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  • 1.
    Bhatti, Muhammad Khurram
    et al.
    Informat Technol Univ, Embedded Comp Lab, 346-B Ferozpur Rd, Lahore, Pakistan..
    Oz, Isil
    Izmir Inst Technol, Comp Engn Dept, Izmir, Turkey..
    Amin, Sarah
    Informat Technol Univ, Embedded Comp Lab, 346-B Ferozpur Rd, Lahore, Pakistan..
    Mushtaq, Maria
    Informat Technol Univ, Embedded Comp Lab, 346-B Ferozpur Rd, Lahore, Pakistan..
    Farooq, Umer
    Dhofar Univ, Dept Elect & Comp Engn, Salalah 211, Oman..
    Popov, Konstantin
    SICS, Isafjordsgatan 22, S-16429 Kista, Sweden..
    Brorsson, Mats
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics. , S-16429 Kista, Sweden..
    Locality-aware task scheduling for homogeneous parallel computing systems2018In: Computing, ISSN 0010-485X, E-ISSN 1436-5057, Vol. 100, no 6, p. 557-595Article in journal (Refereed)
    Abstract [en]

    In systems with complex many-core cache hierarchy, exploiting data locality can significantly reduce execution time and energy consumption of parallel applications. Locality can be exploited at various hardware and software layers. For instance, by implementing private and shared caches in a multi-level fashion, recent hardware designs are already optimised for locality. However, this would all be useless if the software scheduling does not cast the execution in a manner that promotes locality available in the programs themselves. Since programs for parallel systems consist of tasks executed simultaneously, task scheduling becomes crucial for the performance in multi-level cache architectures. This paper presents a heuristic algorithm for homogeneous multi-core systems called locality-aware task scheduling (LeTS). The LeTS heuristic is a work-conserving algorithm that takes into account both locality and load balancing in order to reduce the execution time of target applications. The working principle of LeTS is based on two distinctive phases, namely; working task group formation phase (WTG-FP) and working task group ordering phase (WTG-OP). The WTG-FP forms groups of tasks in order to capture data reuse across tasks while the WTG-OP determines an optimal order of execution for task groups that minimizes the reuse distance of shared data between tasks. We have performed experiments using randomly generated task graphs by varying three major performance parameters, namely: (1) communication to computation ratio (CCR) between 0.1 and 1.0, (2) application size, i.e., task graphs comprising of 50-, 100-, and 300-tasks per graph, and (3) number of cores with 2-, 4-, 8-, and 16-cores execution scenarios. We have also performed experiments using selected real-world applications. The LeTS heuristic reduces overall execution time of applications by exploiting inter-task data locality. Results show that LeTS outperforms state-of-the-art algorithms in amortizing inter-task communication cost.

  • 2.
    Brunsell, Per R.
    et al.
    KTH, Superseded Departments (pre-2005), Alfvén Laboratory.
    Bergsåker, Henric
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Brzozowski, Jerzy
    Cecconello, Marco
    Drake, James R.
    Malmberg, Jenny-Ann
    Scheffel, Jan
    KTH, School of Electrical Engineering and Computer Science (EECS).
    Schnack, Dalton
    Mode Dynamics and Confinement in the Reversed-field Pinch2000In: 18th IAEA Fusion Energy Conference in Sorrento, Italy, 4-10 Oct. 2000. Paper IAEA-CN-77/EXP3/14, 2000Conference paper (Refereed)
    Abstract [en]

    Tearing mode dynamics and toroidal plasma flow in the RFP has been experimentally studied in the Extrap T2 device. A toroidally localised, stationary magnetic field perturbation, the ``slinky mode'' is formed in nearly all discharges. There is a tendency of increased phase alignment of different toroidal Fourier modes, resulting in higher localised mode amplitudes, with higher magnetic fluctuation level. The fluctuation level increases slightly with increasing plasma current and plasma density. The toroidal plasma flow velocity and the ion temperature has been measured with Doppler spectroscopy. Both the toroidal plasma velocity and the ion temperature clearly increase with I/N. Initial, preliminary experimental results obtained very recently after a complete change of the Extrap T2 front-end system (first wall, shell, TF coil), show that an operational window with mode rotation most likely exists in the rebuilt device, in contrast to the earlier case discussed above. A numerical code DEBSP has been developed to simulate the behaviour of RFP confinement in realistic geometry, including essential transport physics. Resulting scaling laws are presented and compared with results from Extrap T2 and other RFP experiments.

  • 3.
    Dumont, R. J.
    et al.
    CEA, IRFM, F-13108 St Paul Les Durance, France..
    Mailloux, J.
    Culham Sci Ctr, CCFE, Abingdon OX14 3DB, Oxon, England..
    Aslanyan, V
    MIT, PSFC, 175 Albany St, Cambridge, MA 02039 USA..
    Baruzzo, M.
    Consorzio RFX, Corso Stati Uniti 4, I-35127 Padua, Italy..
    Challis, C. D.
    Culham Sci Ctr, CCFE, Abingdon OX14 3DB, Oxon, England..
    Coffey, I
    Queens Univ, Dept Pure & Appl Phys, Belfast BT7 1NN, Antrim, North Ireland..
    Czarnecka, A.
    Inst Plasma Phys & Laser Microfus, Hery St 23, PL-00908 Warsaw, Poland..
    Delabie, E.
    Oak Ridge Natl Lab, Oak Ridge, TN USA..
    Eriksson, J.
    Uppsala Univ, Dept Phys & Astron, SE-75119 Uppsala, Sweden..
    Faustin, J.
    Ecole Polytech Fed Lausanne, SPC, CH-1015 Lausanne, Switzerland..
    Ferreira, J.
    Univ Lisbon, IST, Inst Plasmas & Fusao Nucl, Lisbon, Portugal..
    Fitzgerald, M.
    Culham Sci Ctr, CCFE, Abingdon OX14 3DB, Oxon, England..
    Garcia, J.
    CEA, IRFM, F-13108 St Paul Les Durance, France..
    Giacomelli, L.
    Univ Milano Bicocca, Piazza Sci 3, I-20126 Milan, Italy..
    Giroud, C.
    Culham Sci Ctr, CCFE, Abingdon OX14 3DB, Oxon, England..
    Hawkes, N.
    Culham Sci Ctr, CCFE, Abingdon OX14 3DB, Oxon, England..
    Jacquet, Ph
    Culham Sci Ctr, CCFE, Abingdon OX14 3DB, Oxon, England..
    Joffrin, E.
    CEA, IRFM, F-13108 St Paul Les Durance, France..
    Johnson, Thomas
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Keeling, D.
    Culham Sci Ctr, CCFE, Abingdon OX14 3DB, Oxon, England..
    King, D.
    Culham Sci Ctr, CCFE, Abingdon OX14 3DB, Oxon, England..
    Kiptily, V
    Culham Sci Ctr, CCFE, Abingdon OX14 3DB, Oxon, England..
    Lomanowski, B.
    Aalto Univ, POB 14100, FIN-00076 Aalto, Finland..
    Lerche, E.
    Ass EUROFUS Belgian State, LPP ERM KMS, TEC Partner, Brussels, Belgium..
    Mantsinen, M.
    Barcelona Supercomp Ctr, Barcelona, Spain.;ICREA, Barcelona, Spain..
    Meneses, L.
    Univ Lisbon, IST, Inst Plasmas & Fusao Nucl, Lisbon, Portugal..
    Menmuir, S.
    Culham Sci Ctr, CCFE, Abingdon OX14 3DB, Oxon, England..
    McClements, K.
    Culham Sci Ctr, CCFE, Abingdon OX14 3DB, Oxon, England..
    Moradi, S.
    Culham Sci Ctr, CCFE, Abingdon OX14 3DB, Oxon, England..
    Nabais, F.
    Univ Lisbon, IST, Inst Plasmas & Fusao Nucl, Lisbon, Portugal..
    Nocente, M.
    Univ Milano Bicocca, Piazza Sci 3, I-20126 Milan, Italy..
    Patel, A.
    Culham Sci Ctr, CCFE, Abingdon OX14 3DB, Oxon, England..
    Patten, H.
    Ecole Polytech Fed Lausanne, SPC, CH-1015 Lausanne, Switzerland..
    Puglia, P.
    Ecole Polytech Fed Lausanne, SPC, CH-1015 Lausanne, Switzerland..
    Scannell, R.
    Culham Sci Ctr, CCFE, Abingdon OX14 3DB, Oxon, England..
    Sharapov, S.
    Culham Sci Ctr, CCFE, Abingdon OX14 3DB, Oxon, England..
    Solano, E. R.
    CIEMAT, Lab Nacl Fus, Madrid, Spain..
    Tsalas, M.
    FOM Inst DIFFER, NL-3430 BE Nieuwegein, Netherlands.;ITER Org, Route Vinon Sur Verdon, F-13067 St Paul Les Durance, France..
    Vallejos, Pablo
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Weisen, H.
    Ecole Polytech Fed Lausanne, SPC, CH-1015 Lausanne, Switzerland..
    Scenario development for the observation of alpha-driven instabilities in JET DT plasmas2018In: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 58, no 8, article id 082005Article in journal (Refereed)
    Abstract [en]

    In DT plasmas, toroidal Alfven eigenmodes (TAEs) can be made unstable by the alpha particles resulting from fusion reactions, and may induce a significant redistribution of fast ions. Recent experiments have been conducted in JET deuterium plasmas in order to prepare scenarios aimed at observing alpha-driven TAEs in a future JET DT campaign. Discharges at low density, large core temperatures associated with the presence of internal transport barriers and characterised by good energetic ion confinement have been performed. ICRH has been used in the hydrogen minority heating regime to probe the TAE stability. The consequent presence of MeV ions has resulted in the observation of TAEs in many instances. The impact of several key parameters on TAE stability could therefore be studied experimentally. Modeling taking into account NBI and ICRH fast ions shows good agreement with the measured neutron rates, and has allowed predictions for DT plasmas to be performed.

  • 4.
    Fazinic, Stjepko
    et al.
    Rudjer Boskovic Inst, Bijenicka 54, Zagreb 10000, Croatia..
    Tadic, Tonic
    Rudjer Boskovic Inst, Bijenicka 54, Zagreb 10000, Croatia..
    Vuksic, Marin
    Rudjer Boskovic Inst, Bijenicka 54, Zagreb 10000, Croatia..
    Rubel, Marek
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Petersson, Per
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Fortuna-Zalesna, Elibieta
    Warsaw Univ Technol, Fac Mat Sci & Technol, Woloska 141, PL-02507 Warsaw, Poland..
    Widdowson, Anna
    Culham Sci Ctr, Culham Ctr Fus Energy, Abingdon OX14 3DB, Oxon, England..
    Ion Microbeam Analyses of Dust Particles and Codeposits from JET with the ITER-Like Wall2018In: Analytical Chemistry, ISSN 0003-2700, E-ISSN 1520-6882, Vol. 90, no 9, p. 5744-5752Article in journal (Refereed)
    Abstract [en]

    Generation of metal dust in the JET tokamak with the ITER-like wall (ILW) is a topic of vital interest to next-step fusion devices because of safety issues with plasma operation. Simultaneous Nuclear Reaction Analysis (NRA) and Particle Induced X-ray Emission (PIXE) with a focused four MeV He-3 microbeam was used to determine the composition of dust particles related to the JET operation with the ILW. The focus was on "Be-rich particles" collected from the deposition zone on the inner divertor tile. The particles found are composed of a mix of codeposited species up to 120 m in size with a thickness of 30-40 mu m, The main constituents are D from the fusion fuel, Be and W from the main plasma-facing components, and Ni and Cr from the Inconel grills of the antennas for auxiliary plasma heating. Elemental concentrations were estimated by iterative NRA-PIXE analysis. Two types of dust particles were found: (i) larger Be-rich particles with Be concentrations above 90 at% with a deuterium presence of up to 3.4 at% and containing Ni (1-3 at%), Cr (0.4-0.8 at%), W (0.2-0.9 at%), Fe (0.3-0.6 at%), and Cu and Ti in lower concentrations and (ii) small particles rich in Al and/or Si that were in some cases accompanied by other elements, such as Fe, Cu, or Ti or W and Mo.

  • 5.
    Garcia Carrasco, Alvaro
    et al.
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Petersson, Per
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Rubel, Marek
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Widdowson, A.
    Fortuna-Zalesna, E.
    Jachmich, S.
    Brix, M.
    Marot, L.
    Plasma impact on diagnostic mirrors in JET2017In: NUCLEAR MATERIALS AND ENERGY, ISSN 2352-1791, Vol. 12, p. 506-512Article in journal (Refereed)
    Abstract [en]

    Metallic mirrors will be essential components of all optical systems for plasma diagnosis in ITER. This contribution provides a comprehensive account on plasma impact on diagnostic mirrors in JET with the ITER-Like Wall. Specimens from the First Mirror Test and the lithium-beam diagnostic have been studied by spectrophotometry, ion beam analysis and electron microscopy. Test mirrors made of molybdenum were retrieved from the main chamber and the divertor after exposure to the 2013-2014 experimental campaign. In the main chamber, only mirrors located at the entrance of the carrier lost reflectivity (Be deposition), while those located deeper in the carrier were only slightly affected. The performance of mirrors in the JET divertor was strongly degraded by deposition of beryllium, tungsten and other species. Mirrors from the lithium-beam diagnostic have been studied for the first time. Gold coatings were severely damaged by intense arcing. As a consequence, material mixing of the gold layer with the stainless steel substrate occurred. Total reflectivity dropped from over 90% to less than 60%, i.e. to the level typical for stainless steel.

  • 6.
    Garcia Carrasco, Alvaro
    et al.
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Petersson, Per
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Schwarz-Selinger, T.
    Wauters, T.
    Douai, D.
    Bobkov, V.
    Cavazzana, R.
    Krieger, K.
    Lyssoivan, A.
    Moeller, S.
    Spolaore, M.
    Rohde, V.
    Rubel, M.
    Investigation of probe surfaces after ion cyclotron wall conditioning in ASDEX upgrade2017In: NUCLEAR MATERIALS AND ENERGY, ISSN 2352-1791, Vol. 12, p. 733-735Article in journal (Refereed)
    Abstract [en]

    For the first time, material analysis techniques have been applied to study the effect of ion cyclotron wall conditioning (ICWC) on probe surfaces in a metal-wall machine. ICWC is a technique envisaged to contribute to the removal of fuel and impurities from the first wall of ITER. The objective of this work was to assess impurity migration under ICWC operation. Tungsten probes were exposed in ASDEX Upgrade to discharges in helium. After wall conditioning, the probes were covered with a co-deposited layer containing D, B, C, N, O and relatively high amount of He. The concentration ratio He/C+B was 0.7. The formation of the co-deposited layer indicates that a fraction of the impurities desorbed from the wall under ICWC operation are transported by plasma and deposited away from their original location.

  • 7.
    Hoelzl, M.
    et al.
    Max Planck Inst Plasma Phys, Boltzmannstr 2, D-85748 Garching, Germany..
    Huijsmans, G. T. A.
    CEA, IRFM, St Paul Les Durance, France.;Eindhoven Univ Technol, Eindhoven, Netherlands..
    Orain, F.
    Max Planck Inst Plasma Phys, Boltzmannstr 2, D-85748 Garching, Germany..
    Artola, F. J.
    Aix Marseille Univ, CNRS, Marseille 20, France..
    Pamela, S.
    Culham Sci Ctr, CCFE, Abingdon, Oxon, England..
    Becoulet, M.
    CEA, IRFM, St Paul Les Durance, France..
    van Vugt, D.
    Eindhoven Univ Technol, Eindhoven, Netherlands..
    Liu, F.
    CEA, IRFM, St Paul Les Durance, France.;Univ Cote dAzur, Lab JA Dieudonne, CNRS, UMR 7351,UNS, Nice 02, France..
    Futatani, S.
    Barcelona Supercomp Ctr, Barcelona, Spain..
    Lessig, A.
    Max Planck Inst Plasma Phys, Boltzmannstr 2, D-85748 Garching, Germany..
    Wolfrum, E.
    Max Planck Inst Plasma Phys, Boltzmannstr 2, D-85748 Garching, Germany..
    Mink, F.
    Max Planck Inst Plasma Phys, Boltzmannstr 2, D-85748 Garching, Germany..
    Trier, E.
    Max Planck Inst Plasma Phys, Boltzmannstr 2, D-85748 Garching, Germany..
    Dunne, M.
    Max Planck Inst Plasma Phys, Boltzmannstr 2, D-85748 Garching, Germany..
    Viezzer, E.
    Max Planck Inst Plasma Phys, Boltzmannstr 2, D-85748 Garching, Germany..
    Eich, T.
    Max Planck Inst Plasma Phys, Boltzmannstr 2, D-85748 Garching, Germany..
    Vanovac, B.
    Dutch Inst Fundamental Energy Res, DIFFER, Eindhoven, Netherlands..
    Frassinetti, Lorenzo
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Guenter, S.
    Max Planck Inst Plasma Phys, Boltzmannstr 2, D-85748 Garching, Germany..
    Lackner, K.
    Max Planck Inst Plasma Phys, Boltzmannstr 2, D-85748 Garching, Germany..
    Krebs, I.
    Princeton Plasma Phys Lab, POB 451, Princeton, NJ 08543 USA..
    Insights into type-I edge localized modes and edge localized mode control from JOREK non-linear magneto-hydrodynamic simulations2018In: Contributions to Plasma Physics, ISSN 0863-1042, E-ISSN 1521-3986, Vol. 58, no 6-8, p. 518-528Article in journal (Refereed)
    Abstract [en]

    Edge localized modes (ELMs) are repetitive instabilities driven by the large pressure gradients and current densities in the edge of H-mode plasmas. Type-I ELMs lead to a fast collapse of the H-mode pedestal within several hundred microseconds to a few milliseconds. Localized transient heat fluxes to divertor targets are expected to exceed tolerable limits for ITER, requiring advanced insights into ELM physics and applicable mitigation methods. This paper describes how non-linear magneto-hydrodynamic (MHD) simulations can contribute to this effort. The JOREK code is introduced, which allows the study of large-scale plasma instabilities in tokamak X-point plasmas covering the main plasma, the scrape-off layer, and the divertor region with its finite element grid. We review key physics relevant for type-I ELMs and show to what extent JOREK simulations agree with experiments and help reveal the underlying mechanisms. Simulations and experimental findings are compared in many respects for type-I ELMs in ASDEX Upgrade. The role of plasma flows and non-linear mode coupling for the spatial and temporal structure of ELMs is emphasized, and the loss mechanisms are discussed. An overview of recent ELM-related research using JOREK is given, including ELM crashes, ELM-free regimes, ELM pacing by pellets and magnetic kicks, and mitigation or suppression by resonant magnetic perturbation coils (RMPs). Simulations of ELMs and ELM control methods agree in many respects with experimental observations from various tokamak experiments. On this basis, predictive simulations become more and more feasible. A brief outlook is given, showing the main priorities for further research in the field of ELM physics and further developments necessary.

  • 8. Koslowski, H.R.
    et al.
    Bhattacharyya, S.R.
    Hansen, P.
    Linsmeier, Ch.
    Rasinski, M.
    Ström, Petter
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Temperature-dependent in-situ LEIS measurement of W surface enrichment by 250 eV D sputtering of EUROFER2018In: Nuclear Materials and Energy, E-ISSN 2352-1791, Vol. 16, p. 181-190Article in journal (Refereed)
  • 9.
    Lindvall, Kristoffer
    et al.
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Scheffel, Jan
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    A time-spectral method for initial-value problems using a novel spatial subdomain scheme2018In: COGENT MATHEMATICS, ISSN 2331-1835, Vol. 5, no 1, article id 1529280Article in journal (Refereed)
    Abstract [en]

    We analyse a novel subdomain scheme for time-spectral solution of initial-value partial differential equations. In numerical modelling spectral methods are commonplace for spatially dependent systems, whereas finite difference schemes are typically applied for the temporal domain. The Generalized Weighted Residual Method (GWRM) is a fully spectral method that spectrally decomposes all specified domains, including the temporal domain, using multivariate Chebyshev polynomials. The Common Boundary-Condition method (CBC) here proposed is a spatial subdomain scheme for the GWRM. It solves the physical equations independently from the global connection of subdomains in order to reduce the total number of modes. Thus, it is a condensation procedure in the spatial domain that allows for a simultaneous global temporal solution. It is here evaluated against the finite difference methods of Crank-Nicolson and Lax-Wendroff for two example linear PDEs. The CBC-GWRM is also applied to the linearised ideal magnetohydrodynamic (MHD) equations for a screw pinch equilibrium. The growth rate of the most unstable mode was efficiently computed with an error <0.1%.

  • 10. Louche, F.
    et al.
    Wauters, T.
    Ragona, R.
    Moeller, S.
    Durodie, F.
    Litnovsky, A.
    Lyssoivan, A.
    Messiaen, A.
    Ongena, J.
    Petersson, Per
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Rubel, Marek
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Brezinsek, S.
    Linsmeier, Ch.
    Van Schoor, M.
    Design of an ICRF system for plasma-wall interactions and RF plasma production studies on TOMAS2017In: Fusion engineering and design, ISSN 0920-3796, E-ISSN 1873-7196, Vol. 123, p. 317-320Article in journal (Refereed)
    Abstract [en]

    Ion cyclotron wall conditioning (ICWC) is being developed for ITER and W7-X as a baseline conditioning technique in which the ion cyclotron heating and current drive system will be employed to produce and sustain the currentless conditioning plasma. The TOMAS project (TOroidal MAgnetized System, operated at the FZ-juelich, Germany) proposes to explore several key aspects of ICWC. For this purpose we have designed an ICRF system made of a single strap antenna within a metallic box, connected to a feeding port and a pre-matching system. We discuss the design work of the antenna system with the help of the commercial electromagnetic software CST Microwave Studio (R). The simulation results for a given geometry provide input impedance matrices for the two-port system. These matrices are afterwards inserted into various circuit models to assess the accessibility of the required frequency range. The sensitivity of the matching system to uncertainties on plasma loading and capacitance values is notably addressed. With a choice of three variable capacitors we show that the system can cope with such uncertainties. We also demonstrate that the system can cope as well with the high reflected power levels during the short breakdown phase of the RF discharge, but at the cost of a significantly reduced coupled power.

  • 11.
    Otsuka, T.
    et al.
    Kindai Univ, 3-4-1 Kowakae, Higashiosaka, Osaka 5778502, Japan..
    Masuzaki, S.
    Natl Inst Fus Sci, 322-6 Oroshi Cho, Toki, Gifu 5095292, Japan..
    Ashikawa, N.
    Natl Inst Fus Sci, 322-6 Oroshi Cho, Toki, Gifu 5095292, Japan..
    Hatano, Y.
    Univ Toyama, Gofuku 3190, Toyama 9308555, Japan..
    Asakura, Y.
    Natl Inst Quantum, Aomori 0393212, Japan.;Natl Inst Radiol Sci & Technol, Aomori 0393212, Japan..
    Suzuki, Tatsuya
    Natl Inst Quantum, Aomori 0393212, Japan.;Natl Inst Radiol Sci & Technol, Aomori 0393212, Japan..
    Suzuki, Takumi
    Natl Inst Quantum, Aomori 0393212, Japan.;Natl Inst Radiol Sci & Technol, Aomori 0393212, Japan..
    Isobe, K.
    Natl Inst Quantum, Aomori 0393212, Japan.;Natl Inst Radiol Sci & Technol, Aomori 0393212, Japan..
    Hayashi, T.
    Natl Inst Quantum, Aomori 0393212, Japan.;Natl Inst Radiol Sci & Technol, Aomori 0393212, Japan..
    Tokitani, M.
    Natl Inst Fus Sci, 322-6 Oroshi Cho, Toki, Gifu 5095292, Japan..
    Oya, Y.
    Shizuoka Univ, Shizuoka 4228529, Japan..
    Hamaguchi, D.
    Natl Inst Quantum, Aomori 0393212, Japan.;Natl Inst Radiol Sci & Technol, Aomori 0393212, Japan..
    Kurotaki, H.
    Natl Inst Quantum, Aomori 0393212, Japan.;Natl Inst Radiol Sci & Technol, Aomori 0393212, Japan..
    Sakamoto, R.
    Natl Inst Fus Sci, 322-6 Oroshi Cho, Toki, Gifu 5095292, Japan..
    Tanigawa, Hiroyasu
    Natl Inst Quantum, Aomori 0393212, Japan.;Natl Inst Radiol Sci & Technol, Aomori 0393212, Japan..
    Nakamichi, M.
    Natl Inst Quantum, Aomori 0393212, Japan.;Natl Inst Radiol Sci & Technol, Aomori 0393212, Japan..
    Widdowson, A.
    Culham Sci Ctr, Culham Ctr Fus Energy, Abingdon OX14 3DB, Oxon, England..
    Rubel, Marek
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Tritium retention characteristics in dust particles in JET with ITER-like wall2018In: Nuclear Materials and Energy, E-ISSN 2352-1791, Vol. 17, p. 279-283Article in journal (Refereed)
    Abstract [en]

    A tritium imaging plate technique (TIPT) in combination with an electron-probe microscopic analysis (EPMA) were applied to examine tritium (T) retention characteristics in individual dust particles collected in the Joint European Torus with the ITER-like Wall (JET-ILW) after the first campaign in 2011-2012. A lot of carbon pre-existing carbon deposits in the JET-C or released carbon particles from the remaining carbon-fiber components in the JET-ILW. Most of T was retained at the surface of and/or in the C-dominated dust particles. The retention in tungsten, beryllium and other metal-dominated dust particles is relatively lower by a factor of 10-100 in comparison with that in the Cdominated particles.

  • 12. Oya, Y.
    et al.
    Masuzaki, S.
    Tokitani, M.
    Azuma, K.
    Oyaidzu, M.
    Isobe, K.
    Asakura, N.
    Widdowson, A. M.
    Heinola, K.
    Jachmich, S.
    Rubel, Marek
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Contributors, JET
    Correlation of surface chemical states with hydrogen isotope retention in divertor tiles of JET with ITER-Like Wall2018In: Fusion engineering and design, ISSN 0920-3796, E-ISSN 1873-7196, Vol. 132, p. 24-28Article in journal (Refereed)
    Abstract [en]

    To understand the fuel retention mechanism correlation of surface chemical states and hydrogen isotope retention behavior determined by XPS (X-ray photoelectron spectroscopy) and TDS (Thermal desorption spectroscopy), respectively, for JET ITER-Like Wall samples from operational period 2011–2012 were investigated. It was found that the deposition layer was formed on the upper part of the inner vertical divertor area. At the inner plasma strike point region, the original surface materials, W or Mo, were found, indicating to an erosion-dominated region, but deposition of impurities was also found. Higher heat load would induce the formation of metal carbide. At the outer horizontal divertor tile, mixed material layer was formed with iron as an impurity. TDS showed the H and D desorption behavior and the major D desorption temperature for the upper part of the inner vertical tile was located at 370 °C and 530 °C. At the strike point region, the D desorption temperature was clearly shifted toward higher release temperatures, indicating the stabilization of D trapping by higher heat load.

  • 13.
    Scheffel, Jan
    et al.
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Lindvall, Kristoffer
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    SIR—An efficient solver for systems of equations2018In: Software Quality Professional, ISSN 1522-0540, E-ISSN 2352-7110, Vol. 7, p. 59-62Article in journal (Refereed)
    Abstract [en]

    The Semi-Implicit Root solver (SIR) is an iterative method for globally convergent solution of systems of nonlinear equations. We here present MATLAB and MAPLE codes for SIR, that can be easily implemented in any application where linear or nonlinear systems of equations need be solved efficiently. The codes employ recently developed efficient sparse matrix algorithms and improved numerical differentiation. SIR convergence is quasi-monotonous and approaches second order in the proximity of the real roots. Global convergence is usually superior to that of Newton's method, being a special case of the method. Furthermore the algorithm cannot land on local minima, as may be the case for Newton's method with line search. 

  • 14.
    Sheikh, U. A.
    et al.
    Ecole Polytech Fed Lausanne, SPC, CH-1015 Lausanne, Switzerland..
    Dunne, M.
    Max Planck Inst Plasma Phys, Garching, Germany..
    Frassinetti, Lorenzo
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Blanchard, P.
    Ecole Polytech Fed Lausanne, SPC, CH-1015 Lausanne, Switzerland..
    Duval, B. P.
    Ecole Polytech Fed Lausanne, SPC, CH-1015 Lausanne, Switzerland..
    Labit, B.
    Ecole Polytech Fed Lausanne, SPC, CH-1015 Lausanne, Switzerland..
    Merle, A.
    Ecole Polytech Fed Lausanne, SPC, CH-1015 Lausanne, Switzerland..
    Sauter, O.
    Ecole Polytech Fed Lausanne, SPC, CH-1015 Lausanne, Switzerland..
    Theiler, C.
    Ecole Polytech Fed Lausanne, SPC, CH-1015 Lausanne, Switzerland..
    Tsui, C.
    Ecole Polytech Fed Lausanne, SPC, CH-1015 Lausanne, Switzerland..
    Pedestal structure and energy confinement studies on TCV2019In: Plasma Physics and Controlled Fusion, ISSN 0741-3335, E-ISSN 1361-6587, Vol. 61, no 1, article id 014002Article in journal (Refereed)
    Abstract [en]

    High external gas injection rates are foreseen for future devices to reduce divertor heat loads and this can influence pedestal stability. Fusion yield has been estimated to vary as strongly as T-e,ped(2) so an understanding of the underlying pedestal physics in the presence of additional fuelling and seeding is required. To address this, a database scanning plasma triangularity, fuelling and nitrogen seeding rates in neutral beam (NBH) heated ELM-y H-mode plasmas was constructed on TCV. Low nitrogen seeding was observed to increase pedestal top pressure but all other gas injection rates led to a decrease. Lower triangularity discharges were found to be less sensitive to variations in gas injection rates. No clear trend was measured between plasma top P-e and stored energy which is attributed to the non-stiffness of core plasma pressure profiles. Peeling ballooning stability analysis put these discharges close to the ideal MHD stability boundary. A constant for D in the relation pedestal width w = D root beta(Ped)(theta), was not found. Experimentally inferred values of D were used in EPED1 simulations and gave good agreement for pedestal width. Pedestal height agreed well for high triangularity but was overestimated for low triangularity. IPED simulations showed that relative shifts in pedestal position were contributing significantly to the pedestal height and were able to reproduce the measured profiles more accurately.

  • 15.
    Stefániková, Estera
    et al.
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics. EUROfus Consortium, JET, Culham Sci Ctr, Abingdon OX14 3DB, Oxon, England..
    Frassinetti, Lorenzo
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics. EUROfus Consortium, JET, Culham Sci Ctr, Abingdon OX14 3DB, Oxon, England..
    Saarelma, S.
    EUROfus Consortium, JET, Culham Sci Ctr, Abingdon OX14 3DB, Oxon, England.;Culham Sci Ctr, CCFE, Abingdon OX14 3DB, Oxon, England..
    Loarte, A.
    EUROfus Consortium, JET, Culham Sci Ctr, Abingdon OX14 3DB, Oxon, England.;ITER Org, Route Vinon Sur Verdon, F-13067 St Paul Les Durance, France..
    Nunes, I.
    EUROfus Consortium, JET, Culham Sci Ctr, Abingdon OX14 3DB, Oxon, England.;Inst Plasmas & Fusao Nucl, IST, Lisbon, Portugal..
    Garzotti, L.
    EUROfus Consortium, JET, Culham Sci Ctr, Abingdon OX14 3DB, Oxon, England.;Culham Sci Ctr, CCFE, Abingdon OX14 3DB, Oxon, England..
    Lomas, P.
    EUROfus Consortium, JET, Culham Sci Ctr, Abingdon OX14 3DB, Oxon, England.;Culham Sci Ctr, CCFE, Abingdon OX14 3DB, Oxon, England..
    Rimini, F.
    EUROfus Consortium, JET, Culham Sci Ctr, Abingdon OX14 3DB, Oxon, England.;Culham Sci Ctr, CCFE, Abingdon OX14 3DB, Oxon, England..
    Drewelow, P.
    EUROfus Consortium, JET, Culham Sci Ctr, Abingdon OX14 3DB, Oxon, England.;Max Planck Inst Plasma Phys, Garching, Germany..
    Kruezi, U.
    EUROfus Consortium, JET, Culham Sci Ctr, Abingdon OX14 3DB, Oxon, England.;Culham Sci Ctr, CCFE, Abingdon OX14 3DB, Oxon, England..
    Lomanowski, B.
    EUROfus Consortium, JET, Culham Sci Ctr, Abingdon OX14 3DB, Oxon, England.;Ctr Adv Instrumentat, Dept Phys, Durham DH1 3LE, England..
    de la Luna, E.
    EUROfus Consortium, JET, Culham Sci Ctr, Abingdon OX14 3DB, Oxon, England.;Lab Nacl Fus CIEMAT, Madrid, Spain..
    Meneses, L.
    EUROfus Consortium, JET, Culham Sci Ctr, Abingdon OX14 3DB, Oxon, England.;Inst Plasmas & Fusao Nucl, IST, Lisbon, Portugal..
    Peterka, M.
    EUROfus Consortium, JET, Culham Sci Ctr, Abingdon OX14 3DB, Oxon, England.;Inst Plasma Phys AS CR, Prague, Czech Republic.;Charles Univ Prague, Fac Math & Phys, Prague, Czech Republic..
    Viola, B.
    EUROfus Consortium, JET, Culham Sci Ctr, Abingdon OX14 3DB, Oxon, England.;CR Frascati, ENEA, Via E Fermi 45, Rome, Italy..
    Giroud, C.
    EUROfus Consortium, JET, Culham Sci Ctr, Abingdon OX14 3DB, Oxon, England.;Culham Sci Ctr, CCFE, Abingdon OX14 3DB, Oxon, England..
    Maggi, C.
    EUROfus Consortium, JET, Culham Sci Ctr, Abingdon OX14 3DB, Oxon, England.;Culham Sci Ctr, CCFE, Abingdon OX14 3DB, Oxon, England..
    Effect of the relative shift between the electron density and temperature pedestal position on the pedestal stability in JET-ILW and comparison with JET-C2018In: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 58, no 5, article id 056010Article in journal (Refereed)
    Abstract [en]

    The electron temperature and density pedestals tend to vary in their relative radial positions, as observed in DIII-D (Beurskens et al 2011 Phys. Plasmas 18 056120) and ASDEX Upgrade (Dunne et al 2017 Plasma Phys. Control. Fusion 59 14017). This so-called relative shift has an impact on the pedestal magnetohydrodynamic (MHD) stability and hence on the pedestal height (Osborne et al 2015 Nucl. Fusion 55 063018). The present work studies the effect of the relative shift on pedestal stability of JET ITER-like wall (JET-ILW) baseline low triangularity (d) unseeded plasmas, and similar JET-C discharges. As shown in this paper, the increase of the pedestal relative shift is correlated with the reduction of the normalized pressure gradient, therefore playing a strong role in pedestal stability. Furthermore, JET-ILW tends to have a larger relative shift compared to JET carbon wall (JET-C), suggesting a possible role of the plasma facing materials in affecting the density profile location. Experimental results are then compared with stability analysis performed in terms of the peeling-ballooning model and with pedestal predictive model EUROPED (Saarelma et al 2017 Plasma Phys. Control. Fusion). Stability analysis is consistent with the experimental findings, showing an improvement of the pedestal stability, when the relative shift is reduced. This has been ascribed mainly to the increase of the edge bootstrap current, and to minor effects related to the increase of the pedestal pressure gradient and narrowing of the pedestal pressure width. Pedestal predictive model EUROPED shows a qualitative agreement with experiment, especially for low values of the relative shift.

  • 16.
    Ström, Petter
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Material characterization for magnetically confined fusion: Surface analysis and method development2019Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The dream of abundant clean energy has brought scientists and laypeople alike to ponder the possibilities of nuclear fusion since it was established as the energy source of the stars in 1939. Starting from the mid the 20th century, significant effort has been put into overcoming the technological challenges related to the construction of a power plant, but initial optimism has faded somewhat due to a notable absence of practical outcomes. Nevertheless, the research continues and progress is made slowly but surely.

    The present work deals with a small part of the fusion puzzle, namely the materials to be used in the first wall surrounding a magnetically confined plasma. Carbon, which has historically been considered as the most viable element for this role, has been ruled out due to issues with plasma-induced erosion, hydrocarbon formation and a buildup of thick deposited material layers on wall components. The latter two lead to an unacceptable accumulation of radioactive tritium, both in the deposited layers and in dust particles. A metal wall, which would alleviate these particular problems but increase the severity of others, is therefore envisioned for a future demonstration reactor.

    Three contributions to the overall research effort are made though this thesis. First, an increased understanding of plasma-induced erosion of so-called reduced activation ferritic-martensitic steels and preferential sputtering of light material components is provided. High-resolution ion beam analysis and microscopy methods are used to examine samples of such a steel after exposure to plasma under controlled circumstances. Model films consisting of a mixture of iron and tungsten deposited on silicon substrates are also studied as they constitute simpler systems where the effects of interest may be simulated. The knowledge obtained is necessary for an assessment of the possibility to use reduced activation steel as a plasma-facing material in specific regions of a reactor wall.

    The second contribution consists of reports on the composition of deposited material layers on wall components retrieved from the plasma confinement experiments JET and TEXTOR. These provide limited conclusions on the range and rate of material erosion, transport and deposition in two cases.

    Finally, a detection system for the ion beam technique elastic recoil detection analysis has been assembled, tested and put into operation. In addition to improving the quality of analyses performed on fusion-related materials, the system has become an established tool available for users of the 5 MV electrostatic pelletron accelerator at Uppsala University’s Tandem Laboratory.

  • 17.
    Ström, Petter
    et al.
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Petersson, P
    Rubel, M
    Fortuna-Zaleśna, E
    Widdowson, A
    Sergienko, G
    Analysis of deposited layers with deuterium and impurity elements on samples from the divertor of JET with ITER-like wallIn: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820Article in journal (Refereed)
    Abstract [en]

    Inconel-600 blocks and stainless steel covers for quartz microbalance crystals from remote corners in the JET-ILW divertor were studied with time-of-flight elastic recoil detection analysis and nuclear reaction analysis to obtain information about the areal densities and depth profiles of elements present in deposited material layers. Surface morphology and the composition of dust particles were examined with scanning electron microscopy and energy-dispersive X-ray spectroscopy. The analyzed components were present in JET during three ITER-like wall campaigns between 2010 and 2017. Deposited layers had a stratified structure, primarily made up of beryllium, carbon and oxygen with varying atomic fractions of deuterium, up to more than 20%. The range of carbon transport from the ribs of the divertor carrier was limited to a few centimeters, and carbon/deuterium co-deposition was indicated on the Inconel blocks. High atomic fractions of deuterium were also found in almost carbon-free layers on the quartz microbalance covers. Layer thicknesses up to more than 1 micrometer were indicated, but typical values were on the order of a few hundred nanometers. Chromium, iron and nickel fractions were less than or around 1% at layer surfaces while increasing close to the layer-substrate interface. The tungsten fraction depended on the proximity of the plasma strike point to the divertor corners. Particles of tungsten, molybdenum and copper with sizes less than or around 1 micrometer were found. Nitrogen, argon and neon were present after plasma edge cooling and disruption mitigation. Oxygen-18 was found on component surfaces after injection, indicating in-vessel oxidation. Compensation of elastic recoil detection data for detection efficiency and ion-induced release of deuterium during the measurement gave quantitative agreement with nuclear reaction analysis, which strengthens the validity of the results.

  • 18.
    Ström, Petter
    et al.
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Petersson, Per
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Arredondo Parra, R.
    Oberkofler, M.
    Schwarz-Selinger, T.
    Primetzhofer, D.
    Sputtering of polished EUROFER97 steel: Surface structure modification and enrichment with tungsten and tantalum2018In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 508, p. 139-146Article in journal (Refereed)
    Abstract [en]

    Surface structure modification and enrichment with tungsten and tantalum were measured for polished EUROFER97 samples after exposure to a deuterium ion beam. Time-of-flight medium energy ion scattering and time-of-flight elastic recoil detection analysis were implemented for measuring atomic composition profiles. Atomic force microscopy and optical microscopy were used to investigate surface morphology. The deuterium particle fluence was varied between 1021 D/m2 and 1024 D/m2, projectile energy was 200 eV/D and exposure temperatures up to 1050 K were applied. The average fraction of tungsten plus tantalum to total metal content in the 2 nm closest to the sample surface was increased from an initial 0.0046 to 0.12 for the sample exposed to the highest fluence at room temperature. The enrichment was accompanied by an increase in surface roughness of one order of magnitude and grain dependent erosion of the material. The appearance of protrusions with heights up to approximately 40 nm after ion beam exposure at room temperature was observed on individual grains. Samples exposed to 1023 D/m2 at temperatures of 900 K and 1050 K displayed recrystallization and cracking while changes to the total surface fraction of tungsten and tantalum were limited to less than a factor of two compared to the sample exposed to the same fluence at room temperature.

  • 19.
    Ström, Petter
    et al.
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Petersson, Per
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Hamberg, Mathias
    Surface oxide and roughness on test samples for the Ultra High Vacuum section of the European XFEL2018In: Vacuum, ISSN 0042-207X, E-ISSN 1879-2715, Vol. 149, p. 83-86Article in journal (Refereed)
    Abstract [en]

    The European X-ray Free Electron Laser has recently started with operation for users. An approximately 3 m long ultra high vacuum laser heater section is implemented to overcome possible electron bunch instabilities. We describe the process of determining the oxide layer thickness and surface roughness on test samples of the internal surface material in the laser heater vacuum chambers using elastic recoil detection analysis and optical surface profiling. The results are compared to specified values and show that surface roughness on the samples is larger than the requested maximum, with RMS deviations from a mean plane of up to 1.76 μm for 0.60 × 0.45 square millimeter scans. The maximum oxide layer thickness is 5.5 nm on non-electropolished surfaces assuming cuprous oxide with density 6.0 g per cubic centimeter and 4.0 nm on electropolished surfaces.

  • 20. Vizvary, Z.
    et al.
    Bourdel, B.
    Garcia Carrasco, Alvaro
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Lam, N.
    Leipold, F.
    Pitts, R. A.
    Reichle, R.
    Riccardo, V.
    Rubel, Marek
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    De Temmerman, G.
    Thompson, V.
    Widdowson, A.
    Engineering design and analysis of an ITER-like first mirror test assembly on JET2017In: Fusion engineering and design, ISSN 0920-3796, E-ISSN 1873-7196, Vol. 123, p. 1054-1057Article in journal (Refereed)
    Abstract [en]

    The ITER first mirrors are the components of optical diagnostic systems closest to the plasma. Deposition may build up on the surfaces of the mirror affecting their ability to fulfil their function. However, physics modelling of this layer growth is fraught with uncertainty. A new experiment is underway on JET, under contract to ITER, with primary objective to test if, under realistic plasma and wall material conditions and with ITER-like first mirror aperture geometry, deposits do grow on first mirrors. This paper describes the engineering design and analysis of this mirror test assembly. The assembly was installed in the 2014-15 shutdown and will be removed in the 2016-17 shutdown.

  • 21. Weckmann, Armin
    et al.
    Petersson, Per
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Kirschner, A.
    Wienhold, P.
    Brezinsek, S.
    Kreter, A.
    Pospieszczyk, A.
    Rubel, Marek
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Whole-machine material migration studies in the TEXTOR tokamak with molybdenum2017In: NUCLEAR MATERIALS AND ENERGY, ISSN 2352-1791, Vol. 12, p. 518-523Article in journal (Refereed)
    Abstract [en]

    MoF6 injection from a localised source into plasma edge in the TEXTOR tokamak was the last experiment before the final shut-down of the TEXTOR machine. During decommissioning all plasma-facing components (PFCs) became available for surface studies. Detailed mapping of Mo deposition was performed in order to determine its migration on global scale. The concentration of Mo on PFC decays exponentially with distance from the source. The decay length is of the order of 0.1 m on the main PFC and 1 m on the receded components. Also the decay lengths modelled with the ERO code are between 0.15-1.3 m, depending on the anomalous cross-field diffusion coefficient. The inner bumper limiter is found to be the major repository for Mo. Material balance measurements show that only up to 22% of the injected Mo was detected on all the PFCs thus indicating that a large fraction of injected Mo may have been pumped out before being deposited.

  • 22.
    Weckmann, Armin
    et al.
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Petersson, Per
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Rubel, Marek
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Ström, Petter
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Kurki-Suonio, T.
    Aalto Univ, Dept Appl Phys, Aalto 00076, Finland..
    Sarkimaki, K.
    Aalto Univ, Dept Appl Phys, Aalto 00076, Finland..
    Kirschner, A.
    Forschungszentrum Julich, Inst Energie & Klimaforsch Plasmaphys, D-52425 Julich, Germany..
    Kreter, A.
    Forschungszentrum Julich, Inst Energie & Klimaforsch Plasmaphys, D-52425 Julich, Germany..
    Brezinsek, S.
    Forschungszentrum Julich, Inst Energie & Klimaforsch Plasmaphys, D-52425 Julich, Germany..
    Romazanov, J.
    Forschungszentrum Julich, Inst Energie & Klimaforsch Plasmaphys, D-52425 Julich, Germany..
    Wienhold, P.
    Forschungszentrum Julich, Inst Energie & Klimaforsch Plasmaphys, D-52425 Julich, Germany..
    Pospieszczyk, A.
    Forschungszentrum Julich, Inst Energie & Klimaforsch Plasmaphys, D-52425 Julich, Germany..
    Hakola, A.
    VTT Tech Res Ctr Finland Ltd, Espoo 02044, Finland..
    Airila, M.
    VTT Tech Res Ctr Finland Ltd, Espoo 02044, Finland..
    Review on global migration, fuel retention and modelling after TEXTOR decommission2018In: NUCLEAR MATERIALS AND ENERGY, Vol. 17, p. 83-112Article, review/survey (Refereed)
    Abstract [en]

    Before decommissioning of the TEXTOR tokamak in 2013, the machine was conditioned with a comprehensive migration experiment where MoF6 and N-15(2) were injected on the very last operation day. Thereafter, all plasmafacing components (PFCs) were available for extensive studies of both local and global migration of impurities - Mo, W, Inconel alloy constituents, 15 N, F - and fuel retention studies. Measurements were performed on 140 limiter tiles out of 864 throughout the whole machine to map global transport. One fifth of the introduced molybdenum could be found. Wherever possible, the findings are compared to results obtained previously in other machines. This review incorporates both published and unpublished results from this TEXTOR study and combines findings with analytical methods as well as modelling results from two codes, ERO and ASCOT. The main findings are: Both local and global molybdenum transport can be explained by toroidal plasma flow and (sic) x (sic) drift. The suggested transport scheme for molybdenum holds also for other analysed species, namely tungsten from previous experiments and medium-Z metals (Cr-Cu) introduced on various occasions. Analytical interpretation of several deposition profile features is possible with basic geometrical and plasma physics considerations. These are deposition profiles on the collector probe, the lower part of the inner bumper limiter, the poloidal cross-section of the inner bumper limiter, and the poloidal limiter. Any deposition pattern found in this TEXTOR study, including fuel retention, has neither poloidal nor toroidal symmetry, which is often assumed when determining deposition profiles on global scale. Fuel retention is highly inhomogeneous due to local variation of plasma parameters - by auxiliary heating systems and impurity injection - and PFC temperature. Local modelling with ERO yields good qualitative agreement but too high local deposition efficiency. Global modelling with ASCOT shows that the radial electric field and source form have a high impact on global deposition patterns, while toroidal flow has little influence. Some of the experimental findings could be reproduced. Still, qualitative differences between simulated and experimental global deposition patterns remain. The review closes with lessons learnt during this extensive TEXTOR study which might be helpful for future scientific exploitation of other tokamaks to be decommissioned.

  • 23. Widdowson, A.
    et al.
    Alves, E.
    Baron-Wiechec, A.
    Barradas, N. P.
    Catarino, N.
    Coad, J. P.
    Corregidor, V.
    Garcia-Carrasco, A.
    Heinola, K.
    Koivuranta, S.
    Krat, S.
    Lahtinen, A.
    Likonen, J.
    Mayer, M.
    Petersson, Per
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Rubel, Marek
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Van Boxel, S.
    Overview of the JET ITER-like wall divertor2017In: NUCLEAR MATERIALS AND ENERGY, ISSN 2352-1791, Vol. 12, p. 499-505Article in journal (Refereed)
    Abstract [en]

    The work presented draws on new analysis of components removed following the second JET ITER-like wall campaign 2013-14 concentrating on the upper inner divertor, inner and outer divertor corners, lifetime issues relating to tungsten coatings on JET carbon fibre composite divertor tiles and dust/particulate generation. The results show that the upper inner divertor remains the region of highest deposition in the JET-ILW. Variations in plasma configurations between the first and second campaign have altered material migration to the corners of the inner and outer divertor. Net deposition is shown to be beneficial in the sense that it reduces W coating erosion, covers small areas of exposed carbon surfaces and even encapsulates particles.

  • 24. Widdowson, A.
    et al.
    Coad, J. P.
    Alves, E.
    Baron-Wiechec, A.
    Barradas, N. P.
    Catarino, N.
    Corregidor, V.
    Heinola, K.
    Krat, S.
    Likonen, J.
    Matthews, G. F.
    Mayer, M.
    Petersson, Per
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Rubel, Marek
    KTH.
    Impurity re-distribution in the corner regions of the JET divertor2017In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. T170, article id 014060Article in journal (Refereed)
    Abstract [en]

    The International Thermonuclear Experimental Reactor (ITER) will use a mixture of deuterium (D) and tritium (T) as the fuel to generate power. Since T is both radioactive and expensive the Joint European Torus (JET) has been at the forefront of research to discover how much T is used and where it may be retained within the main reaction chamber. Until the year 2010 the JET plasma facing components were constructed of carbon fibre composites. During the JET carbon (C) phases impurities accumulated at the corners of the divertor located towards the bottom of the chamber in regions shadowed from the plasma where they are very difficult to reach and remove. This build-up of C and the associated H-isotope (including T) retention were of particular concern for future fusion reactors therefore, in 2010 JET changed the wall protection to (mainly) Be and the divertor to tungsten (W)-the JET ITER-like wall (ILW)-the choice of materials for ITER. This paper reveals that with the JET ILW impurities are still accumulating in the shadowed regions, with Be being the majority element, though the overall quantities are very much reduced from those in the C phases. Material will be transported into the shadowed regions principally when the plasma strike points are on the corner tiles, but particles typically have about a 75% probability of reflection from line-of sight surfaces, and multiple reflection/scattering results in deposition over all surfaces.

  • 25.
    Wiesen, S.
    et al.
    Forschungszentrum Julich, Inst Energie & Klimaforsch Plasmaphys, D-52425 Julich, Germany..
    Brezinsek, S.
    Forschungszentrum Julich, Inst Energie & Klimaforsch Plasmaphys, D-52425 Julich, Germany..
    Bonnin, X.
    ITER Org, Route Vinon Sur Verdon,CS90 046, F-13067 St Paul Les Durance, France..
    Delabie, E.
    Oak Ridge Natl Lab, POB 2009, Oak Ridge, TN 37831 USA..
    Frassinetti, Lorenzo
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Groth, M.
    Aalto Univ, Espoo, Finland..
    Guillemaut, C.
    Univ Lisbon, Inst Plasmas & Fusao Nucl, IST, P-1049001 Lisbon, Portugal..
    Harrison, J.
    Culham Sci Ctr, CCFE, Abingdon OX14 3DB, Oxon, England..
    Harting, D.
    Culham Sci Ctr, CCFE, Abingdon OX14 3DB, Oxon, England..
    Henderson, S.
    Culham Sci Ctr, CCFE, Abingdon OX14 3DB, Oxon, England..
    Huber, A.
    Forschungszentrum Julich, Inst Energie & Klimaforsch Plasmaphys, D-52425 Julich, Germany..
    Kruezi, U.
    ITER Org, Route Vinon Sur Verdon,CS90 046, F-13067 St Paul Les Durance, France..
    Pitts, R. A.
    ITER Org, Route Vinon Sur Verdon,CS90 046, F-13067 St Paul Les Durance, France..
    Wischmeier, M.
    Max Planck Inst Plasma Phys, D-85748 Garching, Germany..
    On the role of finite grid extent in SOLPS-ITER edge plasma simulations for JET H-mode discharges with metallic wall2018In: Nuclear Materials and Energy, E-ISSN 2352-1791, Vol. 17, p. 174-181Article in journal (Refereed)
    Abstract [en]

    The impact of the finite grid size in SOLPS-ITER edge plasma simulations is assessed for JET H-mode discharges with a metal wall. For a semi-horizontal divertor configuration it is shown that the separatrix density is at least 30% higher when a narrow scrape-off layer (SOL) grid width is chosen in SOLPS-ITER compared to the case for which the SOL grid width is maximised. The density increase is caused by kinetic neutrals being not confined inside the divertor region because of the reduced extent of the plasma grid. In this case, an enhanced level of reflections of energetic neutrals at the low-field side (LFS) metal divertor wall is observed. This leads to a shift of the ionisation source further upstream which must be accounted for as a numerical artefact. An overestimate in the cooling at the divertor entrance is observed in this case, identified by a reduced heat flux decay parameters lambda(div)(q). Otherwise and further upstream the mid-plane heat decay length lambda(q) parameter is not affected by any change in divertor dissipation. This confirms the assumptions made for the ITER divertor design studies, i.e. that lambda(q) upstream is essentially set by the assumptions for the ratio radial to parallel heat conductivity. It is also shown that even for attached conditions the decay length relations lambda(ne)>lambda(Te)>lambda(q) hold in the near-SOL upstream. Thus for interpretative edge plasma simulations one must take the (experimental) value of lambda(ne) into account, rather than lambda(q), as the former actually defines the required minimum upstream SOL grid extent.

  • 26.
    Zhou, Yushan
    et al.
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Bergsåker, Henric
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Bykov, Igor
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Petersson, Per
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Possnert, G.
    Likonen, J.
    Pettersson, J.
    Koivuranta, S.
    Widdowson, A. M.
    Microanalysis of deposited layers in the inner divertor of JET with ITER-like wall2017In: NUCLEAR MATERIALS AND ENERGY, ISSN 2352-1791, Vol. 12, p. 412-417Article in journal (Refereed)
    Abstract [en]

    In JET with ITER-like wall, beryllium eroded in the main chamber is transported to the divertor and deposited mainly at the horizontal surfaces of tiles 1 and 0 (high field gap closure, HFGC). These surfaces are tungsten coated carbon fibre composite (CFC). Surface sampleswere collected following the plasma operations in 2011-2012 and 2013-2014 respectively. The surfaces, as well as polished cross sections of the deposited layers at the surfaces have been studied with micro ion beam analysis methods (mu-IBA). Deposition of Beand other impurities, and retention of D is microscopically inhomogeneous. Impurities and trapped deuterium accumulate preferentially in cracks, pits and depressed regions, and at the sides of large pits in the substrate (e.g. arc tracks where the W coating has been removed). With careful overlaying of mu-NRA elemental maps with optical microscopy images, it is possible to separate surface roughness effects from depth profiles at microscopically flat surface regions.

1 - 26 of 26
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