| 31. |
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| 32. |
- Maggi, C. F., et al.
(författare)
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Isotope identity experiments in JET-ILW with H and D L-mode plasmas
- 2019
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Ingår i: Nuclear Fusion. - : Institute of Physics Publishing (IOPP). - 0029-5515 .- 1741-4326. ; 59:7
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Tidskriftsartikel (refereegranskat)abstract
- NBI-heated L-mode plasmas have been obtained in JET with the Be/W ITER-like wall (JET-ILW) in H and D, with matched profiles of the dimensionless plasma parameters, rho*, nu*, beta and q in the plasma core confinement region and same T-i/T-e and Z(eff). The achieved isotope identity indicates that the confinement scale invariance principle is satisfied in the core confinement region of these plasmas, where the dominant instabilities are Ion Temperature Gradient (ITG) modes. The dimensionless thermal energy confinement time, Omega(i) tau(E,th), and the scaled core plasma heat diffusivity, A chi(eff)/B-T, are identical in H and D within error bars, indicating lack of isotope mass dependence of the dimensionless L-mode thermal energy confinement time in JET-ILW. Predictive flux driven simulations with JETTO-TGLF of the H and D identity pair is in very good agreement with experiment for both isotopes: the stiff core heat transport, typical of JET-ILW NBI heated L-modes, overcomes the local gyro-Bohm scaling of gradient-driven TGLF, explaining the lack of isotope mass dependence in the confinement region of these plasmas. The effect of E x B shearing on the predicted heat and particle transport channels is found to be negligible for these low beta and low momentum input plasmas.
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| 33. |
- Makepeace, C., et al.
(författare)
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The effect of beryllium oxide on retention in JET ITER-like wall tiles
- 2019
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Ingår i: Nuclear Materials and Energy. - : Elsevier. - 2352-1791. ; 19, s. 346-351
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Tidskriftsartikel (refereegranskat)abstract
- Preliminary results investigating the microstructure, bonding and effect of beryllium oxide formation on retention in the JET ITER-like wall beryllium tiles, are presented. The tiles have been investigated by several techniques: Scanning Electron Microscopy (SEM) equipped with Energy Dispersive X-ray (EDX), Transmission Electron microscopy (TEM) equipped with EDX and Electron Energy Loss Spectroscopy (EELS), Raman Spectroscopy and Thermal Desorption Spectroscopy (TDS). This paper focuses on results from melted materials of the dump plate tiles in JET. From our results and the literature, it is concluded, beryllium can form micron deep oxide islands contrary to the nanometric oxides predicted under vacuum conditions. The deepest oxides analyzed were up to 2-micron thicknesses. The beryllium Deuteroxide (BeOxDy) bond was found with Raman Spectroscopy. Application of EELS confirmed the oxide presence and stoichiometry. Literature suggests these oxides form at temperatures greater than 700 degrees C where self-diffusion of beryllium ions through the surface oxide layer can occur. Further oxidation is made possible between oxygen plasma impurities and the beryllium ions now present at the wall surface. Under Ultra High Vacuum (UHV) nanometric Beryllium oxide layers are formed and passivate at room temperature. After continual cyclic heating (to the point of melt formation) in the presence of oxygen impurities from the plasma, oxide growth to the levels seen experimentally (approximately two microns) is proposed. This retention mechanism is not considered to contribute dramatically to overall retention in JET, due to low levels of melt formation. However, this mechanism, thought the result of operation environment and melt formation, could be of wider concern to ITER, dependent on wall temperatures.
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| 34. |
- Nabais, F., et al.
(författare)
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Energetic ion losses 'channeling' mechanism and strategy for mitigation
- 2019
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Ingår i: Plasma Physics and Controlled Fusion. - : Institute of Physics Publishing (IOPP). - 0741-3335 .- 1361-6587. ; 61:8
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Tidskriftsartikel (refereegranskat)abstract
- Results from two different sets of JET experiments are presented. In experiments in which toroidicity-induced Alfven eigenmodes (TAEs) localized at different radial locations had the same frequencies and toroidal mode numbers, the occurrence of enhanced losses after the excitation of TAEs in the core of the plasma was observed. On the contrary, enhanced losses were not observed if the TAEs localized at different radial locations had different frequencies and toroidal mode numbers. Numerical modeling indicates that, in the first set of experiments, the enhanced losses were caused by a combined effect of the TAEs localized at different radial locations. The TAEs localized in the plasma core transported energetic ions from the core to outer regions of the plasma. Then, the TAEs localized in outer regions of the plasma interacted with these ions just transported by the core-localized TAEs causing a further radial displacement of the ions to the plasma edge. This process eventually ends up causing the loss of the resonant ions. In the second set of experiments, it was found that TAEs localized in the plasma core and in outer regions did not interact with the same ions and so no enhanced losses were measured. Sheared profiles of the safety factor combined with flat mass density profiles lead to larger differences on the frequencies of the TAEs localized at different radial locations, eventually avoiding loss of energetic ions through the described mechanism.
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| 35. |
- Neverov, V. S., et al.
(författare)
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Determination of isotope ratio in the divertor of JET-ILW by high-resolution H alpha spectroscopy : H-D experiment and implications for D-T experiment
- 2019
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Ingår i: Nuclear Fusion. - : Institute of Physics Publishing (IOPP). - 0029-5515 .- 1741-4326. ; 59:4
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Tidskriftsartikel (refereegranskat)abstract
- The data of the H alpha high-resolution spectroscopy, collected on the multiple lines of sight, which cover the entire divertor space in poloidal cross-section, during the recent hydrogen-deuterium experiments in JET-ILW (ITER-like wall), are processed. A strong spatial inhomogeneity of the hydrogen concentration, H/(H + D), in divertor is found in many pulses. Namely, the H/(H + D) ratio may be lower in the inner divertor than that in the outer divertor by the values of 0.15-0.35, depending on the conditions of gas puffing and plasma heating. This effect suggests the necessity of spatially-resolved measurements of isotope ratio in the divertor in the upcoming deuterium-tritium experiments. Also, separation of the overlapped T alpha and D alpha spectral lines is shown to be a challenging task especially when the local Doppler-broadened (Gaussian) line shapes are noticeably distorted by the net inward flux of fast non-Maxwellian neutral atoms. We use the respective, formerly developed model of an asymmetric spectral line shape, while analysing the data of the first deuterium-tritium experiment in JET-C (carbon wall), and test the model via comparing the isotope ratio results with another diagnostic's measurements. This model is shown to increase the accuracy of tritium concentration measurements in the divertor.
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| 36. |
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| 37. |
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| 38. |
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| 39. |
- Pamela, S., et al.
(författare)
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A wall-aligned grid generator for non-linear simulations of MHD instabilities in tokamak plasmas
- 2019
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Ingår i: Computer Physics Communications. - : Elsevier. - 0010-4655 .- 1879-2944. ; 243, s. 41-50
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Tidskriftsartikel (refereegranskat)abstract
- Block-structured mesh generation techniques have been well addressed in the CFD community for automobile and aerospace studies, and their applicability to magnetic fusion is highly relevant, due to the complexity of the plasma-facing wall structures inside a tokamak device. Typically applied to non-linear simulations of MHD instabilities relevant to magnetically confined fusion, the JOREK code was originally developed with a 2D grid composed of isoparametric bi-cubic Bezier finite elements, that are aligned to the magnetic equilibrium of tokamak plasmas (the third dimension being represented by Fourier harmonics). To improve the applicability of these simulations, the grid-generator has been generalised to provide a robust extension method, using a block-structured mesh approach, which allows the simulations of arbitrary domains of tokamak vacuum vessels. Such boundary-aligned grids require the adaptation of boundary conditions along the edge of the new domain. Demonstrative non-linear simulations of plasma edge instabilities are presented to validate the robustness of the new grid, and future potential physics applications for tokamak plasmas are discussed. The methods presented here may be of interest to the wider community, beyond tokamak physics, wherever imposing arbitrary boundaries to quadrilateral finite elements is required.
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| 40. |
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