The Experts below are selected from a list of 11517 Experts worldwide ranked by ideXlab platform

Fulvio Zonca - One of the best experts on this subject based on the ideXlab platform.

  • 2D continuous spectrum of shear Alfvén waves in the presence of a Magnetic Island
    Plasma Physics and Controlled Fusion, 2011
    Co-Authors: Alessandro Biancalani, Francesco Pegoraro, Liu Chen, Fulvio Zonca
    Abstract:

    The radial structure of the continuous spectrum of shear Alfven modes is calculated in the presence of a Magnetic Island in tokamak plasmas. Modes with the same helicity as the Magnetic Island are considered in a slab model approximation. In this framework, with an appropriate rotation of the coordinates the problem reduces to two dimensions. Geometrical effects due to the shape of the flux surface's cross-section are retained to all orders. On the other hand, we neglect toroidal couplings but fully take into account curvature effects responsible for the beta-induced gap in the low-frequency part of the continuous spectrum. New continuum accumulation points are found at the O-point of the Magnetic Island. The beta-induced Alfven eigenmodes (BAE) continuum accumulation point is found to be positioned at the separatrix flux surface. The most remarkable result is the modification of the BAE continuum accumulation point frequency, due to the presence of the Magnetic Island.

  • shear alfven wave continuous spectrum within Magnetic Islands
    Physics of Plasmas, 2010
    Co-Authors: Alessandro Biancalani, Francesco Pegoraro, Liu Chen, Fulvio Zonca
    Abstract:

    The radial structure of the continuous spectrum of shear Alfven waves is calculated in this paper within the separatrix of a Magnetic Island. Geometrical effects due to the noncircularity of the flux surface's cross section are retained to all orders. On the other hand, only curvature effects responsible for the beta-induced gap in the low-frequency part of the continuous spectrum are kept. Modes with different helicity from that of the Magnetic Island are considered. The main result is that, inside a Magnetic Island, there is a continuous spectrum very similar to that of tokamak plasmas, where a generalized safety factor q can be defined and where a wide frequency gap is formed, analogous to the ellipticity induced Alfven eigenmode gap in tokamaks. The presence of this gap is due to the strong eccentricity of the Island cross section. The importance of the existence of such a gap is recognized in potentially hosting Magnetic Island induced Alfven eigenmodes (MiAE). Due to the frequency dependence of the shear Alfven wave continuum on the Magnetic Island size, the possibility of utilizing MiAE frequency scalings as a novel Magnetic Island diagnostic is also discussed.

  • 2d continuous spectrum of shear alfven waves in the presence of a Magnetic Island
    arXiv: Plasma Physics, 2010
    Co-Authors: Alessandro Biancalani, Francesco Pegoraro, Liu Chen, Fulvio Zonca
    Abstract:

    The radial structure of the continuous spectrum of shear Alfven modes is calculated in the presence of a Magnetic Island in tokamak plasmas. Modes with the same helicity of the Magnetic Island are considered in a slab model approximation. In this framework, with an appropriate rotation of the coordinates the problem reduces to 2 dimensions. Geometrical effects due to the shape of the flux surface's cross section are retained to all orders. On the other hand, we keep only curvature effects responsible of the beta induced gap in the low-frequency part of the continuous spectrum. New continuum accumulation points are found at the O-point of the Magnetic Island. The beta-induced Alfven Eigenmodes (BAE) continuum accumulation point is found to be positioned at the separatrix flux surface. The most remarkable result is the nonlinear modification of the BAE continuum accumulation point frequency.

  • continuous spectrum of shear alfven waves within Magnetic Islands
    Physical Review Letters, 2010
    Co-Authors: Alessandro Biancalani, Francesco Pegoraro, Liu Chen, Fulvio Zonca
    Abstract:

    The radial structure of the shear Alfven wave continuous spectrum is calculated inside the separatrix of a Magnetic Island. We find that, within a Magnetic Island, there is a continuous spectrum very similar to that of tokamak plasmas, where a generalized safety factor q can be defined and a wide frequency gap is formed, analogous to the ellipticity induced Alfven eigenmode gap in tokamaks. This is due to the strong eccentricity of the Island cross section. In this gap, a Magnetic-Island induced Alfven eigenmode (MIAE) can exist as a bound state, essentially free of continuum damping, which can be resonantly excited by energetic particles and interact nonlinearly with the Magnetic Island. Because of the frequency dependence of the shear Alfven wave continuum on the Magnetic-Island size, the possibility of utilizing MIAE frequency scalings as a novel Magnetic-Island diagnostic is also discussed.

S Benkadda - One of the best experts on this subject based on the ideXlab platform.

  • convective radial energy flux due to resonant Magnetic perturbations and Magnetic curvature at the tokamak plasma edge
    Physics of Plasmas, 2014
    Co-Authors: Peter Beyer, F A Marcus, G Fuhr, A Monnier, S Benkadda
    Abstract:

    With the resonant Magnetic perturbations (RMPs) consolidating as an important tool to control the transport barrier relaxation, the mechanism on how they work is still a subject to be clearly understood. In this work, we investigate the equilibrium states in the presence of RMPs for a reduced MHD model using 3D electroMagnetic fluid numerical code with a single harmonic RMP (single Magnetic Island chain) and multiple harmonics RMPs in cylindrical and toroidal geometry. Two different equilibrium states were found in the presence of the RMPs with different characteristics for each of the geometries used. For the cylindrical geometry in the presence of a single RMP, the equilibrium state is characterized by a strong convective radial thermal flux and the generation of a mean poloidal velocity shear. In contrast, for toroidal geometry, the thermal flux is dominated by the Magnetic flutter. For multiple RMPs, the high amplitude of the convective flux and poloidal rotation are basically the same in cylindrical geometry, but in toroidal geometry the convective thermal flux and the poloidal rotation appear only with the Islands overlapping of the linear coupling between neighbouring poloidal wavenumbers m, m – 1, and m + 1.

  • convective radial energy flux due to resonant Magnetic perturbations and Magnetic curvature at the tokamak plasma edge
    arXiv: Plasma Physics, 2014
    Co-Authors: Peter Beyer, F A Marcus, G Fuhr, A Monnier, S Benkadda
    Abstract:

    With the resonant Magnetic perturbations (RMPs) consolidating as an important tool to control the transport barrier relaxation, the mechanism on how they work is still a subject to be clearly understood. In this work we investigate the equilibrium states in the presence of RMPs for a reduced MHD model using 3D electroMagnetic fluid numerical code (EMEDGE3D) with a single harmonic RMP (single Magnetic Island chain) and multiple harmonics RMPs in cylindrical and toroidal geometry. Two different equilibrium states were found in the presence of the RMPs with different characteristics for each of the geometries used. For the cylindrical geometry in the presence of a single RMP, the equilibrium state is characterized by a strong convective radial thermal flux and the generation of a mean poloidal velocity shear. In contrast, for toroidal geometry the thermal flux is dominated by the Magnetic flutter. For multiple RMPs, the high amplitude of the convective flux and poloidal rotation are basically the same in cylindrical geometry, but in toroidal geometry the convective thermal flux and the poloidal rotation appear only with the Islands overlapping of the linear coupling between neighbouring poloidal wavenumbers $m$, $m-1$, $m+1$.

  • effect of the curvature and the beta parameter on the nonlinear dynamics of a drift tearing Magnetic Island
    arXiv: Plasma Physics, 2011
    Co-Authors: M Muraglia, K Itoh, O Agullo, M Yagi, S Benkadda, Beyer Peter, X Garbet, Sanae Itoh, Abhijit Sen
    Abstract:

    We present numerical simulation studies of 2D reduced MHD equations investigating the impact of the electronic \beta parameter and of curvature effects on the nonlinear evolution of drift tearing Islands. We observe a bifurcation phenomenon that leads to an amplification of the pressure energy, the generation of E \times B poloidal flow and a nonlinear diaMagnetic drift that affects the rotation of the Magnetic Island. These dynamical modifications arise due to quasilinear effects that generate a zonal flow at the onset point of the bifurcation. Our simulations show that the transition point is influenced by the \beta parameter such that the pressure gradient through a curvature effect strongly stabilizes the transition. Regarding the modified rotation of the Island, a model for the frequency is derived in order to study its origin and the effect of the \beta parameter. It appears that after the transition, an E \times B poloidal flow as well as a nonlinear diaMagnetic drift are generated due to an amplification of the stresses by pressure effects.

  • effect of the curvature and the β parameter on the nonlinear dynamics of a drift tearing Magnetic Island
    Nuclear Fusion, 2009
    Co-Authors: M Muraglia, O Agullo, M Yagi, S Benkadda, P Beyer
    Abstract:

    We present numerical simulation studies of 2D reduced MHD equations investigating the impact of the electronic β parameter and of curvature effects on the nonlinear evolution of drift tearing Islands. We observe a bifurcation phenomenon that leads to an amplification of the pressure energy, the generation of E × B poloidal flow and a nonlinear diaMagnetic drift that affects the rotation of the Magnetic Island. These dynamical modifications arise due to quasilinear effects that generate a zonal flow at the onset point of the bifurcation. Our simulations show that the transition point is influenced by the β parameter such that the pressure gradient through a curvature effect strongly stabilizes the transition. Regarding the modified rotation of the Island, a model for the frequency is derived in order to study its origin and the effect of the β parameter. It appears that after the transition, an E × B poloidal flow as well as a nonlinear diaMagnetic drift are generated due to an amplification of the stresses by pressure effects.

Alessandro Biancalani - One of the best experts on this subject based on the ideXlab platform.

  • 2D continuous spectrum of shear Alfvén waves in the presence of a Magnetic Island
    Plasma Physics and Controlled Fusion, 2011
    Co-Authors: Alessandro Biancalani, Francesco Pegoraro, Liu Chen, Fulvio Zonca
    Abstract:

    The radial structure of the continuous spectrum of shear Alfven modes is calculated in the presence of a Magnetic Island in tokamak plasmas. Modes with the same helicity as the Magnetic Island are considered in a slab model approximation. In this framework, with an appropriate rotation of the coordinates the problem reduces to two dimensions. Geometrical effects due to the shape of the flux surface's cross-section are retained to all orders. On the other hand, we neglect toroidal couplings but fully take into account curvature effects responsible for the beta-induced gap in the low-frequency part of the continuous spectrum. New continuum accumulation points are found at the O-point of the Magnetic Island. The beta-induced Alfven eigenmodes (BAE) continuum accumulation point is found to be positioned at the separatrix flux surface. The most remarkable result is the modification of the BAE continuum accumulation point frequency, due to the presence of the Magnetic Island.

  • shear alfven wave continuous spectrum within Magnetic Islands
    Physics of Plasmas, 2010
    Co-Authors: Alessandro Biancalani, Francesco Pegoraro, Liu Chen, Fulvio Zonca
    Abstract:

    The radial structure of the continuous spectrum of shear Alfven waves is calculated in this paper within the separatrix of a Magnetic Island. Geometrical effects due to the noncircularity of the flux surface's cross section are retained to all orders. On the other hand, only curvature effects responsible for the beta-induced gap in the low-frequency part of the continuous spectrum are kept. Modes with different helicity from that of the Magnetic Island are considered. The main result is that, inside a Magnetic Island, there is a continuous spectrum very similar to that of tokamak plasmas, where a generalized safety factor q can be defined and where a wide frequency gap is formed, analogous to the ellipticity induced Alfven eigenmode gap in tokamaks. The presence of this gap is due to the strong eccentricity of the Island cross section. The importance of the existence of such a gap is recognized in potentially hosting Magnetic Island induced Alfven eigenmodes (MiAE). Due to the frequency dependence of the shear Alfven wave continuum on the Magnetic Island size, the possibility of utilizing MiAE frequency scalings as a novel Magnetic Island diagnostic is also discussed.

  • 2d continuous spectrum of shear alfven waves in the presence of a Magnetic Island
    arXiv: Plasma Physics, 2010
    Co-Authors: Alessandro Biancalani, Francesco Pegoraro, Liu Chen, Fulvio Zonca
    Abstract:

    The radial structure of the continuous spectrum of shear Alfven modes is calculated in the presence of a Magnetic Island in tokamak plasmas. Modes with the same helicity of the Magnetic Island are considered in a slab model approximation. In this framework, with an appropriate rotation of the coordinates the problem reduces to 2 dimensions. Geometrical effects due to the shape of the flux surface's cross section are retained to all orders. On the other hand, we keep only curvature effects responsible of the beta induced gap in the low-frequency part of the continuous spectrum. New continuum accumulation points are found at the O-point of the Magnetic Island. The beta-induced Alfven Eigenmodes (BAE) continuum accumulation point is found to be positioned at the separatrix flux surface. The most remarkable result is the nonlinear modification of the BAE continuum accumulation point frequency.

  • continuous spectrum of shear alfven waves within Magnetic Islands
    Physical Review Letters, 2010
    Co-Authors: Alessandro Biancalani, Francesco Pegoraro, Liu Chen, Fulvio Zonca
    Abstract:

    The radial structure of the shear Alfven wave continuous spectrum is calculated inside the separatrix of a Magnetic Island. We find that, within a Magnetic Island, there is a continuous spectrum very similar to that of tokamak plasmas, where a generalized safety factor q can be defined and a wide frequency gap is formed, analogous to the ellipticity induced Alfven eigenmode gap in tokamaks. This is due to the strong eccentricity of the Island cross section. In this gap, a Magnetic-Island induced Alfven eigenmode (MIAE) can exist as a bound state, essentially free of continuum damping, which can be resonantly excited by energetic particles and interact nonlinearly with the Magnetic Island. Because of the frequency dependence of the shear Alfven wave continuum on the Magnetic-Island size, the possibility of utilizing MIAE frequency scalings as a novel Magnetic-Island diagnostic is also discussed.

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

  • study of slowing down mechanism of locked mode like instability in helical plasmas
    Nuclear Fusion, 2019
    Co-Authors: Y Takemura, K Ida, Y Narushima, S Sakakibara, Kiyomasa Watanabe, S Ohdachi, M Yoshinuma, H Tsuchiya, T Tokuzawa, I Yamada
    Abstract:

    The mode slowing down mechanism of the locked-mode-like instability without a large Magnetic Island is investigated, based on the LHD experimental analysis. The mode frequency coincides with E × B rotation frequency at the resonant surface and the slowing down is caused by two processes. One is the resonant surface moving to the small E × B rotation frequency region and the other is the slowing down of the E × B rotation frequency near the resonant surface. Both processes are almost the same as those of the locked-mode-like instability with a large Magnetic Island. The results suggest that the slowing down occurs even though the precursor does not have a large Magnetic Island. However, when the external RMP is imposed, the mode frequency in the slowing down phase sometimes does not coincide with the E × B rotation frequency. Moreover, the mode amplitude during the slowing down phase increases with the decrease of the mode frequency both with and without the imposed external RMP, which suggests that the instability growth in the slowing down phase is more strongly related to the mode frequency than the E × B rotation frequency because the mode sometimes does not rotate with the E × B rotation.

  • hysteresis relation between turbulence and temperature modulation during the heat pulse propagation into a Magnetic Island in diii d
    Physical Review Letters, 2018
    Co-Authors: K Ida, G R Mckee, T Kobayashi, M Ono, T E Evans, M E Austin
    Abstract:

    The hysteresis relation between turbulence and temperature modulation during the heat pulse propagation into a Magnetic Island is studied for the first time in toroidal plasmas. Lissajous curves of the density fluctuation (n[over ˜]/n) and the electron temperature (T_{e}) modulation show that the (n[over ˜]/n) propagation is faster than the heat pulse propagation near the O point of the Magnetic Island. This faster n[over ˜]/n propagation is experimental evidence of the turbulence spreading from the X point to the O point of the Magnetic Island.

  • observations of sustained phase shifted Magnetic Islands from externally imposed m n 1 1 rmp in lhd
    Nuclear Fusion, 2017
    Co-Authors: Yoshiro Narushima, K Ida, K Y Watanabe, S Sakakibara, Y Takemura, S Ohdachi, M Yoshinuma, Yasuhiro Suzuki, F Castejon, D Lopezbruna
    Abstract:

    New observations in the large helical device (LHD) show that the Magnetic Islands externally imposed by m/n = 1/1 resonant Magnetic perturbation (RMP) can be maintained in an intermediate state with a finite phase shift away from the value present in vacuum. Given the previous experimental observation that the saturated Magnetic Islands show either growth or healing, the intermediate states are realized in the 'healing region' in the beta and collisionality space, which implies that a parameter other than beta and collisionality should exist in order to determine the Island state. Theories based on the competition between electroMagnetic torques and poloidal flow-induced viscous torques provide a prediction for the intermediate state. These two types of torques might be balanced to realize the steadily maintained intermediate state whereas the Islands are placed in the growth state or healing state in the case in which the balance is broken. The experimental observation shows that there is a possibility for the Magnetic Island phase to deviate from its designed position. If the parameters are controlled properly, it is possible to control the phase of the Magnetic Island, which may permit continued utilization of the Island divertor concept.

  • modeling of helium transport and exhaust in the lhd edge
    Plasma Physics and Controlled Fusion, 2016
    Co-Authors: A Bader, C C Hegna, M Kobayashi, O Schmitz, A R Akerson, F Effenberg, H Frerichs, Y Feng, K Ida
    Abstract:

    Experimental results from LHD show a reduction of helium concentration in the plasma with the introduction of a Magnetic Island on the m/n = 1/1 resonant surface in the plasma edge. Simulations of the plasma with and without the Island are carried out with the coupled code EMC3-EIRENE and compared to charge exchange recombination spectroscopy measurements of ionized core helium, and visible spectroscopy measurements of edge neutral helium. The numerical simulations indicate that the experimental parameters lie in a high density regime where the impurity transport is dominated by the outward directed friction force. The EMC3-EIRENE simulations capture the reduction in helium transport well and indicate that: (1) the reduction in core helium is a result of increased outward transport caused by the Magnetic Island and an increased opening of the edge-surface layer to the divertor plates; (2) the dominant source of neutral helium is best modeled by recycled helium at the targets; and (3) ionized helium density profiles are best matched in the simulations when there is a large core helium source in addition to a smaller edge source.

  • self regulated oscillation of transport and topology of Magnetic Islands in toroidal plasmas
    Scientific Reports, 2015
    Co-Authors: K Ida, S Inagaki, S I Itoh, T Kobayashi, T E Evans, M E Austin, S Ohdachi, M W Shafer, Y Suzuki, K Itoh
    Abstract:

    The coupling between the transport and Magnetic topology is an important issue because the structure of Magnetic Islands, embedded in a toroidal equilibrium field, depends on the nature of the transport at the edge of the Islands. Measurements of modulated heat pulse propagation in the DIII-D tokamak have revealed the existence of self-regulated oscillations in the radial energy transport into Magnetic Islands that are indicative of bifurcations in the Island structure and transport near the q = 2 surface. Large amplitude heat pulses are seen in one state followed by small amplitude pulses later in the discharge resulting in a repeating cycle of Island states. These two states are interpreted as a bifurcation of Magnetic Island with high and low heat pulse accessibility. This report describes the discovery of a bifurcation in the coupled dynamics between the transport and topology of Magnetic Islands in tokamak plasmas.

F A Marcus - One of the best experts on this subject based on the ideXlab platform.

  • convective radial energy flux due to resonant Magnetic perturbations and Magnetic curvature at the tokamak plasma edge
    Physics of Plasmas, 2014
    Co-Authors: Peter Beyer, F A Marcus, G Fuhr, A Monnier, S Benkadda
    Abstract:

    With the resonant Magnetic perturbations (RMPs) consolidating as an important tool to control the transport barrier relaxation, the mechanism on how they work is still a subject to be clearly understood. In this work, we investigate the equilibrium states in the presence of RMPs for a reduced MHD model using 3D electroMagnetic fluid numerical code with a single harmonic RMP (single Magnetic Island chain) and multiple harmonics RMPs in cylindrical and toroidal geometry. Two different equilibrium states were found in the presence of the RMPs with different characteristics for each of the geometries used. For the cylindrical geometry in the presence of a single RMP, the equilibrium state is characterized by a strong convective radial thermal flux and the generation of a mean poloidal velocity shear. In contrast, for toroidal geometry, the thermal flux is dominated by the Magnetic flutter. For multiple RMPs, the high amplitude of the convective flux and poloidal rotation are basically the same in cylindrical geometry, but in toroidal geometry the convective thermal flux and the poloidal rotation appear only with the Islands overlapping of the linear coupling between neighbouring poloidal wavenumbers m, m – 1, and m + 1.

  • convective radial energy flux due to resonant Magnetic perturbations and Magnetic curvature at the tokamak plasma edge
    arXiv: Plasma Physics, 2014
    Co-Authors: Peter Beyer, F A Marcus, G Fuhr, A Monnier, S Benkadda
    Abstract:

    With the resonant Magnetic perturbations (RMPs) consolidating as an important tool to control the transport barrier relaxation, the mechanism on how they work is still a subject to be clearly understood. In this work we investigate the equilibrium states in the presence of RMPs for a reduced MHD model using 3D electroMagnetic fluid numerical code (EMEDGE3D) with a single harmonic RMP (single Magnetic Island chain) and multiple harmonics RMPs in cylindrical and toroidal geometry. Two different equilibrium states were found in the presence of the RMPs with different characteristics for each of the geometries used. For the cylindrical geometry in the presence of a single RMP, the equilibrium state is characterized by a strong convective radial thermal flux and the generation of a mean poloidal velocity shear. In contrast, for toroidal geometry the thermal flux is dominated by the Magnetic flutter. For multiple RMPs, the high amplitude of the convective flux and poloidal rotation are basically the same in cylindrical geometry, but in toroidal geometry the convective thermal flux and the poloidal rotation appear only with the Islands overlapping of the linear coupling between neighbouring poloidal wavenumbers $m$, $m-1$, $m+1$.