFC samples retrieved from JET ITER-like wall
- 2016
-
In: Physica Scripta. - : IOP PUBLISHING LTD. - 0031-8949 .- 1402-4896. ; T167
-
Journal article (peer-reviewed)abstract
- x-ray micro-laminography was qualified and implemented as a complementary solution for the 3D microstructural analysis of tungsten coated carbon-fibre reinforced carbon (W/CFC) samples retrieved from JET ITER-like wall. As expected, the W layers spatially correlate with the morphology of the CFC substrate. Three main cases were distinguished; (i) tungsten layers coated parallel to PAN fibre bundles tend to have a quasi-continuous, weakly waved surface (waves amplitude <100 mu m); (ii) tungsten layers coated onto the relatively porous felt region appear to smoothly follow even the surface of the largest pores of around 250 mu m and (iii) samples coated perpendicular to the PAN fibre bundles display frequently and strong crater-like discontinuities of the metal layer. The characteristics dimensions of these gaps range in the order of 300-400 mu m both in the coating plane and perpendicular to it. On some craters the bottom W layer is broken and the generated debris can be found even deeper than one mm into the CFC substrate. These W particles, sized of 20-40 mu m, are always found in the large gaps located between the fibre bundles perpendicular to the coated surface.
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| 538. |
- Tokitani, M., et al.
(author)
-
Micro-/nano-characterization of the surface structures on the divertor tiles from JET ITER-like wall
- 2017
-
In: Fusion engineering and design. - : Elsevier. - 0920-3796 .- 1873-7196. ; 116, s. 1-4
-
Journal article (peer-reviewed)abstract
- Micro-/nano-characterization of the surface structures on the divertor tiles used in the first campaign (2011–2012) of the JET tokamak with the ITER-like wall (JET ILW) were studied. The analyzed tiles were a single poloidal section of the tile numbers of 1, 3 and 4, i.e., upper, vertical and horizontal targets, respectively. A sample from the apron of Tile 1 was deposition-dominated. Stratified mixed-material layers composed of Be, W, Ni, O and C were deposited on the original W-coating. Their total thickness was ∼1.5 μm. By means of transmission electron microscopy, nano-size bubble-like structures with a size of more than 100 nm were identified in that layer. They could be related to deuterium retention in the layer dominated by Be. The surface microstructure of the sample from Tile 4 also showed deposition: a stratified mixed-material layer with the total thickness of 200–300 nm. The electron diffraction pattern obtained with transmission electron microscope indicated Be was included in the layer. No bubble-like structures have been identified. The surface of Tile 3, originally coated by Mo, was identified as the erosion zone. This is consistent with the fact that the strike point was often located on that tile during the plasma operation. The study revealed the micro- and nano-scale modification of the inner tile surface of the JET ILW. In particular, a complex mixed-material deposition layer could affect hydrogen isotope retention and dust formation.
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| 539. |
- Tokitani, M., et al.
(author)
-
Plasma-wall interaction on the divertor tiles of JET ITER-like wall from the viewpoint of micro/nanoscopic observations
- 2018
-
In: Fusion engineering and design. - : Elsevier. - 0920-3796 .- 1873-7196. ; 136, s. 199-204
-
Journal article (peer-reviewed)abstract
- Micro/nanoscopic observations on the surface of the divertor tiles used in the first campaign (2011-2012) of the JET tokamak with ITER-like Wall (JET ILW) have been carried out by means of several material analysis techniques. Previous results from the inner divertor were reported for a single poloidal section of the tile numbers 1, 3 and 4, i.e., upper, vertical and horizontal targets, respectively. The formation of the thick stratified mixed-material deposition layer on tiles 1 and 4, and erosion on tile 3 were identified. This study is mostly focused on the outer divertor: tiles 6, 7 and 8. In contrast to the inner tile, remarkable surface modifications have not been observed on the vertical target (tiles 7 and 8) where sputtering erosion and impurity deposition would have been almost balanced. Only a specific part of tile 6 (horizontal target) located near the exhaust channel was covered with a stratified ("geological-like") mixed-material deposition layer which mainly included Be and Ni with the thickness of similar to 2 mu m. Special feature of this mixed layer was that a certain amount of nitrogen (N) was clearly detected in the layer. Since the concentration of N varied with the depth position, it could be depended on the amount of that gas puffed for plasma edge cooling during the JET experimental campaign. In addition to the outer divertor tiles, a very interesting feature of the local erosion and deposition effects is reported in this paper.
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| 540. |
- Tsavalas, P., et al.
(author)
-
Be ITER-like wall at the JET tokamak under plasma
- 2017
-
In: Physica Scripta. - : IOP PUBLISHING LTD. - 0031-8949 .- 1402-4896. ; T170
-
Journal article (peer-reviewed)abstract
- The JET tokamak is operated with beryllium and tungsten plasma-facing components to prepare for the exploitation of ITER. To determine beryllium erosion and migration in JET a set of markers were installed. Specimens from different beryllium marker tiles of the main wall of the ITER-like wall (ILW) JET tokamak from the first and the second D-D campaign were analyzed with nuclear reaction analysis, x-ray fluorescence spectroscopy, scanning electron microscopy and x-ray diffraction (XRD). Emphasis was on the determination of carbon plasma impurities deposited on beryllium surfaces. The C-12(d, p(0))C-13 reaction was used to quantify carbon deposition and to determine depth profiles. Carbon quantities on the surface of the Be tiles are low, varying from (0.35 +/- 0.07) x 10(17) to (11.8 +/- 0.6) x 10(17) at cm(-2) in the deposition depth from 0.4 to 6.7 mu m, respectively. In the 0.4-0.5 mm wide grooves of castellation sides the carbon content is found up to (14.3 +/- 2.5) x 10(17) at cm(-2) while it is higher (up to (38 +/- 4) x 10(17) at cm(-2)) in wider gaps (0.8 mm) separating tile segments. Oxygen (O), titanium (Ti), chromium (Cr), manganese (Mn), iron (Fe), nickel (Ni) and tungsten (W) were detected in all samples exposed to plasma and the reference one but at lower quantities at the latter. In the central part of the Inner Wall Guard Limiter from the first ILW campaign and in the Outer Poloidal Limiter from the second ILW campaign the Ni interlayer has been completely eroded. XRD shows the formation of BeNi in most specimens.
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