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  • 251. Hakola, A.
    et al.
    Brezinsek, S.
    Douai, D.
    Balden, M.
    Bobkov, V.
    Carralero, D.
    Greuner, H.
    Elgeti, S.
    Kallenbach, A.
    Krieger, K.
    Meisl, G.
    Oberkofler, M.
    Rohde, V.
    Schneider, P.
    Schwarz-Selinger, T.
    Lahtinen, A.
    De Temmerman, G.
    Caniello, R.
    Ghezzi, F.
    Wauters, T.
    Garcia Carrasco, Alvaro
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Petersson, Per
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Radovic, I. Bogdanovic
    Siketic, Z.
    Plasma-wall interaction studies in the full-W ASDEX upgrade during helium plasma discharges2017In: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 57, no 6, article id 066015Article in journal (Refereed)
    Abstract [en]

    Plasma-wall interactions have been studied in the full-W ASDEX Upgrade during its dedicated helium campaign. Relatively clean plasmas with a He content of > 80% could be obtained by applying ion cyclotron wall conditioning (ICWC) discharges upon changeover from D to He. However, co-deposited layers with significant amounts of He and D were measured on W samples exposed to ICWC plasmas at the low-field side (outer) midplane. This is a sign of local migration and accumulation of materials and residual fuel in regions shadowed from direct plasma exposure albeit globally D was removed from the vessel. When exposing W samples to ELMy H-mode helium plasmas in the outer strike-point region, no net erosion was observed but the surfaces had been covered with co-deposited layers mainly consisting of W, B, C, and D and being the thickest on rough and modified surfaces. This is different from the typical erosion-deposition patterns in D plasmas, where usually sharp net-erosion peaks surrounded by prominent net-deposition maxima for W are observed close to the strike point. Moreover, no clear signs of W nanostructure growth or destruction could be seen. The growth of deposited layers may impact the operation of future fusion reactors and is attributed to strong sources in the main chamber that under suitable conditions may switch the balance from net erosion into net deposition, even close to the strike points. In addition, the absence of noticeable chemical erosion in helium plasmas may have affected the thickness of the deposited layers. Retention of He, for its part, remained small and uniform throughout the strike-point region although our results indicate that samples with smooth surfaces can contain an order of magnitude less He than their rough counterparts.

  • 252. Hakola, A.
    et al.
    Koivuranta, S.
    Likonen, J.
    Groth, M.
    Kurki-Suonio, T.
    Lindholm, V.
    Miettunen, J.
    Krieger, K.
    Mayer, M.
    Müller, H. W.
    Neu, R.
    Rohde, V.
    Petersson, Per
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Global migration of C-13 impurities in high-density L-mode plasmas in ASDEX Upgrade2013In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 438, no Suppl., p. S694-S697Article in journal (Refereed)
    Abstract [en]

    We have studied the migration of 13C in ASDEX Upgrade after a global impurity injection experiment in 2011. The main chamber was observed to be the largest deposition region for carbon: almost 35% of the injected atoms end up there. Moreover, gaps between wall tiles account for surface densities which are comparable to those on the plasma-facing surfaces. SOLPS modeling of the experiment produced a set of background plasmas and poloidal flow profiles for simulating the transport of 13C with ASCOT; a match with measured deposition, however, required using an imposed flow profile. ASCOT reproduced the observed localized deposition at the outer midplane but work is needed to explain the measured deposition at the inner side of the torus and at the top of the vessel.

  • 253.
    Hannan, Abdul
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Modelling Ion Cyclotron Resonance Heating and Fast Wave Current Drive in Tokamaks2013Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Fast magnetosonic waves in the ion cyclotron range of frequencies have the potential to heat plasma and drive current in a thermonuclear fusion reactor. A code, SELFO-light, has been developed to study the physics of ion cyclotron resonantheating and current drive in thermonuclear fusion reactors. It uses a global full wave solver LION and a new 1D Fokker-Planck solver for the self-consistent calculations of the wave field and the distribution function of ions.In present day tokamak experiments like DIII-D and JET, fast wave damping by ions at higher harmonic cyclotron frequencies is weak compared to future thermonuclear tokamak reactors like DEMO. The strong damping by deuterium, tritium and thermonuclear alpha-particles and the large Doppler width of fast alpha-particles in DEMO makes it difficult to drive the current when harmonic resonance layers of these ionspecies are located at low field side of the magnetic axis. At higher harmonic frequencies the possibility of fast wave current drive diminishes due to the overlapping of alpha-particle harmonic resonance layers. Narrow frequency bands suitable for the fast wave current drive in DEMO have been identified at lower harmonics of the alpha-particles. For these frequencies the effect of formation of high-energy tails in the distribution function of majority and minority ion species on the current drive have been studied. Some of these frequencies are found to provide efficient ion heating in the start up phase of DEMO. The spectrum where efficient current drive can be obtained is restricted due to weak electron damping at lower toroidal mode numbers and strong trapped electron damping at higher toroidal mode numbers. The width of toroidal mode spectra for which efficient current drive can be obtained have been identified, which has important implications for the antenna design.

  • 254.
    Hannan, Abdul
    et al.
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Hellsten, Torbjörn
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Johnson, Thomas
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    A Fokker-Planck Code for Fast Self-Consistent Calculations of ICRH2010In: 37th EPS Conference on Plasma Physics, 2010Conference paper (Other academic)
    Abstract [en]

    Modeling of ion cyclotron resonant heating, ICRH, requires self-consistent modeling of the distribution function of the resonant ion species and the wave field. A method has been devised for fast self-consistent calculation of the distribution function and the wave field for ICRH modeling. The distribution function is obtained by solving a pitch angle averaged 1D time dependent Fokker Planck equation that includes the Coulomb collision and quasi-linear operators. The quasi-linear operators describing the wave-particle interactions are obtained from the LION code [1]. The time dependent 1D Fokker-Planck equation solved with a cubic finite element method will be presented in this report. The modifications of the susceptibility tensors of the resonant ion species due to changes of the distribution functions caused by heating are calculated by the Fokker-Planck solver and then used in the LION code for calculating the modified wave field

  • 255.
    Hannan, Abdul
    et al.
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Hellsten, Torbjörn
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Johnson, Thomas
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Fast wave current drive scenarios for DEMO2013In: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 53, no 4, p. 043005-Article in journal (Refereed)
    Abstract [en]

    Scenarios for non-inductive current drive using the fast magnetosonic wave in the ion cyclotron range of frequencies in DEMO have been studied. The strong ion cyclotron damping and large Doppler broadening of the alpha particles are shown to limit the possible current drive scenarios to four frequency bands. However, these scenarios may be compromised in the presence of impurities with unfavourable charge to mass ratio. For each frequency the current drive efficiency is optimized with respect to the parallel wave number. The optimized current drive efficiencies are comparable to that from neutral beam injection and electron cyclotron heating, and thus the ion cyclotron range of frequencies should remain a candidate for driving the non-inductive current in DEMO.

  • 256.
    Hannan, Abdul
    et al.
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Hellsten, Torbjörn
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Johnson, Thomas
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    On fast wave current drive at higher cyclotron harmonics2011In: 38th EPS Conference on Plasma Physics 2011, EPS 2011 - Europhysics Conference Abstracts: Volume 35 1, 2011, p. 889-892Conference paper (Other academic)
  • 257.
    Hannan, Abdul
    et al.
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Hellsten, Torbjörn
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Johnson, Thomas
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Self-consistent Ion Cyclotron Resonant Heating and Fast Wave Current Drive for DEMOArticle in journal (Other academic)
  • 258. Hawkes, N. C.
    et al.
    Esposito, B.
    Andrew, Y.
    Biewer, T. M.
    Brzozowski, Jerzy H.
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Brix, M.
    Cardinali, A.
    Crisanti, F.
    Crombé, K.
    Van Eester, D.
    Felton, R.
    Giroud, C.
    Johnson, Thomas
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Lerche, E.
    Meigs, A.
    Parail, V.
    Sharapov, S.
    Sozzi, C.
    Voitsekhovitch, I.
    Zastrow, K. -D
    Ion transport barrier formation with low injected torque in JET2007In: 34th EPS Conference on Plasma Physics 2007, EPS 2007 - Europhysics Conference Abstracts, 2007, no 1, p. 504-507Conference paper (Refereed)
    Abstract [en]

    Ion temperature ITB trigger events have been provoked on JET with very low levels of injected torque using a 3He minority ion heating scheme. The evidence indicates that E x B shear driven by toroidal rotation is not important in these ITB triggers, however the ITBs which form are weak and short lived. Evidence from other experiments [4], suggests that higher torque is necessary to establish and maintain strong ITBs. Future experiments with the increased RF power of the new JET ICRH antenna will be made to explore whether 'strong' ITBs can be created at high power and low applied torque.

  • 259. Heinola, K.
    et al.
    Widdowson, A.
    Likonen, J.
    Alves, E.
    Baron-Wiechec, A.
    Barradas, N.
    Brezinsek, S.
    Catarino, N.
    Coad, P.
    Koivuranta, S.
    Krat, S.
    Matthews, G. F.
    Mayer, M.
    Petersson, Per
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics. EUROfusion Consortium, Culham Science Centre, JET, Abingdon, United Kingdom.
    Contributors, J.
    Long-term fuel retention in JET ITER-like wall2016In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. T167, article id 014075Article in journal (Refereed)
    Abstract [en]

    Post-mortem studies with ion beam analysis, thermal desorption, and secondary ion mass spectrometry have been applied for investigating the long-term fuel retention in the JET ITERlike wall components. The retention takes place via implantation and co-deposition, and the highest retention values were found to correlate with the thickness of the deposited impurity layers. From the total amount of retained D fuel over half was detected in the divertor region. The majority of the retained D is on the top surface of the inner divertor, whereas the least retention was measured in the main chamber on the mid-plane of the inner wall limiter. The recessed areas of the inner wall showed significant contribution to the main chamber total retention. Thermal desorption spectroscopy analysis revealed the energetic T from DD reactions being implanted in the divertor. The total T inventory was assessed to be >0.3 mg.

  • 260. Heinola, K.
    et al.
    Widdowson, A.
    Likonen, J.
    Alves, E.
    Baron-Wiechec, A.
    Barradas, N.
    Brezinsek, S.
    Catarino, N.
    Coad, P.
    Koivuranta, S.
    Matthews, G. F.
    Mayer, M.
    Petersson, Per
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Fuel retention in JET ITER-Like Wall from post-mortem analysis2015In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 463, p. 961-965Article in journal (Refereed)
    Abstract [en]

    Selected Ion Beam Analysis techniques applicable for detecting deuterium and heavier impurities have been used in the post-mortem analyses of tiles removed after the first JET ITER-Like Wall (JET-ILW) campaign. Over half of the retained fuel was measured in the divertor region. The highest figures for fuel retention were obtained from regions with the thickest deposited layers, i.e. in the inner divertor on top of tile 1 and on the High Field Gap Closure tile, which resides deep in the plasma scrape-off layer. Least retention was found in the main chamber high erosion regions, i.e. in the mid-plane of Inner Wall Guard Limiter. The fuel retention values found typically varied with deposition layer thicknesses. The reported retention values support the observed decrease in fuel retention obtained with gas balance experiments of JET-ILW.

  • 261. Hellesen, C.
    et al.
    Johnson, M. G.
    Sunden, E. A.
    Conroy, S.
    Ericsson, G.
    Ronchi, E.
    Sjostrand, H.
    Weiszflog, M.
    Gorini, G.
    Tardocchi, M.
    Johnson, Thomas J.
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Kiptily, V. G.
    Pinches, S. D.
    Sharapov, S. E.
    Johnson, M. Gatu
    Anderson-Sunden, E.
    Neutron emission generated by fast deuterons accelerated with ion cyclotron heating at JET2010In: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 50, no 2Article in journal (Refereed)
    Abstract [en]

    For the first time, the neutron emission from JET plasmas heated with combined deuterium neutral beam injection and third harmonic ion cyclotron radio frequency heating have been studied with neutron emission spectroscopy (NES). Very high DD neutron rates were observed with only modest external heating powers, which was attributed to acceleration of deuterium beam ions to energies of about 2-3 MeV, where the DD reactivity is on a par of that of the DT reaction. Fast deuterium energy distributions were derived from analysis of NES data and confirm acceleration of deuterium beam ions up to energies around 3 MeV, in agreement with theoretical predictions. The high neutron rates allowed for observations of changes in the fast deuterium populations on a time scale of 50 ms. Correlations were seen between fast deuterium ions at different energies and magnetohydrodynamic activities, such as monster sawtooth crashes and toroidal Alfven eigenmodes.

  • 262. Hellesen, C.
    et al.
    Johnson, M. Gatu
    Sunden, E. Andersson
    Conroy, S.
    Ericsson, G.
    Eriksson, J.
    Gorini, G.
    Johnson, Thomas
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Kiptily, V. G.
    Pinches, S. D.
    Sharapov, S. E.
    Sjostrand, H.
    Nocente, M.
    Tardocchi, M.
    Weiszflog, M.
    Measurements of fast ions and their interactions with MHD activity using neutron emission spectroscopy2010In: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 50, no 8, p. 084006-Article in journal (Refereed)
    Abstract [en]

    Ion cyclotron radio frequency (ICRF) heating can produce fast ion populations with energies reaching up to several megaelectronvolts. Here, we present unique measurements of fast ion distributions from an experiment with 3rd harmonic ICRF heating on deuterium beams using neutron emission spectroscopy (NES). From the experiment, very high DD neutron rates were observed, using only modest external heating powers. This was attributed to acceleration of deuterium beam ions to energies up to about 2-3 MeV, where the DD reactivity is on a par with that of the DT reaction. The high neutron rates allowed for observations of changes in the fast deuterium energy distribution on a time scale of 50 ms. Clear correlations were seen between fast deuterium ions in different energy ranges and magnetohydrodynamic activities, such as monster sawteeth and toroidal Alfven eigen modes (TAE). Specifically, NES data showed that the number of deuterons in the region between 1 and 1.5 MeV were decaying significantly during strong TAE activity, while ions with lower energies around 500 keV were not affected. This was attributed to resonances with the TAE modes.

  • 263. Hellesen, C.
    et al.
    Johnson, M. Gatu
    Sundén, E. Andersson
    Conroy, S.
    Ericsson, G.
    Eriksson, J.
    Sjöstrand, H.
    Weiszflog, M.
    Johnson, Thomas
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Gorini, G.
    Nocente, M.
    Tardocchi, M.
    Kiptily, V. G.
    Pinches, S. D.
    Sharapov, S. E.
    Fast-ion distributions from third harmonic ICRF heating studied with neutron emission spectroscopy2013In: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 53, no 11, p. 113009-Article in journal (Refereed)
    Abstract [en]

    The fast-ion distribution from third harmonic ion cyclotron resonance frequency (ICRF) heating on the Joint European Torus is studied using neutron emission spectroscopy with the time-of-flight spectrometer TOFOR. The energy dependence of the fast deuteron distribution function is inferred from the measured spectrum of neutrons born in DD fusion reactions, and the inferred distribution is compared with theoretical models for ICRF heating. Good agreements between modelling and measurements are seen with clear features in the fast-ion distribution function, that are due to the finite Larmor radius of the resonating ions, replicated. Strong synergetic effects between ICRF and neutral beam injection heating were also seen. The total energy content of the fast-ion population derived from TOFOR data was in good agreement with magnetic measurements for values below 350 kJ.

  • 264.
    Hellsten, Torbjörn
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    A statistical model of the wave field in a bounded domain2017In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 24, no 2, article id 022122Article in journal (Refereed)
    Abstract [en]

    Numerical simulations of plasma heating with radiofrequency waves often require repetitive calculations of wave fields as the plasma evolves. To enable effective simulations, bench marked formulas of the power deposition have been developed. Here, a statistical model applicable to waves with short wavelengths is presented, which gives the expected amplitude of the wave field as a superposition of four wave fields with weight coefficients depending on the single pass damping, as. The weight coefficient for the wave field coherent with that calculated in the absence of reflection agrees with the coefficient for strong single pass damping of an earlier developed heuristic model, for which the weight coefficients were obtained empirically using a full wave code to calculate the wave field and power deposition. Antennas launching electromagnetic waves into bounded domains are often designed to produce localised wave fields and power depositions in the limit of strong single pass damping. The reflection of the waves changes the coupling that partly destroys the localisation of the wave field, which explains the apparent paradox arising from the earlier developed heuristic formula that only a fraction a(s)(2)(2-a(s)) and not as of the power is absorbed with a profile corresponding to the power deposition for the first pass of the rays. A method to account for the change in the coupling spectrum caused by reflection for modelling the wave field with ray tracing in bounded media is proposed, which should be applicable to wave propagation in non-uniform media in more general geometries.

  • 265.
    Hellsten, Torbjörn
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Integrated Modelling of Heating, Current Drive and Fast Particle Physics2007In: IAEA Technical Meeting on Fast Particles, 2007Conference paper (Other academic)
  • 266.
    Hellsten, Torbjörn
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics. KTH, School of Electrical Engineering (EES), Centres, Alfvén Laboratory Centre for Space and Fusion Plasma Physics.
    Momentum transport by wave-particle interaction2011In: PLASMA PHYS CONTROL FUSION, 2011, Vol. 53, no 5, p. 054007-Conference paper (Refereed)
    Abstract [en]

    Energy and momentum can be transported across the plasma by waves emitted at one place and absorbed at another. Exchange of momentum and energy between the particles and the waves change the drift orbits, which may give rise to a non-ambipolar particle transport. The main effect of the non-ambipolar transport and quasi-neutrality is a toroidal precession of the trapped particles, which together with the changes in the parallel velocities of the passing resonant particles conserve the toroidal momentum. Non-resonant interactions can give rise to a net change of the local wave number in spatial inhomogeneous plasmas with a resulting force on the medium. Both resonant and non-resonant interactions have to be taken into account in order to have a consistent description of the momentum transported by the waves. The momentum transfer is, in particular, important for waves with short wave length and low frequency, and may explain the enhanced rotation seen in some mode conversion experiments, when the fast magnetosonic wave is converted to an ion-cyclotron wave. The apparent contradiction that the wave momentum may change due to non-resonant wave-particle interactions without changing the energy in geometric optics is explained.

  • 267.
    Hellsten, Torbjörn
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    On self-consistent modelling of energetic particles generated by auxiliary heating and associated instabilities2006Conference paper (Other academic)
  • 268.
    Hellsten, Torbjörn
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Rotation Driven by Rectified RF-sheath Potentials and Spatial Dispersion2009In: RADIO FREQUENCY POWER IN PLASMAS, 2009, Vol. 1187, p. 625-628Conference paper (Refereed)
    Abstract [en]

    Plasma rotation is of interest for improving confinement and stabilising plasma. Effects from fast particles with broad orbits can only partly explain the changes in the rotation profiles during ICRH. The effect on wave-particle interaction of a finite poloidal mode number is discussed and two new RF-mechanisms are proposed: Co-current torque caused by sputtering by rectified RF-sheath potentials and transport of momentum due to spatial dispersion. The latter effect affects the RF-current drive, in particular, in conjunction with mode conversion.

  • 269.
    Hellsten, Torbjörn A. K.
    et al.
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Holmström, Kerstin
    KTH, School of Electrical Engineering (EES), Centres, Alfvén Laboratory Centre for Space and Fusion Plasma Physics.
    Johnson, Thomas J.
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Bergkvist, Tommy
    KTH, School of Electrical Engineering (EES), Centres, Alfvén Laboratory Centre for Space and Fusion Plasma Physics.
    Laxåback, Martin
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    On ion cyclotron emission in toroidal plasmas2006In: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 46, no 7, p. S442-S454Article in journal (Refereed)
    Abstract [en]

    A detailed study of ion cyclotron interactions in a toroidal plasma has been carried out in order to elucidate the role of toroidal effects on ion cyclotron emission. It is well known that non-relaxed distribution functions can give rise to excitation of magnetosonic waves by ion cyclotron interactions when the distribution function increases with respect to the perpendicular velocity. We have extended and clarified the conditions under which even collisionally relaxed distribution function can destabilize magnetosonic eigenmodes. In a toroidal plasma, cyclotron interactions at the plasma boundary with ions having barely co-current passing orbits and marginally trapped orbits can cause destabilisation by the strong inversion of the distribution function along the characteristics of cyclotron interaction by neo-classical effects. The unstable interactions can further be enhanced by tangential interactions, which can also prevent the interactions from reaching the stable part of the characteristics, where they interact with trapped orbits. Conditions on the localization of the magnetosonic eigenmodes for unstable excitation are analysed by studying the anti-Hermitian part of the susceptibility tensor of thermonuclear alpha-particles. The pattern of positive and negative regions of the anti-Hermitian part of the susceptibility tensor of thermonuclear alpha-particles is, in general, consistent with the excitation of edge localized magnetosonic eigenmodes, even though the eigenmodes are usually not localized in the major radius and for distribution functions that have relaxed to steady state.

  • 270.
    Hellsten, Torbjörn
    et al.
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Bergkvist, Tommy
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Johnson, Thomas
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Laxåback, Martin
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Effects of Finite Orbit Width and RF-Induced Spatial Diffusion on Ion Cyclotron Emission2005In: Radio Frequency Power in Plasmas: 16th Topical Conference on Radio Frequency Power in Plasmas / [ed] S. J. Wukitch and P. T. Bonoli, Melville, New York: AIP Conference Proceedings , 2005, p. 50-53Conference paper (Refereed)
    Abstract [en]

    The theory of ion cyclotron emission, ICE, in tokamak plasmas has been revised by including the effects of finite orbit width and RF-induced spatial transport in the wave-particle interactions. Two mechanisms for excitation of edge localised magnetosonic modes are discussed. An inverted distribution function of suprathermal ions near the plasma edge is driving the modes. Counter current propagating waves can be excited by interacting with barely co passing ions. Co current propagating waves interacting at the inner leg only can drive the modes unstable by throwing the fast ions out of the plasma.

  • 271.
    Hellsten, Torbjörn
    et al.
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Bergkvist, Tommy
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Johnson, Thomas
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Laxåback, Martin
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Integrated Modelling of ICRH and AE Dynamics2005In: IEA Burning Plasma Workshop, 2005Conference paper (Other academic)
  • 272.
    Hellsten, Torbjörn
    et al.
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Hannan, Abdul
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Johnson, Thomas
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Eriksson, L. -G
    Höök, Josef
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics. KTH, School of Electrical Engineering (EES), Centres, Alfvén Laboratory Centre for Space and Fusion Plasma Physics.
    Villard, L.
    On Self-Consistent ICRH ModellingArticle in journal (Other academic)
  • 273.
    Hellsten, Torbjörn
    et al.
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Hannan, Abdul
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Johnson, Thomas
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Eriksson, L. -G
    Höök, Josef
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Villard, L.
    Self-consistent ICRH modelling2012In: 39th EPS Conference on Plasma Physics 2012, EPS 2012 and the 16th International Congress on Plasma Physics: Volume 2, 2012, 2012, p. 1106-1109Conference paper (Refereed)
  • 274.
    Hellsten, Torbjörn
    et al.
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Hannan, Abdul
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Johnson, Thomas
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Eriksson, L. -G
    Höök, Josef
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics. KTH, School of Electrical Engineering (EES), Centres, Alfvén Laboratory Centre for Space and Fusion Plasma Physics.
    Villard, L.
    Self-Consistent ICRH Modelling2011In: RADIO FREQUENCY POWER IN PLASMAS: PROCEEDINGS OF THE 19TH TOPICAL CONFERENCE, 2011, p. 365-368Conference paper (Refereed)
    Abstract [en]

    A new code for self-consistent modelling of ion cyclotron heating suitable for routine calculations has been developed.

  • 275.
    Hellsten, Torbjörn
    et al.
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Hannan, Abdul
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Johnson, Thomas
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Eriksson, L-G
    Höök, Lars Josef
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Villard, L.
    A model for self-consistent simulation of ICRH suitable for integrating modelling2013In: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 53, no 9, p. 093004-Article in journal (Refereed)
    Abstract [en]

    A self-consistent modelling of ion cyclotron resonance heating (ICRH) is reviewed with the aim of obtaining a fast robust scheme suitable for routine simulation for transport codes and data analysis. Due to the complexity of calculating the wave field and the distribution function self-consistently simplifications are necessary. To improve modelling of the wave field, methods are developed to include higher order finite Larmor radius terms, up-and downshifts of the parallel wave number and to improve calculations of damping due to the transit time magnetic pumping in finite element wave codes without decomposing the wave locally into planar waves. A new code, SELFO-light, for self-consistent modelling of ion cyclotron heating suitable for routine calculations is developed. The code is based on coupling the global wave code LION with a simple one-dimensional time-dependent Fokker-Planck code. Both the wave and the Fokker-Planck codes use finite element representations. The importance of self-consistent modelling of ion cyclotron heating is illustrated by studying the effect on the power partition for a fast wave current drive scenario at lower harmonic resonances in a deuterium plasma. It is found that the fraction of the power absorbed on the deuterium and the time to reach the steady state vary strongly depending on the position of the resonances. It is found that the deuterium absorption becomes strongly localized to regions where the resonances are tangential to the magnetic flux surfaces.

  • 276.
    Hellsten, Torbjörn
    et al.
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Hannan, Abdul
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Johnson, Thomas
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Höök, Lars Josef
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Villard, L.
    A dielectric response model for FEM solutions of ICRF wave fields2012In: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 401, no 1, p. 012009-Article in journal (Refereed)
    Abstract [en]

    Modelling of fast wave ICRF heating in large machines with high density such as DEMO is challenging because of the short wave lengths. Therefore, fast, efficient global wave solvers are necessary. A major difficulty with calculating the wave field in a spatial dispersive medium is that the dielectric tensor becomes a function of the local wave vector, which in its turn depends on the solution. Furthermore, the solution may consist of several waves co-existing at the same location subjected to separate response functions. In order to model upshift of the parallel wave vector, higher order FLR-effects on the cyclotron absorption and TTMP damping for the electron absorption methods based on iteration, suitable for FEM codes, are proposed.

  • 277.
    Hellsten, Torbjörn
    et al.
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Holmström, Kerstin
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Johnson, Tomas
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Bergkvist, Tommy
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Laxåback, Martin
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    ICE in toroidal plasmas2005In: IAEA Technical Meeting on Fast Particles, 2005Conference paper (Other academic)
  • 278.
    Hellsten, Torbjörn
    et al.
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Johnson, Thomas
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Neoclassical Electric Field in Presence of Large Gradients and Particle Losses2008In: 35th EPS Conference on Plasma Physics 2008, EPS 2008 - Europhysics Conference Abstracts: Volume 32, Issue 3, 2008, p. 1847-1850Conference paper (Other academic)
  • 279.
    Hellsten, Torbjörn
    et al.
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Johnson, Thomas
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    On Current Drive and Wave Induced Bootstrap Current in Toroidal Plasmas2008In: Theory of Fusion Plasmas: Joint Varenna - Lausanne International Workshop, Melville, New York, USA: AMER INST PHYSICS , 2008, p. 88-99Conference paper (Other academic)
    Abstract [en]

    A comprehensive treatment of wave-particle interactions in toroidal plasmas including collisional relaxation. applicable to heating or anomalous wave induced transport, has been obtained by using Monte Carlo operators satisfying quasi-neutrality. This approach enables a self-consistent treatment of wave-particle interactions applicable to the banana regime in the neoclassical theory. It allows an extension into a regime with large temperature and density gradients, losses and transport of particles by wave-particle interactions making the method applicable to transport barriers. It is found that at large gradients the relationship between radial electric field, parallel velocity, temperature and density gradient in the neoclassical theory is modified such that coefficient in front of the logarithmic ion temperature gradient, which in the standard neoclassical theory is small and counteracts the electric field caused by the density gradient, now changes sign and contributes to the built up of the radial electric field. The Possibility to drive current by absorbing the waves on trapped particles has been studied and flow the wave-particle interactions affect the bootstrap current. Two new current drive mechanisms are studied: current drive by wave induced bootstrap current and selective detrapping into passing orbits by directed waves.

  • 280.
    Hellsten, Torbjörn
    et al.
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Johnson, Thomas
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Self-Consistent ICRH Modelling2014In: Radiofrequency power in plasmas, 2014, Vol. 1580, p. 295-297Conference paper (Refereed)
    Abstract [en]

    Self-consistent modelling of ICRH requires calculations of the wave field consistent with the distribution function of the resonant species. Because of the difference in time scales for wave propagation and the evolution of distribution functions this is commonly done by iterations. A robust code SELFO-light, suitable for routine calculations was recently developed, based on coupling a 1D time dependent Fokker-Planck code with the global wave solver LION using a FEM. Here the structure of an upgraded version of the SELFO-light code is presented calculating the distribution function with a 2D Fokker-Planck code. This requires new interfaces calculating the quasi-linear diffusion coefficient from the wave field and the susceptibility tensor from distribution functions.

  • 281.
    Hellsten, Torbjörn
    et al.
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Johnson, Thomas J.
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Van Eester, D.
    Lerche, E.
    Lin, Y.
    Mayoral, M-L
    Ongena, J.
    Calabro, G.
    Crombe, K.
    Frigione, D.
    Giroud, C.
    Lennholm, M.
    Mantica, P.
    Nave, M. F. F.
    Naulin, V.
    Sozzi, C.
    Studholme, W.
    Tala, T.
    Versloot, T.
    Observations of rotation in JET plasmas with electron heating by ion cyclotron resonance heating2012In: Plasma Physics and Controlled Fusion, ISSN 0741-3335, E-ISSN 1361-6587, Vol. 54, no 7, p. 074007-Article in journal (Refereed)
    Abstract [en]

    The rotation of L-mode plasmas in the JET tokamak heated by waves in the ion cyclotron range of frequencies (ICRF) damped on electrons, is reported. The plasma in the core is found to rotate in the counter-current direction with a high shear and in the outer part of the plasma with an almost constant angular rotation. The core rotation is stronger in magnitude than observed for scenarios with dominating ion cyclotron absorption. Two scenarios are considered: the inverted mode conversion scenarios and heating at the second harmonic He-3 cyclotron resonance in H plasmas. In the latter case, electron absorption of the fast magnetosonic wave by transit time magnetic pumping and electron Landau damping (TTMP/ELD) is the dominating absorption mechanism. Inverted mode conversion is done in (He-3)-H plasmas where the mode converted waves are essentially absorbed by electron Landau damping. Similar rotation profiles are seen when heating at the second harmonic cyclotron frequency of He-3 and with mode conversion at high concentrations of He-3. The magnitude of the counter-rotation is found to decrease with an increasing plasma current. The correlation of the rotation with the electron temperature is better than with coupled power, indicating that for these types of discharges the dominating mechanism for the rotation is related to indirect effects of electron heat transport, rather than to direct effects of ICRF heating. There is no conclusive evidence that mode conversion in itself affects rotation for these discharges.

  • 282.
    Hellsten, Torbjörn
    et al.
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Johnson, Thomas
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Sharapov, S. E.
    Kiptily, V.
    Eriksson, J.
    Mantsinen, M.
    Schneider, M.
    Rimini, F.
    Tsalas, M.
    RF Heating for Fusion Product Studies2015In: RADIOFREQUENCY POWER IN PLASMAS, American Institute of Physics (AIP), 2015, article id UNSP 060007Conference paper (Refereed)
    Abstract [en]

    Third harmonic cyclotron heating is an effective tool for accelerating deuterium (D) beams to the MeV energy range, suitable for studying ITER relevant fast particle physics in plasmas without significant tritium content. Such experiments were recently conducted in JET with an ITER like wall in D plasmas with He-3 concentrations up to 30% in order to boost the fusion reactivity by D-He-3 reactions. The harmonic cyclotron heating produces high-energy tails in the MeV range of D ions by on-axis heating and of He-3 ions by tangential off-axis heating. The discharges are characterized by long sawtooth free periods and a rich spectrum of MHD modes excited by the fast D and He-3 ions. The partitions of the power, which depend on the distribution function of D, vary strongly over several slowing down times. Self-consistent modelling of the distribution function with the SELFO-light code are presented and compared with experimental data from fast particle diagnostics.

  • 283.
    Hellsten, Torbjörn
    et al.
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Johnson, Thomas
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Vallejos, Pablo
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    An iterative method for including Doppler shift in global wave solvers using FEM decomposition2014In: Journal of Physics: Conference series, ISSN 1742-6596, Vol. 561Article in journal (Refereed)
    Abstract [en]

    A method for calculating the wave field for spatial dispersive media is proposed suitable for FEM. The method is based on operator splitting by separating the induced current and wave field calculations, and solving the system by means of iterations. In order to take into account several coexisting waves with different poloidal mode numbers when calculating the induced current the wave field is decomposed into wavelets, for which the current is calculated assuming the plasma to be weakly non-uniform.

  • 284.
    Hellsten, Torbjörn
    et al.
    KTH, School of Electrical Engineering (EES), Centres, Alfvén Laboratory Centre for Space and Fusion Plasma Physics.
    Laxåback, Martin
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Bergkvist, Thomas
    KTH, School of Electrical Engineering (EES), Centres, Alfvén Laboratory Centre for Space and Fusion Plasma Physics.
    Johnson, T
    Meo, F
    Nguyen, F
    Petty, C
    Mantsinen, M
    Matthews, G
    Noterdaeme, M
    Tala, T
    Van Eester, D
    Andrew, P
    Beaumont, P
    Bobkov, V
    Brix, M
    Brzozowski, J
    Eriksson, G
    Giroud, C
    Joffrin, E
    Kiptily, V
    Mailloux, J
    Mayoral, L
    Monakhov, I
    Sartori, R
    Staebler, A
    Rachlew, E
    Tennfors, E
    Tuccillo, A
    Walden, A
    Zastrow, D
    On the parasitic absorption in FWCD experiments in JET ITB plasmas2005In: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 45, no 7, p. 706-720Article in journal (Refereed)
  • 285.
    Hellsten, Torbjörn
    et al.
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Laxåback, Martin
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Bergkvist, Tommy
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Johnson, Thomas
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Zastrow, K.-D.
    et, al
    Fast Wave Current Drive and Direct Electron Heating in JET ITB Plasmas2006In: Proc 21st IAEA Fusion Energy Conference, 2006Conference paper (Refereed)
    Abstract [en]

    Experiments with Fast Wave Current Drive, FWCD, and heating have been carried out in JET Internal Transport Barrier (ITB) discharges with strongly reversed magnetic shear. In order to maximize the current drive efficiency and increase the electron damping, and at the same time modifying the current profile in the transport barrier, hot low density ITB plasmas with strongly reversed magnetic shear, close to current hole, were created with Lower Hybrid Current Drive. It was difficult to strongly modify the central plasma current, even though the calculated current drive efficiency in terms of ampere per watts absorbed by the electrons was fairly high, 0.07A/W, because of: the strongly inductive nature of the plasma current due to the high electric conductivity; the interplay between the fast wave driven current and the bootstrap current, which, due to the dependence of the bootstrap current on the poloidal magnetic field, decreases the bootstrap current as the driven current increases; and parasitic absorption of the waves that decreased the power absorbed by the electrons. The power absorbed by the electrons was measured with a power modulation technique and the associated fast wave current drive calculated. Current diffusion simulations using the JETTO transport code, assuming neoclassical resistivity, were then carried out to calculate what changes to the plasma current profile could be expected from the current drive. The simulations showed a much slower response to the current drive compared to the measured central current densities suggesting a faster current penetration in the experiments than expected from neoclassical theory. Whereas the direct electron heating by fast magnetosonic waves using dipole spectra has proven to be an effective method to heat electrons in high-temperature ITB plasmas, even for a single pass damping of only a few percent, the heating in FWCD experiments with + 90o and - 90o antenna phasings were, for similar single pass damping as for the dipole, strongly degraded by parasitic losses, and with a heating efficiency of about half that of the dipole. Observations supporting that the losses are primarily caused by the presence of rectified RF-sheath potentials come from the large differences in performance and in Beryllium-II and Carbon-IV line radiation intensities between the dipole and ±90o phasings.

  • 286. Hirai, T.
    et al.
    Linke, J.
    Rubel, Marek J.
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics. KTH, School of Electrical Engineering (EES), Centres, Alfvén Laboratory Centre for Space and Fusion Plasma Physics.
    Coad, J. P.
    Likonen, J.
    Lungu, C. P.
    Matthews, G. F.
    Philipps, V.
    Wessel, E.
    Thermal load testing of erosion-monitoring beryllium marker tile for the ITER-Like Wall Project at JET2008In: Fusion engineering and design, ISSN 0920-3796, E-ISSN 1873-7196, Vol. 83, no 7-9, p. 1072-1076Article in journal (Refereed)
    Abstract [en]

    ITER-Like Wall Project has been launched at JET in order to perform a fully integrated test of plasma-facing materials. During the next major shutdown a full metal wall will be installed: tungsten in the divertor and beryllium in the main chamber. Beryllium erosion is one of key issues to be addressed. Special marker tiles have been designed for this purpose. Test coupons of such markers have been manufactured and examined. The performance test under high power deposition was carried in the electron beam facility JUDITH. The results of material characterization before and after high heat flux loads are presented. The samples survived, without macroscopic damage, power loads of up to 4.5 MW/m(2) for 10s (surface temperature similar to 650 degrees C) and 50 cyclic loads at 3.5 MW/m(2) lasting 10s each (surface temperature similar to 600 degrees C).

  • 287. Hirai, T.
    et al.
    Linke, J.
    Sundelin, Per
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics. KTH, School of Electrical Engineering (EES), Centres, Alfvén Laboratory Centre for Space and Fusion Plasma Physics.
    Rubel, Marek J.
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics. KTH, School of Electrical Engineering (EES), Centres, Alfvén Laboratory Centre for Space and Fusion Plasma Physics.
    Kuehnlein, W.
    Wessel, E.
    Coad, J. P.
    Lungu, C. P.
    Matthews, G. F.
    Pedrick, L.
    Piazza, G.
    Characterization and heat flux testing of beryllium coatings on Inconel for JET ITER-like wall project2007In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. T128, p. 166-170Article in journal (Refereed)
    Abstract [en]

    In order to perform a fully integrated material test, JET has launched the ITER-like wall project with the aim of installing a full metal wall during the next major shutdown. The material foreseen for the main chamber wall is bulk Be at the limiters and Be coatings on inconel tiles elsewhere. R&D process comprises global characterization ( structure, purity etc) of the evaporated films and testing of their performance under heat loads. The major results are (i) the layers have survived energy loads of 20 MJ m(-2) which is significantly above the required level of 5 - 10 MJ m(-2), (ii) melting limit of beryllium coating would be at the energy level of 30 MJ m(-2), (iii) cyclic thermal load of 10 MJ m(-2) for up to 50 cycles have not induced any noticeable damage such as flaking or detachment.

  • 288. Hirai, T.
    et al.
    Maier, H.
    Rubel, Marek J.
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Mertens, Ph
    Neu, R.
    Gauthier, E.
    Likonen, J.
    Lungu, C.
    Maddaluno, G.
    Matthews, G. F.
    Mitteau, R.
    Neubauer, O.
    Piazza, G.
    Philipps, V.
    Riccardi, B.
    Ruset, C.
    Uytdenhouwen, I.
    R&D on full tungsten divertor and beryllium wall for JET ITER-like wall project2007In: Fusion engineering and design, ISSN 0920-3796, E-ISSN 1873-7196, Vol. 82, no 15-24, p. 1839-1845Article in journal (Refereed)
    Abstract [en]

    The ITER reference materials have been tested separately in tokamaks, plasma simulators, ion beams and high heat flux test beds. In order to perform a fully integrated material test JET has launched the ITER-like Wall Project with the aim of installing a full metal wall during the next major shutdown. As a result of R&D projects in 2005-2006, bulk tungsten tiles are foreseen at the outer horizontal target and tungsten coating at the other divertor tiles. In some regions of the main chamber, beryllium coated Inconel tiles and bulk beryllium tiles are utilised which include marker tiles as erosion diagnostics. This paper gives an overview of the R&D carried out in the frame of the ITER-like Wall Project on the development of an inertially cooled bulk tungsten tile design and the characterization of tungsten and beryllium coating technologies.

  • 289. Horvath, L.
    et al.
    Maggi, C. F.
    Casson, F. J.
    Frassinetti, L.
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Dunne, M. G.
    Hobirk, J.
    Lupelli, I.
    Gibson, K. J.
    Evolution of the bootstrap current profile during the type I ELM cycle of JET-ILWH-mode plasmas2016In: 43rd European Physical Society Conference on Plasma Physics, EPS 2016, European Physical Society (EPS) , 2016Conference paper (Refereed)
  • 290.
    Höök, Josef
    et al.
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics. Association VR-Euratom, Sweden .
    Hellsten, Torbjörn
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics. Association VR-Euratom, Sweden .
    Adaptive delta f Monte Carlo Method for Simulation of RF-heating and Transport in Fusion Plasmas2009In: Radio Frequency Power in Plasmas, American Institute of Physics (AIP), 2009, Vol. 1187, p. 589-592Conference paper (Refereed)
    Abstract [en]

    Essential for modeling heating and transport of fusion plasma is determining the distribution function of the plasma species. Characteristic for RF-heating is creation of particle distributions with a high energy tail. In the high energy region the deviation from a Maxwellian distribution is large while in the low energy region the distribution is close to a Maxwellian due to the velocity dependency of the collision frequency. Because of geometry and orbit topology Monte Carlo methods are frequently used. To avoid simulating the thermal part, delta f methods are beneficial. Here we present a new delta f Monte Carlo method with an adaptive scheme for reducing the total variance and sources, suitable for calculating the distribution function for RF-heating

  • 291.
    Höök, Josef
    et al.
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Hellsten, Torbjörn
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    An Adaptive delta f Monte Carlo Method2010In: IEEE Transactions on Plasma Science, ISSN 0093-3813, E-ISSN 1939-9375, Vol. 38, no 9, p. 2190-2197Article in journal (Refereed)
    Abstract [en]

    A new adaptive delta f Monte Carlo method is presented with an application to radio frequency heating and transport in fusion plasmas. The method is suitable when an initial zeroth-order approximation of the distribution function is known. The difference between our method and earlier delta f methods is that we model the source term, obtained from the delta f ansatz, by adding particles. The rate of particles is defined by the inhomogeneous term in the Fokker-Planck equation. We develop an adaptive scheme for modifying the unperturbed part G(x) such that the number of particles used in the simulation for a fixed weight is minimized. This implicitly reduces the variance and improves computational efficiency. The method is tested on a one-dimensional Fokker-Planck model for RF-heating and compared against the analytical stationary solution.

  • 292.
    Höök, L. Josef
    et al.
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Johnson, Thomas
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Composition schemes for the stochastic differential equation describing collisional pitch-angle diffusion2014In: Computer Physics Communications, ISSN 0010-4655, E-ISSN 1879-2944, Vol. 185, no 2, p. 590-594Article in journal (Refereed)
    Abstract [en]

    Two new second order accurate Monte Carlo integration schemes are derived for the stochastic differential equation describing pitch-angle scattering by Coulomb collisions in magnetized plasmas. Here the pitch-angle is the angle between the magnetic field and the particle velocity vectors. Mathematically this collision process corresponds to diffusion in the polar angle of a spherical coordinate system. The schemes are simple to implement, they are naturally bounded to the solution domain and their convergences are shown to compare favourably against commonly used alternative integration schemes.

  • 293.
    Höök, Lars Josef
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Kernel density estimation using the RBF-Galerkin method2012Report (Other academic)
  • 294.
    Höök, Lars Josef
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Numerical solution of quasilinear kinetic diffusion equations in toroidal plasmas2013Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    One of the main challenges for the realization of a working fusion power plant is an increased detailed understanding of kinetic phenomena in toroidal plasmas. The tokamak is a toroidal, magnetically confined plasma device and is currently the main line towards a power plant. The spatial and temporal scales in a tokamak plasma are extreme and the only tractable path for quantitative studies is to rely on computer simulations. Present day simulation codes can resolve only some of these scales. Nevertheless they still require the largest high performance computing (HPC) resources available in the world. In combination with the increase of computational performance, it is therefore necessary to improve the numerical algorithms used in the simulations.

    In this thesis we have developed new numerical methods designed for Monte Carlo simulation of plasma kinetic diffusion. Examples are simulation of fast-ion thermalization and radio-frequency heating. The aim has been to reduce the statistical random noise in particle codes, produced by a finite number of particles (or markers). Traditionally the statistical noise is improved by increasing the number of particles (N) or by simulating the perturbation of the distribution (with particles) from a known distribution function. This is the well known δf method. In this thesis we have developed a new type of δf method, which minimizes the number of particles used in a simulation. The computational speedup of the new method is substantial. In this thesis, we have further benchmarked quasi-Monte Carlo techniques that improve the convergence rate from N−1/2 to N−1 for some cases.

    In Monte Carlo simulations, error appears also from the time step discretization. Based on the mathematics of operator splitting, a new scheme for the pitch-angle scattering diffusion process has been developed that outperforms the standard methods. Finally this thesis also presents a new code, SELFO-light, for self-consistent simulations of ion cyclotron resonance heating, suitable for routine calculations, which couples a one dimensional Fokker-Planck model with the finite element wave solver LION.

  • 295.
    Höök, Lars Josef
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Variance reduction methods for numerical solution of plasma kinetic diffusion2012Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Performing detailed simulations of plasma kinetic diffusion is a challenging task and currently requires the largest computational facilities in the world. The reason for this is that, the physics in a confined heated plasma occur on a broad range of temporal and spatial scales. It is therefore of interest to improve the computational algorithms together with the development of more powerful computational resources. Kinetic diffusion processes in plasmas are commonly simulated with the Monte Carlo method, where a discrete set of particles are sampled from a distribution function and advanced in a Lagrangian frame according to a set of stochastic differential equations. The Monte Carlo method introduces computational error in the form of statistical random noise produced by a finite number of particles (or markers) N and the error scales as αNβ where β = 1/2 for the standard Monte Carlo method. This requires a large number of simulated particles in order to obtain a sufficiently low numerical noise level. Therefore it is essential to use techniques that reduce the numerical noise. Such methods are commonly called variance reduction methods. In this thesis, we have developed new variance reduction methods with application to plasma kinetic diffusion. The methods are suitable for simulation of RF-heating and transport, but are not limited to these types of problems. We have derived a novel variance reduction method that minimizes the number of required particles from an optimization model. This implicitly reduces the variance when calculating the expected value of the distribution, since for a fixed error the  optimization model ensures that a minimal number of particles are needed. Techniques that reduce the noise by improving the order of convergence, have also been considered. Two different methods have been tested on a neutral beam injection scenario. The methods are the scrambled Brownian bridge method and a method here called the sorting and mixing method of L´ecot and Khettabi[1999]. Both methods converge faster than the standard Monte Carlo method for modest number of time steps, but fail to converge correctly for large number of time steps, a range required for detailed plasma kinetic simulations. Different techniques are discussed that have the potential of improving the convergence to this range of time steps.

  • 296.
    Höök, Lars Josef
    et al.
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Johnson, Thomas
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Hellsten, Torbjörn
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Multilevel Monte Carlo and control-variate simulation of Coulomb collisionsManuscript (preprint) (Other academic)
  • 297.
    Höök, Lars Josef
    et al.
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Johnson, Thomas
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Hellsten, Torbjörn
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Randomized quasi-Monte Carlo simulation of fast-ion thermalization2012In: Computational Science & Discovery, ISSN 1749-4680, E-ISSN 1749-4699, Vol. 5, no 1, p. 014010-Article in journal (Refereed)
    Abstract [en]

    This work investigates the applicability of the randomized quasi-Monte Carlo method for simulation of fast-ion thermalization processes in fusion plasmas, e.g. for simulation of neutral beam injection and radio frequency heating. In contrast to the standard Monte Carlo method, the quasi-Monte Carlo method uses deterministic numbers instead of pseudo-random numbers and has a statistical weak convergence close to O(N -1), where N is the number of markers. We have compared different quasi-Monte Carlo methods for a neutral beam injection scenario, which is solved by many realizations of the associated stochastic differential equation, discretized with the Euler-Maruyama scheme. The statistical convergence of the methods is measured for time steps up to 2 14.

  • 298.
    Höök, Lars Josef
    et al.
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Johnson, Thomas
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Äkäslompolo, S.
    Composition schemes forthe stochastic differential equation describing collisional pitch-angle diffusionArticle in journal (Other academic)
  • 299. Ingesson, L. C.
    et al.
    Campbell, D. J.
    Cecconello, Marco
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Ciattaglia, E.
    Dirken, P.
    Hawkes, N.
    Walsh, M.
    von Hellermann, M.
    Kalvin, S.
    McCarthy, P.
    Neubauer, O.
    Petrizzi, L.
    Progress on common aspects of the EU-supplied ITER diagnostics and prediction of diagnostic performance2006In: Review of Scientific Instruments, ISSN 0034-6748, E-ISSN 1089-7623, Vol. 77, no 10, p. 10F502-Article in journal (Refereed)
    Abstract [en]

    The European Union will supply a number of diagnostics for ITER. Significant progress has been made on the design and integration of these diagnostics. Specific topics, in common for several of the diagnostics, discussed are port-plug integration, simplification of labyrinths against neutron streaming, and design measures to protect and to be able to replace sensitive optical components. Performance analysis to predict the likely capability of the diagnostics to meet the ITER measurement requirements will be an important aspect of the design process. The interpretation of the measurement requirements, specifically of spatial resolution in the case of indirect measurements, is discussed on the basis of two examples, and methods of performance analysis are compared.

  • 300.
    Ivanova, Darya
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Fuel retention and fuel removal from first wall components in tokamaks2011Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Fuel inventory and generation of carbon and metal dust in a tokamak are perceived to be serious safety and economy issues for a steady-state operation of a fusion reactor, e.g. ITER. These topics have been explored in the on-going Ph.D. work in order to contribute to the better understanding and development of methods for controlling and curtailing fuel accumulation and dust formation in controlled fusion devices. The work was carried out with material facing fusion plasmas in three tokamaks: TEXTOR in Forschungszentrum Jülich (Germany), Tore Supra in Cadarache (France) and JET in Culham Centre for Fusion Energy (United Kingdom).

     This thesis provides an account on studies of fuel removal techniques from plasmafacing components (PFCs) and on consequences of dust formation. Following issues are addressed: (a)  properties of carbon and metal dust formed in the TEXTOR tokamak;  (b)  dust generation associated with removal of fuel and co-deposited layers from carbon PFCs from TEXTOR and Tore Supra;  (c)  surface morphology of wall components after different cleaning treatments;  (d)  surface properties of diagnostic mirrors tested at JET for ITER. The study dealt with carbon, tungsten and beryllium, i.e. with the three major elements being used for PFC in present-day devices and foreseen for a next-step machine.

     Some essential results are summarised by the following.

     (i)  The amount of loose dust found on the floor of the TEXTOR liner does not exceed 2 grams with particle size range 0.1 mm – 1 mm. The presence of fine (up to 1 mm) crystalline graphite in the collected matter suggests that brittle destruction of carbon PFC could take place during off-normal events. Carbon is the main component, but there are also magnetic and non-magnetic metal agglomerates. The results obtained strongly indicate that in a carbon wall machine the disintegration of flaking co-deposits on PFC is the main source of dust:  (ii)  The fuel content in dust and co-deposits varies from 10% on the main limiters to 0.03% on the neutralizer plates as determined by thermal desorption and ionbeam methods:  (iii)  Fuel removal by annealing in vacuum or by oxidative methods disintegrates codeposits. In the case of thick layers, the treatment makes them brittle thus reducing the adherence to the target and, as a consequence, this leads to the formation of dust:  (iv)   Application of thermal methods for fuel removal from carbon-rich layers is effective only at high temperatures (above 800 K), i.e. in the range exceeding the allowed baking temperature of the ITER divertor:  (v)   Photonic cleaning by laser pulses effectively removes fuel-rich deposited layers, but it also produces debris, especially under ablation conditions:  (vi)  Photonic cleaning of mirrors exposed in JET results in partial recovery of reflectivity, but surfaces are modified by laser pulses.

    The presentation of results is accompanied by a discussion of their consequences for the future development and the application of fuel and dust removal methods in a next-step fusion device.

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