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  • 1. Asunta, O.
    et al.
    Coelho, R.
    Kalupin, D.
    Johnson, Thomas
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Franke, T.
    Predictions of neutral beam current drive in DEMO using BBNBI and ASCOT within the European Transport Simulator2015Ingår i: 42nd European Physical Society Conference on Plasma Physics, EPS 2015, European Physical Society (EPS) , 2015Konferensbidrag (Refereegranskat)
    Abstract [en]

    It was demonstrated that BBNBI and ASCOT are capable of flexible and sophisticated modelling of NBI as a part of an ETS simulation. The simulations showed that tilting the DEMO beam vertically results in a few per cent higher current drive than comparable horizontal tilts.

  • 2. Baiocchi, B.
    et al.
    Calabrò, G.
    Lauro-Taroni, L.
    Mantica, P.
    Cardinali, A.
    Corrigan, G.
    Crisanti, F.
    Farina, D.
    Figini, L.
    Giruzzi, G.
    Johnson, Thomas
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Marinucci, M.
    Parail, V.
    Predictive modelling of H-mode and steady-state scenarios in FAST2010Ingår i: 37th EPS Conference on Plasma Physics 2010, EPS 2010: Volume 1, 2010, s. 293-296Konferensbidrag (Refereegranskat)
  • 3. Baiocchi, B.
    et al.
    Mantica, P.
    Giroud, C.
    Johnson, Thomas
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Naulin, V.
    Salmi, A.
    Tala, T.
    Tsalas, M.
    Discriminating the role of rotation and its gradient in determining ion stiffness mitigation in JET2013Ingår i: Plasma Physics and Controlled Fusion, ISSN 0741-3335, E-ISSN 1361-6587, Vol. 55, nr 2, s. 025010-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Starting from recent JET experimental results that show a significant reduction of ion stiffness in the plasma core region due to plasma rotation in the presence of low magnetic shear, an experiment was carried out at JET in order to separate the role of rotation and rotation gradient in mitigating the ion stiffness level. Enhanced toroidal field ripple (up to 1.5%) and external resonant magnetic fields are the two mechanisms used to try and decouple the rotation value from its gradient. In addition, shots with reversed toroidal field and plasma current, yielding counter-current neutral beam injection, were compared with standard co-injection cases. These tools also allowed varying the rotation independently of the injected power. Shots with high rotation gradient are found to maintain their low stiffness level even when the absolute value of the rotation was significantly reduced. Conversely, high but flat rotation yields much less peaked ion temperature profiles than a peaked rotation profile with lower values. This behaviour suggests the rotation gradient as the main player in reducing the ion stiffness level. In addition, it is found that inverting the rotation gradient sign does not suppress its effect on ion stiffness.

  • 4.
    Bergkvist, Tommy
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Hellsten, Torbjörn
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Johnson, Thomas
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Self-consistent study of fast particle redistribution by Alfvén eigenmodes during Ion cyclotron resonance heating2005Ingår i: Proceedings of the 9th IAEA Technical Meeting on Energetic Particles in Magnetic Confinement Systems, 2005, s. 14-20Konferensbidrag (Refereegranskat)
  • 5. Berk, H. L.
    et al.
    Boswell, C. J.
    Borba, D.
    Figueiredo, A. C. A.
    Johnson, Thomas J.
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Nave, M. F. F.
    Pinches, S. D.
    Sharapov, S. E.
    Explanation of the JET n=0 chirping mode2006Ingår i: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 46, nr 10, s. S888-S897Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Persistent rapid up and down frequency chirping modes with a toroidal mode number of zero (n = 0) are observed in the JET tokamak when energetic ions, in the range of several hundred keV, are created by high field side ion cyclotron resonance frequency heating. Fokker-Planck calculations demonstrate that the heating method enables the formation of an energetically inverted ion distribution which supplies the free energy for the ions to excite a mode related to the geodesic acoustic mode. The large frequency shifts of this mode are attributed to the formation of phase space structures whose frequencies, which are locked to an ion orbit bounce resonance frequency, are forced to continually shift so that energetic particle energy can be released to counterbalance the energy dissipation present in the background plasma.

  • 6. Bilato, R.
    et al.
    Bertelli, N.
    Brambilla, M.
    Dumont, R.
    Jaeger, E. F.
    Johnson, Thomas
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Lerche, E.
    Sauter, O.
    Van Eester, D.
    Villard, L.
    Status of the benchmark activity of ICRF full-wave codes within EUROfusion WPCD and beyond2015Ingår i: RADIOFREQUENCY POWER IN PLASMAS, American Institute of Physics (AIP), 2015, artikel-id UNSP 060001Konferensbidrag (Refereegranskat)
    Abstract [en]

    As follow-up of the benchmark activity of ICRF full-wave codes within the EUROfusion Code Development for Integrated Modelling project (WPCD), a simple-to-complex approach has been devised for verification of the European ICRF codes, imported in the European-Integrated Modelling infrastructure, which represents a unique environment for input-data sharing and result analysis. This benchmark activity has been recently extended to non-European codes, in particular the ICRF full-wave AORSA code. Here we discussed the results of this benchmark.

  • 7. Bonanomi, N.
    et al.
    Mantica, P.
    Di Siena, A.
    Delabie, E.
    Giroud, C.
    Johnson, Thomas
    KTH.
    Lerche, E.
    Menmuir, S.
    Tsalas, M.
    Van Eester, D.
    Turbulent transport stabilization by ICRH minority fast ions in low rotating JET ILW L-mode plasmas2018Ingår i: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 58, nr 5, artikel-id 056025Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The first experimental demonstration that fast ion induced stabilization of thermal turbulent transport takes place also at low values of plasma toroidal rotation has been obtained in JET ILW (ITER-like wall) L-mode plasmas with high (He-3)-D ICRH (ion cyclotron resonance heating) power. A reduction of the gyro-Bohm normalized ion heat flux and higher values of the normalized ion temperature gradient have been observed at high ICRH power and low NBI (neutral beam injection) power and plasma rotation. Gyrokinetic simulations indicate that ITG (ion temperature gradient) turbulence stabilization induced by the presence of high-energetic He-3 ions is the key mechanism in order to explain the experimental observations. Two main mechanisms have been identified to be responsible for the turbulence stabilization: a linear electrostatic wave-fast particle resonance mechanism and a nonlinear electromagnetic mechanism. The dependence of the stabilization on the He-3 distribution function has also been studied.

  • 8. Boswell, C. J.
    et al.
    Berk, H. L.
    Borba, D. N.
    Johnson, Thomas J.
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Pinches, S. D.
    Sharapov, S. E.
    Observation and explanation of the JET n=0 chirping mode2006Ingår i: Physics Letters A, ISSN 0375-9601, E-ISSN 1873-2429, Vol. 358, nr 2, s. 154-158Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Persistent rapid up and down frequency chirping modes with a toroidal mode number of zero (n = 0) have been observed in the JET tokamak when energetic ions, with a mean energy similar to 500 keV, were created by high field side ion cyclotron resonance frequency heating. This heating method enables the formation of an energetically inverted ion distribution function that allows ions to spontaneously excite the observed instability, identified as a global geodesic acoustic mode. The interpretation is that phase space structures form and interact with the fluid zonal flow to produce the pronounced frequency chirping.

  • 9. Budny, R. V.
    et al.
    Indireshkumar, K.
    McCune, D.
    Mayoral, M. -L
    Ongena, J.
    Van Eester, D.
    Conboy, J.
    Voitsekhovitch, I.
    Johnson, Thomas Joe
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Sartori, R.
    Progress testing TRANSP-TORIC simulations of ICRH in JET2009Ingår i: 36th EPS Conference on Plasma Physics 2009, EPS 2009 - Europhysics Conference Abstracts, 2009, s. 1455-1458Konferensbidrag (Refereegranskat)
  • 10. Chapman, I. T.
    et al.
    Graves, J. P.
    Johnson, Thomas
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Asunta, O.
    Bonoli, P.
    Choi, M.
    Jaeger, E. F.
    Jucker, M.
    Sauter, O.
    Sawtooth control in ITER using ion cyclotron resonance heating2011Ingår i: Plasma Physics and Controlled Fusion, ISSN 0741-3335, E-ISSN 1361-6587, Vol. 53, nr 12, s. 124003-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Numerical modelling of the effects of ion cyclotron resonance heating (ICRH) on the stability of the internal kink mode suggests that ICRH should be considered as an essential sawtooth control tool in ITER. Sawtooth control using ICRH is achieved by directly affecting the energy of the internal kink mode rather than through modification of the magnetic shear by driving localized currents. Consequently, ICRH can be seen as complementary to the planned electron cyclotron current drive actuator, and indeed will improve the efficacy of current drive schemes. Simulations of the ICRH distribution using independent RF codes give confidence in numerical predictions that the stabilizing influence of the fusion-born alphas can be negated by appropriately tailored minority (3)He ICRH heating in ITER. Finally, the effectiveness of all sawtooth actuators is shown to increase as the q = 1 surface moves towards the manetic axis, whilst the passive stabilization arising from the alpha and NBI particles decreases.

  • 11. Chapman, I. T.
    et al.
    Graves, J. P.
    Johnson, Thomas
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Jaeger, E. F.
    Jucker, M.
    Sauter, O.
    Sawtooth Control in ITER using Ion Cyclotron Resonance Heating2011Ingår i: Proceedings of the EPS Conference on Plasma Physics, 2011Konferensbidrag (Övrigt vetenskapligt)
    Abstract [en]

    Numerical modelling of the effects of ion cyclotron resonance heating (ICRH) on the stability of the internal kink mode suggests that ICRH should be considered as an essential sawtooth control tool in ITER. Sawtooth control using ICRH is achieved by directly affecting the energy of the kink mode rather than through modification of the magnetic shear by driving localised currents. Consequently, ICRH can be seen as complementary to the planned electron cyclotron current drive actuator. Simulations of the ICRH distribution using independent RF codes give confidence in numerical predictions that the stabilising influence of the fusion-born alphas can be negated by appropriately tailored minority 3He ICRH heating in ITER.

  • 12. Chapman, I. T.
    et al.
    Graves, J. P.
    Lennholm, M.
    Faustin, J.
    Lerche, E.
    Johnson, Thomas
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik. EUROfusion Consortium, England.
    Tholerus, Simon
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik. EUROfusion Consortium, England.
    The merits of ion cyclotron resonance heating schemes for sawtooth control in tokamak plasmas2015Ingår i: Journal of Plasma Physics, ISSN 0022-3778, E-ISSN 1469-7807, Vol. 81, nr 06, artikel-id 365810601Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    JET experiments have compared the efficacy of low-and high field side ion cyclotron resonance heating (ICRH) as an actuator to deliberately minimise the sawtooth period. It is found that low-field side ICRH with low minority concentration is optimal for saw tooth control for two main reasons. Firstly, low-field side heating means that any toroidal phasing of the ICRH (-90 degrees, +90 degrees or dipole) has a destabilising effect on the sawteeth, meaning that dipole phasing can be employed, since tins is preferable due to less plasma wall interaction from Resonant Frequency (RI) sheaths. Secondly, the resonance position of the low field side ICRH does not have to be very accurately placed to achieve saw tooth control, relaxing the requirement for real-time control of the RF frequency. These empirical observations have been confirmed by hybrid kinetic-magnetohydrodynamic modelling, and suggest that the ICRH antenna design for ITER is well positioned to provide a control actuator capable of having a significant effect on the sawtooth behaviour.

  • 13. Chapman, I. T.
    et al.
    Graves, J. P.
    Sauter, O.
    Zucca, C.
    Asunta, O.
    Buttery, R. J.
    Coda, S.
    Goodman, T.
    Igochine, V.
    Johnson, Thomas
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Jucker, M.
    La Haye, R. J.
    Lennholm, M.
    Power requirements for electron cyclotron current drive and ion cyclotron resonance heating for sawtooth control in ITER2013Ingår i: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 53, nr 6, s. 066001-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    13MW of electron cyclotron current drive (ECCD) power deposited inside the q = 1 surface is likely to reduce the sawtooth period in ITER baseline scenario below the level empirically predicted to trigger neoclassical tearing modes (NTMs). However, since the ECCD control scheme is solely predicated upon changing the local magnetic shear, it is prudent to plan to use a complementary scheme which directly decreases the potential energy of the kink mode in order to reduce the sawtooth period. In the event that the natural sawtooth period is longer than expected, due to enhanced a particle stabilization for instance, this ancillary sawtooth control can be provided from >10MW of ion cyclotron resonance heating (ICRH) power with a resonance just inside the q = 1 surface. Both ECCD and ICRH control schemes would benefit greatly from active feedback of the deposition with respect to the rational surface. If the q = 1 surface can be maintained closer to the magnetic axis, the efficacy of ECCD and ICRH schemes significantly increases, the negative effect on the fusion gain is reduced, and off-axis negative-ion neutral beam injection (NNBI) can also be considered for sawtooth control. Consequently, schemes to reduce the q = 1 radius are highly desirable, such as early heating to delay the current penetration and, of course, active sawtooth destabilization to mediate small frequent sawteeth and retain a small q = 1 radius. Finally, there remains a residual risk that the ECCD + ICRH control actuators cannot keep the sawtooth period below the threshold for triggering NTMs (since this is derived only from empirical scaling and the control modelling has numerous caveats). If this is the case, a secondary control scheme of sawtooth stabilization via ECCD + ICRH + NNBI, interspersed with deliberate triggering of a crash through auxiliary power reduction and simultaneous pre-emptive NTM control by off-axis ECCD has been considered, permitting long transient periods with high fusion gain. The power requirements for the necessary degree of sawtooth control using either destabilization or stabilization schemes are expected to be within the specification of anticipated ICRH and ECRH heating in ITER, provided the requisite power can be dedicated to sawtooth control.

  • 14. Chapman, I. T.
    et al.
    Liu, Y. Q.
    Asunta, O.
    Graves, J. P.
    Johnson, Thomas
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Jucker, M.
    Kinetic damping of resistive wall modes in ITER2012Ingår i: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 19, nr 5, s. 052502-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Full drift kinetic modelling including finite orbit width effects has been used to assess the passive stabilisation of the resistive wall mode (RWM) that can be expected in the ITER advanced scenario. At realistic plasma rotation frequency, the thermal ions have a stabilising effect on the RWM, but the stability limit remains below the target plasma pressure to achieve Q = 5. However, the inclusion of damping arising from the fusion-born alpha particles, the NBI ions, and ICRH fast ions extends the RWM stability limit above the target beta for the advanced scenario. The fast ion damping arises primarily from finite orbit width effects and is not due to resonance between the particle frequencies and the instability.

  • 15. Chapman, I. T.
    et al.
    Pinches, S. D.
    Graves, J. P.
    Akers, R. J.
    Appel, L. C.
    Budny, R. V.
    Coda, S.
    Conway, N. J.
    de Bock, M.
    Eriksson, L-G
    Hastie, R. J.
    Hender, T. C.
    Huysmans, G. T. A.
    Johnson, Thomas J.
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Koslowski, H. R.
    Kraemer-Flecken, A.
    Lennholm, M.
    Liang, Y.
    Saarelma, S.
    Sharapov, S. E.
    Voitsekhovitch, I.
    The physics of sawtooth stabilization2007Ingår i: Plasma Physics and Controlled Fusion, ISSN 0741-3335, E-ISSN 1361-6587, Vol. 49, nr 12B, s. B385-B394Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Long period sawteeth have been observed to result in low-beta triggering of neo-classical tearing modes, which can significantly degrade plasma confinement. Consequently, a detailed physical understanding of sawtooth behaviour is critical, especially for ITER where fusion-born a particles are likely to lead to very long sawtooth periods. Many techniques have been developed to control, and in particular to destabilize the sawteeth. The application of counter-current neutral beam injection (NBI) in JET has resulted in shorter sawtooth periods than in Ohmic plasmas. This result has been explained because, firstly, the counter-passing fast ions give a destabilizing contribution to the n=1 internal kink mode-which is accepted to be related to sawtooth oscillations-and secondly, the flow shear strongly influences the stabilizing trapped particles. A similar experimental result has been observed in counter-NBI heated plasmas in MAST. However, the strong toroidal flows in spherical tokamaks mean that the sawtooth behaviour is determined by the gyroscopic flow stabilization of the kink mode rather than kinetic effects. In NBI heated plasmas in smaller conventional aspect-ratio tokamaks, such as TEXTOR, the flow and kinetic effects compete to give different sawtooth behaviour. Other techniques applied to destabilize sawteeth are the application of electron cyclotron current drive (ECCD) or ion cyclotron resonance heating (ICRH). In JET, it has been observed that localized ICRH is able to destabilize sawteeth which were otherwise stabilized by a co-existing population of energetic trapped ions in the core. This is explained through the dual role of the ICRH in reducing the critical magnetic shear required to trigger a sawtooth crash, and the increase in the local magnetic shear which results from driving current near the q=1 rational surface. Sawtooth control in ITER could be provided by a combination of ECCD and co-passing off-axis negative-NBI fast ions.

  • 16. Citrin, J.
    et al.
    Garcia, J.
    Görier, T.
    Jenko, F.
    Mantica, P.
    Told, D.
    Bourdelle, C.
    Hatch, D. R.
    Hogeweij, G. M. D.
    Johnson, Thomas
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Pueschel, M. J.
    Schneider, M.
    Electromagnetic stabilization of tokamak microturbulence in a high-β regime2014Ingår i: 41st EPS Conference on Plasma Physics, EPS 2014, European Physical Society (EPS) , 2014Konferensbidrag (Refereegranskat)
    Abstract [en]

    The impact of electromagnetic stabilization and flow shear stabilization on ITG turbulence is investigated. Analysis of a low-β JET L-mode discharge illustrates the relation between ITG stabilization, and proximity to the electromagnetic instability threshold. This threshold is reduced by suprathermal pressure gradients, highlighting the effectiveness of fast ions in ITG stabilization. Extensive linear and nonlinear gyrokinetic simulations are then carried out for the high-β JET hybrid discharge 75225, at two separate locations at inner and outer radii. It is found that at the inner radius, nonlinear electromagnetic stabilization is dominant, and is critical for achieving simulated heat fluxes in agreement with the experiment. The enhancement of this effect by suprathermal pressure also remains significant. It is also found that flow shear stabilization is not effective at the inner radii. However, at outer radii the situation is reversed. Electromagnetic stabilization is negligible while the flow shear stabilization is significant. These results constitute the high-β generalization of comparable observations found at low-β at JET. This is encouraging for the extrapolation of electromagnetic ITG stabilization to future devices. An estimation of the impact of this effect on the ITER hybrid scenario leads to a 20% fusion power improvement.

  • 17. Citrin, J.
    et al.
    Garcia, J.
    Görler, T.
    Jenko, F.
    Mantica, P.
    Told, D.
    Bourdelle, C.
    Hatch, D. R.
    Hogeweij, G. M. D.
    Johnson, Thomas
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Pueschel, M. J.
    Schneider, M.
    Electromagnetic stabilization of tokamak microturbulence in a high-beta regime2015Ingår i: Plasma Physics and Controlled Fusion, ISSN 0741-3335, E-ISSN 1361-6587, Vol. 57, nr 1, s. 014032-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The impact of electromagnetic stabilization and flow shear stabilization on ITG turbulence is investigated. Analysis of a low-beta JET L-mode discharge illustrates the relation between ITG stabilization and proximity to the electromagnetic instability threshold. This threshold is reduced by suprathermal pressure gradients, highlighting the effectiveness of fast ions in ITG stabilization. Extensive linear and nonlinear gyrokinetic simulations are then carried out for the high-beta JET hybrid discharge 75225, at two separate locations at inner and outer radii. It is found that at the inner radius, nonlinear electromagnetic stabilization is dominant and is critical for achieving simulated heat fluxes in agreement with the experiment. The enhancement of this effect by suprathermal pressure also remains significant. It is also found that flow shear stabilization is not effective at the inner radii. However, at outer radii the situation is reversed. Electromagnetic stabilization is negligible while the flow shear stabilization is significant. These results constitute the high-beta generalization of comparable observations found at low-beta at JET. This is encouraging for the extrapolation of electromagnetic ITG stabilization to future devices. An estimation of the impact of this effect on the ITER hybrid scenario leads to a 20% fusion power improvement.

  • 18. Citrin, J.
    et al.
    Jenko, F.
    Mantica, P.
    Told, D.
    Bourdelle, C.
    Dumont, R.
    Garcia, J.
    Haverkort, J. W.
    Hogeweij, G. M. D.
    Johnson, Thomas
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Pueschel, M. J.
    Ion temperature profile stiffness: non-linear gyrokinetic simulations and comparison with experiment2014Ingår i: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 54, nr 2, s. 023008-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Recent experimental observations at JET show evidence of reduced ion temperature profile stiffness. An extensive set of nonlinear gyrokinetic simulations are performed based on the experimental discharges, investigating the physical mechanism behind the observations. The impact on the ion heat flux of various parameters that differ within the data-set are explored. These parameters include the safety factor, magnetic shear, toroidal flow shear, effect of rotation on the magnetohydrodynamic equilibrium, R/L-n, beta(e), Z(eff), T-e/T-i, and the fast-particle content. While previously hypothesized to be an important factor in the stiffness reduction, the combined effect of toroidal flow shear and low magnetic shear is not predicted by the simulations to lead to a significant reduction in ion heat flux, due both to an insufficient magnitude of flow shear and significant parallel velocity gradient destabilization. It is however found that nonlinear electromagnetic effects due to both thermal and fast-particle pressure gradients, even at low beta(e), can significantly reduce the ion heat flux, and is a key factor in explaining the experimental observations. A total of four discharges are examined, at both inner and outer radii. For all cases studied, the simulated and experimental ion heat flux values agree within reasonable variations of input parameters around the experimental uncertainties.

  • 19. Citrin, J.
    et al.
    Jenko, F.
    Mantica, P.
    Told, D.
    Bourdelle, C.
    Garcia, J.
    Haverkort, J. W.
    Hogeweij, G. M. D.
    Johnson, Thomas
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Pueschel, M. J.
    Nonlinear stabilization of tokamak microturbulence by fast ions2013Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 111, nr 15, s. 155001-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Nonlinear electromagnetic stabilization by suprathermal pressure gradients found in specific regimes is shown to be a key factor in reducing tokamak microturbulence, augmenting significantly the thermal pressure electromagnetic stabilization. Based on nonlinear gyrokinetic simulations investigating a set of ion heat transport experiments on the JET tokamak, described by Mantica et al., this result explains the experimentally observed ion heat flux and stiffness reduction. These findings are expected to improve the extrapolation of advanced tokamak scenarios to reactor relevant regimes.

  • 20. Coda, S.
    et al.
    Eriksson, L. -G
    Lennholm, M.
    Graves, J.
    Johnson, Thomas J.
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Brzozowski, Jerzy H.
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    DeBaar, M.
    Howell, D. F.
    Jachmich, S.
    Kiptily, V.
    Koslowski, R.
    Mayoral, M. -L
    Mueck, A.
    Pinches, S.
    Saibene, G.
    Santala, M. I. K.
    Stamp, M. F.
    Valisa, M.
    NTM prevention by ICCD control of fast-ion-stabilised sawteeth2007Ingår i: 34th EPS Conference on Plasma Physics 2007, EPS 2007 - Europhysics Conference Abstracts, 2007, nr 3, s. 2138-2141Konferensbidrag (Refereegranskat)
  • 21. de Vries, P. C.
    et al.
    Salmi, A.
    Parail, V.
    Giroud, C.
    Andrew, Y.
    Biewer, T. M.
    Crombe, K.
    Jenkins, I.
    Johnson, Thomas J.
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Kiptily, V.
    Loarte, A.
    Lonnroth, J.
    Meigs, A.
    Oyama, N.
    Sartori, R.
    Saibene, G.
    Urano, H.
    Zastrow, K. D.
    Effect of toroidal field ripple on plasma rotation in JET2008Ingår i: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 48, nr 3Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Dedicated experiments on TF ripple effects on the performance of tokamak plasmas have been carried out at JET. The TF ripple was found to have a profound effect on the plasma rotation. The central Mach number, M, defined as the ratio of the rotation velocity and the thermal velocity, was found to drop as a function of TF ripple amplitude (3) from an average value of M = 0.40-0.55 for operations at the standard JET ripple of 6 = 0.08% to M = 0.25-0.40 for 6 = 0.5% and M = 0.1-0.3 for delta = 1%. TF ripple effects should be considered when estimating the plasma rotation in ITER. With standard co-current injection of neutral beam injection (NBI), plasmas were found to rotate in the co-current direction. However, for higher TF ripple amplitudes (delta similar to 1%) an area of counter rotation developed at the edge of the plasma, while the core kept its co-rotation. The edge counter rotation was found to depend, besides on the TF ripple amplitude, on the edge temperature. The observed reduction of toroidal plasma rotation with increasing TF ripple could partly be explained by TF ripple induced losses of energetic ions, injected by NBI. However, the calculated torque due to these losses was insufficient to explain the observed counter rotation and its scaling with edge parameters. It is suggested that additional TF ripple induced losses of thermal ions contribute to this effect.

  • 22. De Vries, P. C.
    et al.
    Salmi, A.
    Parail, V.
    Giroud, C.
    Andrew, Y.
    Biewer, T. M.
    Crombé, K.
    Jenkins, I.
    Johnson, Thomas J.
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Kiptily, V.
    Loarte, A.
    Lönnroth, J.
    Meigs, A.
    Oyama, N.
    Sartori, R.
    Saibene, G.
    Urano, H.
    Zastrow, K. -D
    Effect of toroidal field ripple on plasma rotation in JET2007Ingår i: 34th EPS Conference on Plasma Physics 2007, EPS 2007 - Europhysics Conference Abstracts, 2007, nr 1, s. 603-606Konferensbidrag (Refereegranskat)
  • 23. Di Siena, A.
    et al.
    Görier, T.
    Doerk, H.
    Citrin, J.
    Johnson, Thomas
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Schneider, M.
    Poli, E.
    Non-Maxwellian background effects in gyrokinetic simulations with GENE2016Ingår i: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 775, nr 1, artikel-id 012003Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The interaction between fast particles and core turbulence has been established as a central issue for a tokamak reactor. Recent results predict significant enhancement of electromagnetic stabilisation of ITG turbulence in the presence of fast ions. However, most of these simulations were performed with the assumption of equivalent Maxwellian distributed particles, whereas to rigorously model fast ions, a non-Maxwellian background distribution function is needed. To this aim, the underlying equations in the gyrokinetic code GENE have been re-derived and implemented for a completely general background distribution function. After verification studies, a previous investigation on a particular JET plasma has been revised with linear simulations. The plasma is composed by Deuterium, electron, Carbon impurities, NBI fast Deuterium and ICRH 3He. Fast particle distributions have been modelled with a number of different analytic choices in order to study the impact of non-Maxwellian distributions on the plasma turbulence: slowing down and anisotropic Maxwellian. Linear growth rates are studied as a function of the wave number and compared with those obtained using an equivalent Maxwellian. Generally, the choice of the 3He distribution seems to have a stronger impact on the microinstabilities than that of the fast Deuterium.

  • 24. Eriksson, J.
    et al.
    Nocente, M.
    Binda, F.
    Cazzaniga, C.
    Conroy, S.
    Ericsson, G.
    Giacomelli, L.
    Gorini, G.
    Hellesen, C.
    Hellsten, Torbjörn
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Hjalmarsson, A.
    Jacobsen, A. S.
    Johnson, Thomas
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Kiptily, V.
    Koskela, T.
    Mantsinen, M.
    Salewski, M.
    Schneider, M.
    Sharapov, S.
    Skiba, M.
    Tardocchi, M.
    Weiszflog, M.
    Dual sightline measurements of MeV range deuterons with neutron and gamma-ray spectroscopy at JET2015Ingår i: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 55, nr 12, artikel-id 123026Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Observations made in a JET experiment aimed at accelerating deuterons to the MeV range by third harmonic radio-frequency (RF) heating coupled into a deuterium beam are reported. Measurements are based on a set of advanced neutron and gamma-ray spectrometers that, for the first time, observe the plasma simultaneously along vertical and oblique lines of sight. Parameters of the fast ion energy distribution, such as the high energy cut-off of the deuteron distribution function and the RF coupling constant, are determined from data within a uniform analysis framework for neutron and gamma-ray spectroscopy based on a one-dimensional model and by a consistency check among the individual measurement techniques. A systematic difference is seen between the two lines of sight and is interpreted to originate from the sensitivity of the oblique detectors to the pitch-angle structure of the distribution around the resonance, which is not correctly portrayed within the adopted one dimensional model. A framework to calculate neutron and gamma-ray emission from a spatially resolved, two-dimensional deuteron distribution specified by energy/pitch is thus developed and used for a first comparison with predictions from ab initio models of RF heating at multiple harmonics. The results presented in this paper are of relevance for the development of advanced diagnostic techniques for MeV range ions in high performance fusion plasmas, with applications to the experimental validation of RF heating codes and, more generally, to studies of the energy distribution of ions in the MeV range in high performance deuterium and deuterium-tritium plasmas.

  • 25. Eriksson, L. -G
    et al.
    Hellsten, Torbjörn A K
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Holmström, Kerstin
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Johnson, Thomas Joe
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Brzozowski, Jerzy H.
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Nave, F.
    Ongena, J.
    Zastrow, K. -D
    Simulation of fast ion contribution to toroidal rotation in ICRF heated jet plasmas2008Ingår i: EPS Conf. Plasma Phys., EPS - Europhys. Conf. Abstr., 2008, nr 3, s. 1679-1682Konferensbidrag (Refereegranskat)
  • 26. Eriksson, L. G.
    et al.
    Hellsten, Torbjörn A. K.
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Nave, M. F. F.
    Brzozowski, Jerzy H.
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Holmström, Kerstin
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Johnson, Thomas
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Ongena, J.
    Zastrow, K. D.
    Holmström, K.
    Toroidal rotation in RF heated JET plasmas2009Ingår i: Plasma Physics and Controlled Fusion, ISSN 0741-3335, E-ISSN 1361-6587, Vol. 51, nr 4Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Observations of bulk plasma rotation in radio frequency (RF) heated JET discharges are reported. This study is concentrated on RF heated L-mode plasmas. In particular, the toroidal rotation profiles in plasmas heated by ion cyclotron resonance frequency (ICRF) waves and lower hybrid (LH) waves have been analysed. It is the first time that rotation profiles in JET plasmas with LH waves have been measured in dedicated discharges. It is found that the toroidal plasma rotation in the outer region of the plasmas is in the co-current direction irrespective of the heating scenario. An interesting feature is that the toroidal rotation profile appears to be hollow in many discharges at low plasma current, but a low current in itself does not seem to be a sufficient condition for finding such profiles. Fast ion transport and finite orbit width effects are mechanisms that could explain hollow rotation profiles. This possibility has been investigated by numerical simulations of the torque on the bulk plasma due to fast ICRF accelerated ions. The obtained torque is used in a transport equation for the toroidal momentum density to estimate the effect on the thermal bulk plasma rotation profile.

  • 27. Eriksson, L. -G
    et al.
    Hellsten, Torbjörn
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Nave, F.
    Brzozowski, Jerzy
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Holmström, Kerstin
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Johnson, Thomas J.
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Ongena, J.
    Zastrow, K. -D
    Toroidal rotation in RF heated JET plasmas2007Ingår i: RADIO FREQUENCY POWER IN PLASMAS, 2007, Vol. 933, s. 59-62Konferensbidrag (Refereegranskat)
    Abstract [en]

    Experiments have been carried out on JET aimed at studying rotation in RF heated plasmas with low external momentum input. Both plasmas with Ion Cyclotron Resonance Frequency (ICRF) heating and Lower Hybrid Current Drive (LHCD) have been investigated. The rotation profiles are measured by Charge Exchange recombination spectroscopy, using short diagnostic Neutral Beam Injection (NBI) pulses. Moreover, the temporal evolution of the central rotation could in some cases be deduced from MHD activity. While most of the measurements were focussed on ICRF heating, the profiles measured in plasmas with LHCD are interesting since they are the first reported from JET in such plasmas. In particular, they allowed for studies of rotation in RF heated plasmas with q>1. The experimental results are presented together with an analysis of the torque from ICRF heated fast ions.

  • 28. Eriksson, L. G.
    et al.
    Johnson, Thomas J.
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Mayoral, M. L.
    Coda, S.
    Sauter, O.
    Buttery, R. J.
    McDonald, D.
    Hellsten, Torbjörn A. K.
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Mantsinen, M. J.
    Mueck, A.
    Noterdaeme, J. M.
    Santala, M.
    Westerhor, E.
    de Vries, P.
    On ion cyclotron current drive for sawtooth control2006Ingår i: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 46, nr 10, s. S951-S964Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Experiments using ion cyclotron current drive (ICCD) to control sawteeth are presented. In particular, discharges demonstrating shortening of fast ion induced long sawteeth reported in (Eriksson et al 2004 Phys. Rev. Lett. 92 235004) by ICCD have been analysed in detail. Numerical simulations of the ICCD driven currents are shown to be consistent with the experimental observations. They support the hypothesis that an increase in the magnetic shear, due to the driven current, at the surface where the safety factor is unity was the critical factor for the shortening of the sawteeth. In view of the potential utility of ICCD, the mechanisms for the current drive have been further investigated experimentally. This includes the influence of the averaged energy of the resonating ions carrying the current and the spectrum of the launched waves. The results of these experiments are discussed in the light of theoretical considerations.

  • 29. Eriksson, L-G
    et al.
    Johnson, Thomas
    KTH, Tidigare Institutioner, Alfvénlaboratoriet.
    Hellsten, Torbjörn
    KTH, Tidigare Institutioner, Alfvénlaboratoriet.
    Giroud, C
    Kiptily, G
    Kirov, K
    Brzozowski, Jerzy H.
    KTH, Tidigare Institutioner, Alfvénlaboratoriet.
    DeBaar, M
    DeGrassie, J
    Mantsinen, M
    Meigs, A
    Noterdaeme, M
    Staebler, A
    Testa, D
    Tuccillo, A
    Zastrow, D
    Plasma rotation induced by directed waves in the ion-cyclotron range of frequencies2004Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 92, nr 23Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Changes of the toroidal plasma rotation induced by directed waves in the ion-cyclotron range of frequencies (ICRF) have been identified experimentally for the first time on the JET tokamak. The momentum carried by the waves is initially absorbed by fast resonating ions, which subsequently transfer it to the bulk plasma. Thus, the results provide evidence for the influence of ICRF heated fast ions on plasma rotation.

  • 30. Eriksson, L-G
    et al.
    Mueck, A
    Sauter, O
    Coda, S
    Mantsinen, M J
    Mayoral, M-L
    Westerhof, E
    Buttery, R J
    McDonald, D
    Johnson, Thomas
    Euratom-VR Association, Stockholm, Sweden.
    Noterdaeme, J-M
    De Vries, P
    Destabilization of fast-ion-induced long sawteeth by localized current drive in the JET tokamak.2004Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 92, nr 23Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In a tokamak fusion reactor the energetic alpha particles will transiently stabilize the magnetohydrodynamic activity causing sawtooth oscillations. The crash events terminating long sawtooth free periods can provide seed islands for neoclassical tearing modes [Phys. Rev. Lett. 88, 105001 (2002)]]. To shorten the sawtooth periods localized current drive near the q=1 surface is a possibility. This Letter provides the first experimental evidence for the effectiveness of this method in the different physics regime associated with fast-ion-induced long sawteeth on the JET tokamak.

  • 31. Falchetto, G. L.
    et al.
    Coster, D.
    Coelho, R.
    Scott, B. D.
    Figini, L.
    Kalupin, D.
    Nardon, E.
    Nowak, S.
    Alves, L. L.
    Artaud, J. F.
    Basiuk, V.
    Bizarro, Jao P. S.
    Boulbe, C.
    Dinklage, A.
    Farina, D.
    Faugeras, B.
    Ferreira, J.
    Figueiredo, A.
    Huynh, Ph
    Imbeaux, F.
    Ivanova-Stanik, I.
    Johnson, Thomas
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Klingshirn, H-J
    Konz, C.
    Kus, A.
    Marushchenko, N. B.
    Pereverzev, G.
    Owsiak, M.
    Poli, E.
    Peysson, Y.
    Reimer, R.
    Signoret, J.
    Sauter, O.
    Stankiewicz, R.
    Strand, P.
    Voitsekhovitch, I.
    Westerhof, E.
    Zok, T.
    Zwingmann, W.
    The European Integrated Tokamak Modelling (ITM) effort: achievements and first physics results2014Ingår i: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 54, nr 4, s. 043018-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A selection of achievements and first physics results are presented of the European Integrated Tokamak Modelling Task Force (EFDA ITM-TF) simulation framework, which aims to provide a standardized platform and an integrated modelling suite of validated numerical codes for the simulation and prediction of a complete plasma discharge of an arbitrary tokamak. The framework developed by the ITM-TF, based on a generic data structure including both simulated and experimental data, allows for the development of sophisticated integrated simulations (workflows) for physics application.The equilibrium reconstruction and linear magnetohydrodynamic (MHD) stability simulation chain was applied, in particular, to the analysis of the edgeMHDstability of ASDEX Upgrade type-I ELMy H-mode discharges and ITER hybrid scenario, demonstrating the stabilizing effect of an increased Shafranov shift on edge modes. Interpretive simulations of a JET hybrid discharge were performed with two electromagnetic turbulence codes within ITM infrastructure showing the signature of trapped-electron assisted ITG turbulence. A successful benchmark among five EC beam/ray-tracing codes was performed in the ITM framework for an ITER inductive scenario for different launching conditions from the equatorial and upper launcher, showing good agreement of the computed absorbed power and driven current. Selected achievements and scientific workflow applications targeting key modelling topics and physics problems are also presented, showing the current status of the ITM-TF modelling suite.

  • 32. Giacomelli, L.
    et al.
    Hjalmarsson, A.
    Hellesen, C.
    Gatu Johnson, M.
    Källne, J.
    Weiszflog, M.
    Sharapov, S.
    Andersson Sundén, E.
    Conroy, S.
    Ericsson, G.
    Ronchi, E.
    Sjöstrand, H.
    Gorini, G.
    Tardocchi, M.
    Popovichev, S.
    Johnson, Thomas
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Neutron emission spectroscopy diagnosis of fast ions in RF D( 3He) heated plasmas at JET2007Ingår i: 34th EPS Conference on Plasma Physics 2007, EPS 2007 - Europhysics Conference Abstracts, 2007, nr 2, s. 1158-1161Konferensbidrag (Refereegranskat)
  • 33. Giacomelli, L.
    et al.
    Hjalmarsson, A.
    Källne, J.
    Hellesen, C.
    Tardocchi, M.
    Gorini, G.
    Van Eester, D.
    Lerche, E.
    Johnson, Thomas J.
    EURATOM-UKAEA Association, Culham Science Centre, OX14 3DB Abingdon, United Kingdom .
    Kiptily, V.
    Conroy, S.
    Sundén, A.
    Ericsson, G.
    Johnson, G.
    Sjöstrand, H.
    Weiszflog, M.
    Neutron emission spectroscopy results for internal transport barrier and mode conversion ion cyclotron resonance heating experiments at JET2008Ingår i: Review of Scientific Instruments, ISSN 0034-6748, E-ISSN 1089-7623, ISSN 0034-6748, Vol. 79, nr 10, s. 10E514-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The effect of ion cyclotron resonance heating (ICRH) on ((3)He)D plasmas at JET was studied with the time of flight optimized rate (TOFOR) spectrometer dedicated to 2.5 MeV dd neutron measurements. In internal transport barrier (ITB) plasma experiments with large (3)He concentrations (X((3)He)>15%) an increase in neutron yield was observed after the ITB disappeared but with the auxiliary neutral beam injection and ICRH power still applied. The analysis of the TOFOR data revealed the formation of a high energy (fast) D population in this regime. The results were compared to other mode conversion experiments with similar X((3)He) but slightly different heating conditions. In this study we report on the high energy neutron tails originating from the fast D ions and their correlation with X((3)He) and discuss the light it can shed on ICRH-plasma power coupling mechanisms.

  • 34. Graves, J. P.
    et al.
    Chapman, I.
    Coda, S.
    Crombe, K.
    Eriksson, L. -G
    Johnson, Thomas Joe
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Koslowski, R.
    Lennholm, M.
    Mayoral, M. -L
    Nunes, I.
    Sawtooth control mechanism on JET using off-axis toroidally propagating ICRF waves2009Ingår i: 36th EPS Conference on Plasma Physics 2009, EPS 2009 - Europhysics Conference Abstracts, 2009, s. 1511-1514Konferensbidrag (Refereegranskat)
  • 35. Graves, J. P.
    et al.
    Chapman, I.
    Coda, S.
    Eriksson, L. G.
    Johnson, Thomas J.
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Sawtooth-Control Mechanism using Toroidally Propagating Ion-Cyclotron-Resonance Waves in Tokamaks2009Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 102, nr 6Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The sawtooth control mechanism in plasmas employing off-axis toroidally propagating ion cyclotron resonance waves in tokamaks is reinvestigated. The radial drift excursion of energetic passing ions distributed asymmetrically in the velocity parallel to the magnetic field determines stability when the rational q=1 surface resides within a narrow region centered about the shifted fundamental cyclotron resonance.

  • 36. Graves, J. P.
    et al.
    Chapman, I.
    Coda, S.
    Eriksson, L. -G
    Johnson, Thomas Joe
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Sawtooth control mechanism using counter current propagating ICRH in JET2008Ingår i: EPS Conf. Plasma Phys., EPS - Europhys. Conf. Abstr., 2008, nr 3, s. 2010-2013Konferensbidrag (Refereegranskat)
    Abstract [en]

    The sawtooth control mechanism in plasmas employing off-axis ICCD is reinvestigated. In particular, for counter propagating waves on the high field side, asymmetrically distributed energetic passing ions destabilise the ideal internal kink mode when the q=1 surface resides within a narrow region centred about the shifted fundamental cyclotron resonance.

  • 37. Graves, J. P.
    et al.
    Chapman, I. T.
    Coda, S.
    Johnson, Thomas J.
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Lennholm, M.
    A new sawtooth control mechanism relying on toroidally propagating ion cyclotron resonance frequency waves: Theory and Joint European Torus tokamak experimental evidence2010Ingår i: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 17, nr 5, s. 056118-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The sawtooth control mechanism in plasmas employing toroidally propagating ion cyclotron resonance waves is extended. The asymmetrically distributed energetic passing ions are shown to modify the ideal internal kink mode when the position of the minority ion cyclotron resonance resides within a narrow region close to the q=1 surface. An analytical treatment of the internal kink mode in the presence of model distribution function with parallel velocity asymmetry is developed. The fast ion mechanism explains the strong sensitivity of sawteeth to resonance position, and moreover is consistent with dedicated Joint European Torus [F. Romanelli, Nucl. Fusion 49, 104006 (2009)] experiments which controlled sawteeth despite negligible current drive. [doi:10.1063/1.3363201]

  • 38. Graves, J. P.
    et al.
    Chapman, I. T.
    Coda, S.
    Johnson, Thomas J.
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Lennholm, M.
    Paley, J. I.
    Sauter, O.
    RECENT ADVANCES IN SAWTOOTH CONTROL2011Ingår i: Fusion science and technology, ISSN 1536-1055, E-ISSN 1943-7641, Vol. 59, nr 3, s. 539-548Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Important advances have been made recently in the invention and application of experimental methods to control the sawtooth instability in tokamak plasmas. The primary means of control involves the application of either ion cyclotron resonance heating (ICRH), or electron cyclotron heating, with resonance very close to the q = 1 radius in the plasma core. Reported here are experiments that have successfully applied these methods to either shorten or lengthen the sawteeth deliberately, in a variety of plasma conditions, in three tokamaks: Joint European Torus (JET), TCV, and Tore Supra. It is shown that despite the sensitivity of the sawtooth period to the resonance position, sawteeth can be controlled using either real-time control of the electron cyclotron deposition, or in the case of ion cyclotron heating, very careful adjustment of the magnetic field strength and minority ion concentration. The latter technique has been guided by theoretical advances that have enabled the control of sawteeth in JET with ITER-relevant ICRH scenarios.

  • 39. Graves, J. P.
    et al.
    Chapman, I. T.
    Coda, S.
    Johnson, Thomas
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Lennholm, M.
    Alper, B.
    de Baar, M.
    Crombe, K.
    Eriksson, L. -G
    Felton, R.
    Howell, D.
    Kiptily, V.
    Koslowski, H. R.
    Mayoral, M. -L
    Monakhov, I.
    Nunes, I.
    Pinches, S. D.
    Experimental verification of sawtooth control by energetic particles in ion cyclotron resonance heated JET tokamak plasmas2010Ingår i: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 50, nr 5, s. 052002-Artikel i tidskrift (Övrigt vetenskapligt)
    Abstract [en]

    Experimental evidence from the JET tokamak is presented supporting the predictions of a recent theory (Graves et al 2009 Phys. Rev. Lett. 102 065005) on sawtooth instability control by toroidally propagating ion cyclotron resonance waves. Novel experimental conditions minimized a possible alternate effect of magnetic shear modification by ion cyclotron current drive, and enabled the dependence of the new energetic ion mechanism to be tested over key variables. The results have favourable implications on sawtooth control by ion cyclotron resonance waves in a fusion reactor.

  • 40. Graves, J. P.
    et al.
    Chapman, I. T.
    Coda, S.
    Johnson, Thomas
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Lennholm, M.
    Alper, B.
    de Baar, M.
    Crombe, K.
    Eriksson, L.-G.
    Felton, R.
    Howell, D.
    Kiptily, V.
    Koslowski, H. R.
    Mayoral, M.-L.
    Monakov, I.
    Nunes, I.
    Pinches, S. D.
    Sawtooth Control Relying on Toroidally Propagating ICRF Waves2010Konferensbidrag (Övrigt vetenskapligt)
  • 41. Graves, J. P.
    et al.
    Cooper, W. A.
    Coda, S.
    Eriksson, L. -G
    Johnson, Thomas J.
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Modelling ICCD experiments for sawtooth control in JET2006Ingår i: Theory of Fusion Plasmas , 2006, Vol. 871, s. 350-355Konferensbidrag (Refereegranskat)
  • 42. Graves, J. P.
    et al.
    Lennholm, M.
    Chapman, I. T.
    Lerche, E.
    Reich, M.
    Alper, B.
    Bobkov, V.
    Dumont, R.
    Faustin, J. M.
    Jacquet, P.
    Jaulmes, F.
    Johnson, Thomas
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Keeling, D. L.
    Liu, Y.
    Nicolas, T.
    Tholerus, Simon
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Blackman, T.
    Carvalho, I. S.
    Coelho, R.
    Van Eester, D.
    Felton, R.
    Goniche, M.
    Kiptily, V.
    Monakhov, I.
    Nave, M. F. F.
    Von Thun, C. P.
    Sabot, R.
    Sozzi, C.
    Tsalas, M.
    Sawtooth control in JET with ITER relevant low field side resonance ICRH and ITER like wall2014Ingår i: 41st EPS Conference on Plasma Physics, EPS 2014, European Physical Society (EPS) , 2014Konferensbidrag (Refereegranskat)
    Abstract [en]

    New experiments at JET with the ITER like wall show for the first time that ITER-relevant low field side resonance first harmonic ICRH with can be used to control sawteeth that have been initially lengthened by fast particles. In contrast to previous [J. P. Graves et al, Nature Communs 3, 624 (2012)] high field side resonance sawtooth control experiments undertaken at JET, it is found that the sawteeth of L-mode plasmas can be controlled with less accurate alignment between the resonance layer and the sawtooth inversion radius. This advantage, as well as the discovery that sawteeth can be shortened with various antenna phasings, including dipole, indicates that ICRH is a particularly effective and versatile tool that can be used in future fusion machines for controlling sawteeth. Without sawtooth control, NTMs and locked modes were triggered at very low normalised beta. High power H-mode experiments show the extent to which ICRH can be tuned to control sawteeth and NTMs while simultaneously providing effective electron heating with improved flushing of high Z core impurities. Dedicated ICRH simulations using SELFO, SCENIC and EVE, including wide drift orbit effects, explain why sawtooth control is effective with various antenna phasings, and show that the sawtooth control mechanism cannot be explained by enhancement of the magnetic shear. Hybrid kinetic-MHD stability calculations using MISHKA and HAGIS unravel the optimal sawtooth control regimes in these ITER relevant plasma conditions.

  • 43. Graves, J. P.
    et al.
    Lennholm, M.
    Chapman, I. T.
    Lerche, E.
    Reich, M.
    Alper, B.
    Bobkov, V.
    Dumont, R.
    Faustin, J. M.
    Jacquet, P.
    Jaulmes, F.
    Johnson, Thomas
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Keeling, D. L.
    Liu, Yueqiang
    Nicolas, T.
    Tholerus, Simon
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Blackman, T.
    Carvalho, I. S.
    Coelho, R.
    Van Eester, D.
    Felton, R.
    Goniche, M.
    Kiptily, V.
    Monakhov, I.
    Nave, M. F. F.
    von Thun, C. Perez
    Sabot, R.
    Sozzi, C.
    Tsalas, M.
    Sawtooth control in JET with ITER relevant low field side resonance ion cyclotron resonance heating and ITER-like wall2015Ingår i: Plasma Physics and Controlled Fusion, ISSN 0741-3335, E-ISSN 1361-6587, Vol. 57, nr 1, s. 014033-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    New experiments at JET with the ITER-like wall show for the first time that ITER-relevant low field side resonance first harmonic ion cyclotron resonance heating (ICRH) can be used to control sawteeth that have been initially lengthened by fast particles. In contrast to previous (Graves et al 2012 Nat. Commun. 3 624) high field side resonance sawtooth control experiments undertaken at JET, it is found that the sawteeth of L-mode plasmas can be controlled with less accurate alignment between the resonance layer and the sawtooth inversion radius. This advantage, as well as the discovery that sawteeth can be shortened with various antenna phasings, including dipole, indicates that ICRH is a particularly effective and versatile tool that can be used in future fusion machines for controlling sawteeth. Without sawtooth control, neoclassical tearing modes (NTMs) and locked modes were triggered at very low normalised beta. High power H-mode experiments show the extent to which ICRH can be tuned to control sawteeth and NTMs while simultaneously providing effective electron heating with improved flushing of high Z core impurities. Dedicated ICRH simulations using SELFO, SCENIC and EVE, including wide drift orbit effects, explain why sawtooth control is effective with various antenna phasings and show that the sawtooth control mechanism cannot be explained by enhancement of the magnetic shear. Hybrid kinetic-magnetohydrodynamic stability calculations using MISHKA and HAGIS unravel the optimal sawtooth control regimes in these ITER relevant plasma conditions.

  • 44.
    Hannan, Abdul
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Hellsten, Torbjörn
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Johnson, Thomas
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    A Fokker-Planck Code for Fast Self-Consistent Calculations of ICRH2010Ingår i: 37th EPS Conference on Plasma Physics, 2010Konferensbidrag (Övrigt vetenskapligt)
    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

  • 45.
    Hannan, Abdul
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Hellsten, Torbjörn
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Johnson, Thomas
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Fast wave current drive scenarios for DEMO2013Ingår i: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 53, nr 4, s. 043005-Artikel i tidskrift (Refereegranskat)
    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.

  • 46.
    Hannan, Abdul
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Hellsten, Torbjörn
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Johnson, Thomas
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    On fast wave current drive at higher cyclotron harmonics2011Ingår i: 38th EPS Conference on Plasma Physics 2011, EPS 2011 - Europhysics Conference Abstracts: Volume 35 1, 2011, s. 889-892Konferensbidrag (Övrigt vetenskapligt)
  • 47.
    Hannan, Abdul
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Hellsten, Torbjörn
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Johnson, Thomas
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Self-consistent Ion Cyclotron Resonant Heating and Fast Wave Current Drive for DEMOArtikel i tidskrift (Övrigt vetenskapligt)
  • 48. Hawkes, N. C.
    et al.
    Esposito, B.
    Andrew, Y.
    Biewer, T. M.
    Brzozowski, Jerzy H.
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Brix, M.
    Cardinali, A.
    Crisanti, F.
    Crombé, K.
    Van Eester, D.
    Felton, R.
    Giroud, C.
    Johnson, Thomas
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Lerche, E.
    Meigs, A.
    Parail, V.
    Sharapov, S.
    Sozzi, C.
    Voitsekhovitch, I.
    Zastrow, K. -D
    Ion transport barrier formation with low injected torque in JET2007Ingår i: 34th EPS Conference on Plasma Physics 2007, EPS 2007 - Europhysics Conference Abstracts, 2007, nr 1, s. 504-507Konferensbidrag (Refereegranskat)
    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.

  • 49. Hedin, J.
    et al.
    Hellsten, Torbjörn A. K.
    KTH, Tidigare Institutioner                               , Alfvénlaboratoriet.
    Eriksson, L. G.
    Johnson, Thomas J.
    KTH, Tidigare Institutioner                               , Alfvénlaboratoriet.
    The influence of finite drift orbit width on ICRF heating in toroidal plasmas2002Ingår i: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 42, nr 5, s. 527-540Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Ion cyclotron resonance heating in a toroidal plasma not only increases the perpendicular energy of the resonating ions but also results in their spatial transport. Depending on the direction of propagation of the waves, the ions will either drift inwards or outwards giving rise to an RF induced rotation with the toroidal torque component in the co-current or counter-current directions, respectively. It is found that the spatial transport induced by the RF field, the topology of the ion drift orbits and a wave field consistent with ion absorption are important for determining the distribution function of the heated species. Studies of ICRF heating with the self-consistent code SELFO reveal new features such as the formation of non-standard passing orbits residing on the low field side of the magnetic axis. For a symmetric spectrum the drift terms will in general not cancel. Some classes of orbit will be subjected only to an inward drift and others only to an outward drift. The lack of cancellation of the drift terms is further enhanced by the self-consistent coupling, increasing the absorption for waves propagating parallel to the plasma current, but not for waves propagating in the antiparallel direction. This results in a strong inward pinch also for symmetric wave spectra as well as for typical experimental spectra, with the dominant peak in the counter-plasma-direction.

  • 50. 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, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    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 JET2010Ingår i: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 50, nr 2Artikel i tidskrift (Refereegranskat)
    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.

1234 1 - 50 av 170
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