The Experts below are selected from a list of 75966 Experts worldwide ranked by ideXlab platform

C D Challis - One of the best experts on this subject based on the ideXlab platform.

  • Internal Transport barrier dynamics with plasma rotation in JET
    Nuclear Fusion, 2009
    Co-Authors: P. De Vries, C D Challis, N. C. Hawkes, Basilio Esposito, C. Giroud, E. Joffrin, J. Hobirk, M. Brix, Kristel Crombé, Johnny Lönnroth
    Abstract:

    At JET the dynamics of Internal Transport barriers (ITBs) has been explored by trying to decouple the effects of heating on the one hand and torque on the other with the ultimate objective of identifying the minimum torque required for the formation of Transport barriers. The experiments shed light on the physics behind the initial trigger for ITBs, which often shows to be linked to the shape of the q profile and magnetic shear, while the further development was influenced by the strength of the rotational shear. In discharges with a small amount of rotational shear ITBs were triggered, which suggest that the overall rotational shear is not the dominant factor in the triggering process. However, the subsequent growth of the barrier was limited if the rotational shear was too low at the time of triggering. This growth phase may be highly non-linear, with several possible positive feedback loops, such as the increases in the toroidal and poloidal component of the rotational shear caused by the ITB itself.

  • Effect of toroidal field ripple on the formation of Internal Transport barriers
    Plasma Physics and Controlled Fusion, 2008
    Co-Authors: P. De Vries, X Litaudon, C D Challis, N. C. Hawkes, E. Joffrin, M. Brix, Y. Andrew, M. Beurskens, J. Brzozowski, Kristel Crombé
    Abstract:

    The effect of a toroidal field (TF) ripple on the formation and performance of Internal Transport barriers (ITBs) has been studied in JET. It was found that the TF ripple had a profound effect on the toroidal plasma rotation. An increased TF ripple up to delta = 1% led to a lower rotation and reduced the rotational shear in the region where the ITBs were formed. ITB triggering events were observed in all cases and it is thought that the rotational shear may be less important for this process than, for example, the q-profile. However, the increase in the pressure gradient following the ITB trigger was reduced in discharges with a larger TF ripple and consequently a lower rotational shear. This suggests that toroidal rotation and its shear play a role in the growth of the ITB once it has been triggered.

  • The use of Internal Transport barriers in tokamak plasmas
    Plasma Physics and Controlled Fusion, 2004
    Co-Authors: C D Challis
    Abstract:

    Internal Transport barriers (ITBs) can provide high tokamak confinement at modest plasma current. This is desirable for operation with most of the current driven non-inductively by the bootstrap mechanism, as currently envisaged for steady-state power plants. Maintaining such plasmas in steady conditions with high plasma purity is challenging, however, due to MHD instabilities and impurity Transport effects. Significant progress has been made in the control of ITB plasmas: the pressure profile has been varied using the barrier location; q-profile modification has been achieved with non-inductive current drive, and means have been found to affect density peaking and impurity accumulation. All these features are, to some extent, interdependent and must be integrated self-consistently to demonstrate a sound basis for extrapolation to future devices.

  • On the link between the q-profile and Internal Transport barriers
    Plasma Physics and Controlled Fusion, 2004
    Co-Authors: Yu F Baranov, C D Challis, Xavier Garbet, M. J. Mantsinen, N. C. Hawkes, C. Giroud, E. Joffrin, B. Alper, R. Barnsley, F. P. Orsitto
    Abstract:

    Numerous experiments were performed on JET to clarify the link between rational q, magnetic shear and Internal Transport barriers (ITBs) by varying the q-profile from a monotonic one to one with a central reversed shear (Challis et al 2002 Plasma Phys. Control. Fusion 44 1031). The q-profile was found to be crucial in two cases: for ITB formation in the presence of a strong negative magnetic shear and for ITBs in the vicinity of low order rational q surfaces with a small magnetic shear. The ITBs of these two types have been modelled and analysed in this work using Transport codes (TRANSP, JETTO, TRB). Transport coefficients were calculated using several widely used theories of micro-turbulence and compared with data deduced from the experiment to verify the applicability of the theories. Different underlying physical mechanisms were found to be responsible for ITB formation in a negative magnetic shear and in the vicinity of a rational minimum q.

  • Internal Transport barrier triggering by rational magnetic flux surfaces in tokamaks
    Nuclear Fusion, 2003
    Co-Authors: E. Joffrin, C D Challis, G D Conway, N. C. Hawkes, X. Garbet, T. C. Hender, D. F. Howell, A. Gude, S Gunter, G. T. A. Huysmans
    Abstract:

    The formation of Internal Transport barriers (ITBs) has been experimentally associated with the presence of rational q surfaces in both JET and ASDEX Upgrade. The triggering mechanisms are related to the occurrence of magneto-hydrodynamic (MHD) instabilities such as mode coupling and fishbone activity. These events could locally modify the poloidal velocity and increase transiently the shearing rate to values comparable with the linear growth rate of ion temperature gradient modes. For JET reversed magnetic shear scenarios, ITB emergence occurs preferentially when the minimum q reaches an integral value. In this case, Transport effects localized in the vicinity of zero magnetic shear and close to rational q values may be at the origin of ITB formation. The role of rational q surfaces in ITB triggering stresses the importance of q profile control for an advanced tokamak scenario and could assist in substantially lowering the access power to these scenarios in next step facilities.

Didier Mazon - One of the best experts on this subject based on the ideXlab platform.

  • Internal Transport barrier formation in the Tore Supra tokamak
    Plasma Physics and Controlled Fusion, 2009
    Co-Authors: F. Turco, G. Giruzzi, J.f. Artaud, Didier Mazon
    Abstract:

    The characteristics of the safety factor profile (q) have been studied for a database of discharges featuring Internal Transport barrier phenomena under different heating and current drive conditions in the Tore Supra tokamak. The presence of a recurrent link between the time of formation of an Internal barrier and the central safety factor (q0) crossing a low order rational surface is reported on and discussed for a database of ~40 discharges. Other relevant features of the current profile (e.g. the minimal q value, the magnetic shear and the presence of magnetic islands in the plasma) have been investigated, yielding to little or no correlation with the time and location of the barrier.

  • Probing Internal Transport Barriers with Heat Pulses in JET
    Physical review letters, 2006
    Co-Authors: P. Mantica, Xavier Garbet, F. Imbeaux, Alessandro Marinoni, D. Van Eester, L. Laborde, M. J. Mantsinen, Didier Mazon, D. Moreau, N. C. Hawkes
    Abstract:

    The first electron temperature modulation experiments in plasmas characterized by strong and long-lasting electron and ion Internal Transport barriers (ITB) have been performed in JET using ion cyclotron resonance heating in mode conversion scheme. The ITB is shown to be a well localized narrow layer with low heat diffusivity, characterized by subcritical Transport and loss of stiffness. In addition, results from cold pulse propagation experiments suggest a second order transition process for ITB formation.

  • Statistical analysis of Internal Transport barriers in JET
    Plasma Physics and Controlled Fusion, 2003
    Co-Authors: Patrick Maget, Yu F Baranov, Didier Mazon, N. C. Hawkes, Basilio Esposito, C. Fourment, E. Joffrin, G. T. Hoang
    Abstract:

    The potential role of magnetic shear, rational safety factor surfaces and shearing rate on the confinement is investigated on the basis of a database analysis of JET Internal Transport barriers (ITBs). The ITB is quantified using the JET ITB criterion (Tresset G et al 2002 Nucl. Fusion 42 520). The relation between the shearing rate and the magnetic shear, already established for positive magnetic shear barrier (Tala T J J et al 2001 Plasma Phys. Control. Fusion 43 507), is investigated at low and negative magnetic shear, where the threshold in shearing rate is found to be lower than for positive magnetic shear barriers. Defining the ITB from the departure from profile stiffness is found to be consistent with the database results, and the critical gradient length is found to be minimum at low magnetic shear. Finally, ITBs are found to be localized in the vicinity of integer safety factor surfaces in positive magnetic shear plasmas, whereas no correlation is found with integer values when the barrier is localized in negative magnetic shear regions.

  • Triggering of Internal Transport barrier in JET
    Plasma Physics and Controlled Fusion, 2002
    Co-Authors: E. Joffrin, C D Challis, P. Mantica, Didier Mazon, N. C. Hawkes, Patrick Maget, Giuseppe Gorini, T. C. Hender, D. F. Howell, S. E. Sharapov
    Abstract:

    Internal Transport barriers (ITBs) can be produced in JET by the application of strong additional heating during the current rise phase of the plasma discharge. Using up to 3 MW of lower hybrid power to tailor the q-profile prior to the main heating phase, a large variety of q-profiles ranging from low positive to strong negative central shear have been obtained during the current rise (0.4 MA s−1). With negative central magnetic shear s = (r/q)(dr/dq), the analysis of ITB triggering reveals a correlation between the formation of the ITB and q min reaching an integer value (q = 2 or q = 3). This observation is confirmed by the analysis of the Alfven cascades. The minimum power required to access regimes with ITBs is probably related to the Transport and magnetohydrodynamic properties of integer magnetic surfaces. Laser ablation and shallow pellet injection have also been attempted in recent JET ITB triggering experiments. This article was scheduled to appear in issue 7 of Plasma Phys. Control. Fusion. To access this special issue on advanced Tokamak research in EFDA-JET please follow this link: http://stacks.iop.org/0741-3335/44/7

N. C. Hawkes - One of the best experts on this subject based on the ideXlab platform.

  • Internal Transport barrier dynamics with plasma rotation in JET
    Nuclear Fusion, 2009
    Co-Authors: P. De Vries, C D Challis, N. C. Hawkes, Basilio Esposito, C. Giroud, E. Joffrin, J. Hobirk, M. Brix, Kristel Crombé, Johnny Lönnroth
    Abstract:

    At JET the dynamics of Internal Transport barriers (ITBs) has been explored by trying to decouple the effects of heating on the one hand and torque on the other with the ultimate objective of identifying the minimum torque required for the formation of Transport barriers. The experiments shed light on the physics behind the initial trigger for ITBs, which often shows to be linked to the shape of the q profile and magnetic shear, while the further development was influenced by the strength of the rotational shear. In discharges with a small amount of rotational shear ITBs were triggered, which suggest that the overall rotational shear is not the dominant factor in the triggering process. However, the subsequent growth of the barrier was limited if the rotational shear was too low at the time of triggering. This growth phase may be highly non-linear, with several possible positive feedback loops, such as the increases in the toroidal and poloidal component of the rotational shear caused by the ITB itself.

  • Influence of rotational shear on triggering and sustainment of Internal Transport barriers on JET
    2009
    Co-Authors: Kristel Crombé, N. C. Hawkes, C. Giroud, P. De Vries, M. Brix, Y. Andrew, T. M. Biewer, E. Blanco, A. Fonseca, E. Joffrin
    Abstract:

    Plasmas with improved confinement by an Internal Transport Barrier (ITB) are considered in the Advanced Tokamak scenarios for ITER. Dedicated pulses to study the effect of poloidal (vq) and toroidal (vf) rotation velocities on the triggering and sustainment of ITBs have been carried out on JET. A new instrument has recently been added to the suite of charge exchange spectrometers allowing vq measurements up to 10 ms in the ITB region [1], which is the same temporal resolution as for the vf and ion temperature (Ti). A torque and power scan was performed. Pulses with reversed and monotonic q-profiles are compared to investigate the role of magnetic shear on the ITB triggering and growth.

  • Effect of toroidal field ripple on the formation of Internal Transport barriers
    Plasma Physics and Controlled Fusion, 2008
    Co-Authors: P. De Vries, X Litaudon, C D Challis, N. C. Hawkes, E. Joffrin, M. Brix, Y. Andrew, M. Beurskens, J. Brzozowski, Kristel Crombé
    Abstract:

    The effect of a toroidal field (TF) ripple on the formation and performance of Internal Transport barriers (ITBs) has been studied in JET. It was found that the TF ripple had a profound effect on the toroidal plasma rotation. An increased TF ripple up to delta = 1% led to a lower rotation and reduced the rotational shear in the region where the ITBs were formed. ITB triggering events were observed in all cases and it is thought that the rotational shear may be less important for this process than, for example, the q-profile. However, the increase in the pressure gradient following the ITB trigger was reduced in discharges with a larger TF ripple and consequently a lower rotational shear. This suggests that toroidal rotation and its shear play a role in the growth of the ITB once it has been triggered.

  • Probing Internal Transport Barriers with Heat Pulses in JET
    Physical review letters, 2006
    Co-Authors: P. Mantica, Xavier Garbet, F. Imbeaux, Alessandro Marinoni, D. Van Eester, L. Laborde, M. J. Mantsinen, Didier Mazon, D. Moreau, N. C. Hawkes
    Abstract:

    The first electron temperature modulation experiments in plasmas characterized by strong and long-lasting electron and ion Internal Transport barriers (ITB) have been performed in JET using ion cyclotron resonance heating in mode conversion scheme. The ITB is shown to be a well localized narrow layer with low heat diffusivity, characterized by subcritical Transport and loss of stiffness. In addition, results from cold pulse propagation experiments suggest a second order transition process for ITB formation.

  • On the link between the q-profile and Internal Transport barriers
    Plasma Physics and Controlled Fusion, 2004
    Co-Authors: Yu F Baranov, C D Challis, Xavier Garbet, M. J. Mantsinen, N. C. Hawkes, C. Giroud, E. Joffrin, B. Alper, R. Barnsley, F. P. Orsitto
    Abstract:

    Numerous experiments were performed on JET to clarify the link between rational q, magnetic shear and Internal Transport barriers (ITBs) by varying the q-profile from a monotonic one to one with a central reversed shear (Challis et al 2002 Plasma Phys. Control. Fusion 44 1031). The q-profile was found to be crucial in two cases: for ITB formation in the presence of a strong negative magnetic shear and for ITBs in the vicinity of low order rational q surfaces with a small magnetic shear. The ITBs of these two types have been modelled and analysed in this work using Transport codes (TRANSP, JETTO, TRB). Transport coefficients were calculated using several widely used theories of micro-turbulence and compared with data deduced from the experiment to verify the applicability of the theories. Different underlying physical mechanisms were found to be responsible for ITB formation in a negative magnetic shear and in the vicinity of a rational minimum q.

Katsumi Ida - One of the best experts on this subject based on the ideXlab platform.

  • Reversal of Intrinsic Torque Associated with the Formation of an Internal Transport Barrier
    Physical Review Letters, 2013
    Co-Authors: Katsumi Ida, M. Yokoyama, Hang Lee, K. Nagaoka, Masaki Osakabe, Chihiro Suzuki, M. Yoshinuma, R. Seki, T. Akiyama
    Abstract:

    A reversal of intrinsic torque is observed during the formation of an Internal Transport barrier (ITB) in the Large Helical Device. The intrinsic torque evaluated from the disparity of rotation between coinjection and counterinjection changes sign from counter to codirection inside the ITB after the ITB formation. Both the Internal Transport barrier and the reversal of the intrinsic torque propagate inwards. This experiment demonstrates that the sign of the nondiffusive term of momentum Transport is sensitive to the confinement mode of heat Transport.

  • Ion Internal Transport Barrier in the Large Helical Device
    Contributions to Plasma Physics, 2010
    Co-Authors: Katsumi Ida, K. Nagaoka, Masaki Osakabe, Masayuki Yokoyama, Mikiro Yoshinuma, Shigeru Morita, Motoshi Goto, H. Funaba, Katsunori Ikeda, H. Nakano
    Abstract:

    The Internal Transport barrier (ITB) appears in ion root plasmas with NBI heating, when the Te/Ti ratio is held below unity in LHD. The improvement of Transport is observed only in the ion heat Transport, not in the electron heat Transport nor in the particle Transport. During the formation of ion ITB, the region of low ion thermal diffusivity expands toward the plasma center and results in a wide weak ITB, which is in contrast to the narrow strong ITB typically observed in tokamaks (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

  • Internal Transport barrier formation induced by edge perturbation on LHD
    Nuclear Fusion, 2010
    Co-Authors: Shigeru Inagaki, Katsumi Ida, Akihide Fujisawa, Naoki Tamura, Takashi Shimozuma, Shin Kubo, Yoshio Nagayama, Kazuo Kawahata, Shigeru Sudo, Kimitaka Itoh
    Abstract:

    The electron Internal Transport barrier (ITB) is formed by an edge perturbation induced by a tracer encapsulated solid pellet injection in a net-current free LHD plasma. The ITB trigger by an edge perturbation takes place in the low density regime and the ITB is not accompanied by a significant change in the density. At the onset of the core temperature rise, there is a strong correlation between the heat flux in the central region and the temperature gradient at a distance half of the plasma radius away. A radial temperature profile with a concave curvature abruptly appears in the growing phase of temperature gradient. Turbulence suppression mechanisms via flow and magnetic shears are not sufficient to explain these features.

  • Physics of Internal Transport barrier of toroidal helical plasmas
    Physics of Plasmas, 2007
    Co-Authors: Kimitaka Itoh, Atsushi Fukuyama, Masatoshi Yagi, Akihide Fujisawa, Shinichiro Toda, Sanae Itoh, Patrick Diamond, Katsumi Ida
    Abstract:

    The role of zonal flows (ZFs) in the formation of an Internal Transport barrier in a toroidal helical plasma is analyzed. The turbulent Transport coefficient is shown to be suppressed when the plasma state changes from the branch of a weak negative radial electric field to the strong positive one. This new transition of turbulent Transport is caused by the change of the damping rate of the ZFs. It is clearly demonstrated, theoretically and experimentally, that the damping rate of the ZFs governs the global confinement of toroidal plasmas.

  • Edge and Internal Transport barrier formations in CHS
    Nuclear Fusion, 2005
    Co-Authors: Shoichi Okamura, Katsumi Ida, Akihide Fujisawa, Takashi Minami, Tsuyoshi Akiyama, Tetsutarou Oishi, Harukazu Iguchi, Mitsutaka Isobe, Shinichiro Kado, K. Nagaoka
    Abstract:

    Edge Transport barrier (ETB) formation was observed in a compact helical system. A sharp decrease in Hα emission indicates the quick transition of edge particle Transport. The increase in the density gradient at the edge was measured by using various profile diagnostics. The transition process has a slowly developing pre-phase, and a quick transition for which the fastest case is less than 0.2 ms. There exists a heating power threshold which is roughly proportional to the density and the magnetic field. The transition and back transition is controlled by the heating power. The magnetic configuration effect on ETB formation was also studied. The local density measurement by beam emission spectroscopy shows intermittent bursts of low frequency fluctuations during the ETB formation phase. When the initial density profile is very hollow, ETB formation together with the electron temperature increase (electron Internal Transport barrier) in the core region, was observed for the NBI discharges without ECH.

X Litaudon - One of the best experts on this subject based on the ideXlab platform.

  • Effect of toroidal field ripple on the formation of Internal Transport barriers
    Plasma Physics and Controlled Fusion, 2008
    Co-Authors: P. De Vries, X Litaudon, C D Challis, N. C. Hawkes, E. Joffrin, M. Brix, Y. Andrew, M. Beurskens, J. Brzozowski, Kristel Crombé
    Abstract:

    The effect of a toroidal field (TF) ripple on the formation and performance of Internal Transport barriers (ITBs) has been studied in JET. It was found that the TF ripple had a profound effect on the toroidal plasma rotation. An increased TF ripple up to delta = 1% led to a lower rotation and reduced the rotational shear in the region where the ITBs were formed. ITB triggering events were observed in all cases and it is thought that the rotational shear may be less important for this process than, for example, the q-profile. However, the increase in the pressure gradient following the ITB trigger was reduced in discharges with a larger TF ripple and consequently a lower rotational shear. This suggests that toroidal rotation and its shear play a role in the growth of the ITB once it has been triggered.

  • Computational Images of Internal-Transport-Barrier Oscillations in Tokamak Plasmas
    IEEE Transactions on Plasma Science, 2008
    Co-Authors: João P. S. Bizarro, X Litaudon, T. Tala
    Abstract:

    A well-known benchmarked code, where a Bohm-gyro-Bohm Transport model is complemented with an empirical scaling for the dynamics of Internal Transport barriers (ITBs), is used to model the ITB oscillations that are often seen in advanced tokamak scenarios with a dominant fraction of bootstrap current.

  • Internal Transport barriers critical physics issues
    Plasma Physics and Controlled Fusion, 2006
    Co-Authors: X Litaudon
    Abstract:

    Plasmas regimes with improved core energy confinement properties, i.e. with Internal Transport barriers (ITB), provide a possible route towards simultaneous high fusion performance and continuous tokamak reactor operation in a non-inductive current drive state. High core confinement regimes should be made compatible with a dominant fraction of the plasma current self-generated (pressure-driven) by the bootstrap effect while operating at high normalized pressure and moderate current. Furthermore, ITB regimes with 'non-stiff' plasma core pressure break the link observed in standard inductive operation between fusion performances and plasma pressure at the edge, thus offering a new degree of freedom in the tokamak operational space. Prospects and critical issues for using plasmas with enhanced thermal core insulation as a basis for steady tokamak reactor operation are reviewed in the light of the encouraging experimental and modelling results obtained recently (typically in the last two years). An extensive set of data from experiments carried out worldwide has been gathered on ITB regimes covering a wide range of parameters (q-profile, T i /T e , gradient length, shaping, normalized toroidal Larmor radius, collisionality, Mach number, etc). In the light of the progress made recently, the following critical physics issues relevant to the extrapolation of ITB regimes to next-step experiments, such as ITER, are addressed: (i) conditions for ITB formation and existence of a power threshold, (ii) ITB sustainment at T i ∼ T e , with low toroidal torque injection, low central particle fuelling but at high density and low impurity concentration, (iii) control of confinement for sustaining wide ITBs that encompass a large volume at high β N , (iv) real time profile control (q and pressure) with high bootstrap current and large fraction of alpha-heating and (v) compatibility of core with edge Transport barriers or with external core perturbations (such as frozen hydrogen isotope pellets injection). It is shown that the present experimental results provide some valuable and promising answers to these critical issues.

  • Electron heated Internal Transport barriers in JET
    Plasma Physics and Controlled Fusion, 2002
    Co-Authors: G. M. D. Hogeweij, X Litaudon, J Mailloux, G D Conway, Y. Baranov, S. R. Cortes, M. De Baar, N. Hawkes, F. Imbeaux, F. Rimini
    Abstract:

    By applying lower hybrid current drive (LHCD) for strong electron heating and off-axis current drive starting very early in the discharge, an electron Internal Transport barrier (eITB) can be generated. The barrier is formed just inside the location of minimum q, and slowly moves inward with this location. During the current flat-top the barrier can be sustained during many seconds, either with continued LHCD, or by ion cyclotron resonance heating. In this paper both scenarios are analysed and compared.

  • A dimensionless criterion for characterizing Internal Transport barriers in JET
    Nuclear Fusion, 2002
    Co-Authors: G. Tresset, X Litaudon, D. Moreau, X. Garbet
    Abstract:

    Analysis of experiments performed at different heating powers and magnetic field intensities shows that the existence of Internal Transport barriers in JET can be inferred in regions of space–time where the ratio of the ion gyroradius to the local gradient scale length exceeds some critical value. A possible interpretation leading to the theoretical relevance of this dimensionless parameter as a local indicator of a bifurcated plasma state is the stabilization of turbulence by the E × B shear flows associated with large pressure gradients and plasma rotation. Large database analysis and real time plasma control are envisaged as attractive applications.