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Dimitrios Psaltis - One of the best experts on this subject based on the ideXlab platform.
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the signature of the magnetorotational instability in the reynolds and maxwell stress tensors in accretion discs
Monthly Notices of the Royal Astronomical Society, 2006Co-Authors: Martin E Pessah, Chikwan Chan, Dimitrios PsaltisAbstract:The magnetorotational instability is thought to be responsible for the generation of magnetohydrodynamic turbulence that leads to enhanced Outward angular momentum transport in accretion discs. Here, we present the first formal analytical proof showing that, during the exponential growth of the instability, the mean (averaged over the disc scaleheight) Reynolds stress is always positive, the mean Maxwell stress is always negative, and hence the mean total stress is positive and leads to a net Outward Flux of angular momentum. More importantly, we show that the ratio of the Maxwell to the Reynolds stresses during the late times of the exponential growth of the instability is determined only by the local shear and does not depend on the initial spectrum of perturbations or the strength of the seed magnetic field. Even though we derived this property of the stress tensors for the exponential growth of the instability in incompressible flows, numerical simulations of shearing boxes show that this characteristic is qualitatively preserved under more general conditions, even during the saturated turbulent state generated by the instability.
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the signature of the magnetorotational instability in the reynolds and maxwell stress tensors in accretion discs
arXiv: Astrophysics, 2006Co-Authors: Martin E Pessah, Chikwan Chan, Dimitrios PsaltisAbstract:The magnetorotational instability is thought to be responsible for the generation of magnetohydrodynamic turbulence that leads to enhanced Outward angular momentum transport in accretion discs. Here, we present the first formal analytical proof showing that, during the exponential growth of the instability, the mean (averaged over the disc scale-height) Reynolds stress is always positive, the mean Maxwell stress is always negative, and hence the mean total stress is positive and leads to a net Outward Flux of angular momentum. More importantly, we show that the ratio of the Maxwell to the Reynolds stresses during the late times of the exponential growth of the instability is determined only by the local shear and does not depend on the initial spectrum of perturbations or the strength of the seed magnetic. Even though we derived these properties of the stress tensors for the exponential growth of the instability in incompressible flows, numerical simulations of shearing boxes show that this characteristic is qualitatively preserved under more general conditions, even during the saturated turbulent state generated by the instability.
H M Smith - One of the best experts on this subject based on the ideXlab platform.
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impurity transport and bulk ion flow in a mixed collisionality stellarator plasma
Journal of Plasma Physics, 2017Co-Authors: Sarah Newton, P Helander, A Mollen, H M SmithAbstract:The accumulation of impurities in the core of magnetically confined plasmas, resulting from standard collisional transport mechanisms, is a known threat to their performance as fusion energy sources. Whilst the axisymmetric tokamak systems have been shown to benefit from the effect of temperature screening, that is an Outward Flux of impurities driven by the temperature gradient, impurity accumulation in stellarators was thought to be inevitable, driven robustly by the inward pointing electric field characteristic of hot fusion plasmas. We have shown in Helander et al. (Phys. Rev. Lett, vol. 118, 2017a, 155002) that such screening can in principle also appear in stellarators, in the experimentally relevant mixed collisionality regime, where a highly collisional impurity species is present in a low collisionality bulk plasma. Details of the analytic calculation are presented here, along with the effect of the impurity on the bulk ion flow, which will ultimately affect the bulk contribution to the bootstrap current.
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impurity transport in a mixed collisionality stellarator plasma
Physical Review Letters, 2017Co-Authors: P Helander, Sarah Newton, A Mollen, H M SmithAbstract:A potential threat to the performance of magnetically confined fusion plasmas is the problem of impurity accumulation, which causes the concentration of highly charged impurity ions to rise uncontrollably in the center of the plasma and spoil the energy confinement by excessive radiation. It has long been thought that the collisional transport of impurities in stellarators always leads to such an accumulation (if the electric field points inwards, which is usually the case), whereas tokamaks, being axisymmetric, can benefit from "temperature screening," i.e., an Outward Flux of impurities driven by the temperature gradient. Here it is shown, using analytical techniques supported by results from a new numerical code, that such screening can arise in stellarator plasmas, too, and indeed does so in one of the most relevant operating regimes, where the impurities are highly collisional while the bulk plasma is at low collisionality.
Il Millar - One of the best experts on this subject based on the ideXlab platform.
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The Anatomy of an Alkalic Porphyry Cu-Au System: Geology and Alteration at Northparkes Mines, New South Wales, Australia
'Society of Economic Geologists', 2020Co-Authors: Pacey A, Jj Wilkinson, Owens J, Priest D, Cooke Dr, Il MillarAbstract:The Late Ordovician-early Silurian (~455–435 Ma) Northparkes system is a group of silica-saturated, alkalic porphyry deposits and prospects that developed within the Macquarie island arc. The system is host to a spectacular and diverse range of rocks and alteration-mineralization textures that facilitate a detailed understanding of its evolution, in particular the nature and controls of porphyry-related propylitic alteration. The first intrusive phase at Northparkes is a pre- to early-mineralization pluton that underlies all the deposits and varies in composition from a biotite quartz monzonite to alkali feldspar granite. Prior to total crystallization, this pluton was intruded by a more primitive quartz monzonite that marks the onset of a fertile fractionation series. Toward its upper levels, the quartz monzonite is porphyritic and locally rich in Cu sulfides. Subsequently, a complex series of synmineralization quartz monzonite porphyries was emplaced. The quartz monzonite porphyry intrusions have a distinct pipe-like morphology and are ubiquitously K-feldspar–altered with a crystal-crowded porphyritic texture. The textures of the quartz monzonite porphyries and common occurrence of porphyry-cemented contact breccias indicate they were forcibly emplaced and of relatively low viscosity. The quartz monzonite porphyries are therefore interpreted as crystal-bearing, silicate melt-aqueous fluid slurries that represent the conduits through which deep-seated magmatic-derived ore fluid was discharged into the shallow crust (1–2 km depth). Each deposit is centered on a multiphase cluster of quartz monzonite porphyry intrusions that drove discrete hydrothermal systems. Initial fluid evolution was similar in all the deposits, with three major alteration facies developed as largely concentric zones around the quartz monzonite porphyry complexes. The innermost zone is host to Cu sulfide ore and dominated by K-feldspar alteration. This transitions Outward through a shell of magnetite ± biotite alteration, with pyrite and minor chalcopyrite, to an outer halo of propylitic alteration. Generally, epidote, chlorite, and pyrite are abundant in the most deposit-proximal propylitic zone, with a decrease in the abundance of pyrite, and then epidote, with increasing distance away from deposit centers. Propylitic alteration, particularly within relatively low permeability rocks, is fracture-controlled and a hierarchy of veins is observed. Veins of chlorite-quartz-pyrite ± calcite ± hematite ± epidote ± chalcopyrite (P1) appear to represent the principal fluid conduits. They are surrounded by pervasive and intense alteration halos with a distinct mineralogical zonation from vein-proximal chlorite-sericite (phengite) ± epidote ± pyrite, through hematite-sericite-chlorite ± epidote, ultimately to a vein-distal hematite-albite ± chlorite ± epidote assemblage. These P1 veins are surrounded by regions in which smaller epidote-chlorite ± calcite ± quartz ± pyrite veins (P2) are abundant, again with zoned alteration envelopes: vein-proximal chlorite-sericite (phengite) ± epidote ± pyrite grades out into an epidote-rich zone, which in turn transitions into vein-distal albite-hematite ± chlorite ± epidote. Areas of weakest propylitic alteration, distant from both P1 and P2 veins, are characterized by small epidote-only veinlets (P3) with albite-hematite halos. Mineralogical transitions across the propylitic zone are therefore repeated in the evolution from P1 to P3 veins, as well as in the halos around these veins. It is the overall vein abundance and overlap of associated alteration halos that controls the intensity and appearance of propylitic alteration in most rocks. Such scale invariance and spatial relationships strongly suggest the transition from P1 to P3 veins reflects a broadly decreasing Outward Flux of (magmatic-derived?) fluid that passed through the fracture network and progressively reacted with country rocks. Further support for this hypothesis comes from crosscutting relationships and Rb-Sr dating of epidote (returning an age of 450 ± 11 Ma), which demonstrate the bulk of propylitic alteration was coeval with mineralization and potassic alteration. Late-stage fluid evolution at each deposit was unique. Much of the E48 orebody, and locally the GRP314 deposit, was overprinted by texturally destructive, white sericite-albite-quartz-alunite ± chlorite alteration. In the E26 deposit and in regions of the GRP314 deposit a series of quartz-anhydrite ± pyrite ± Cu sulfide veins with distinctive, vein-proximal, sericite-dominant alteration halos cuts the primary, deposit-concentric alteration facies. The vein-distal mineralogy of these alteration halos is controlled by their distance from deposit centers, changing from K-feldspar ± biotite in deposit-proximal veins to chlorite ± epidote-albite in depositdistal veins. Late-mineralization quartz monzonite porphyries at E26 and GRP314 also appear to be related to the generation of anhydrite-quartz ± sphalerite veins and a set of quartz-calcite-pyrite-sphalerite ± chalcopyrite ± galena veins. Postmineralization magmatic activity produced relatively primitive and barren monzonite porphyries and younger alkali basalt dikes.The attached document is the author(’s’) final accepted/submitted version of the journal article. You are advised to consult the publisher’s version if you wish to cite from it
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The Anatomy of an Alkalic Porphyry Cu-‐Au System: Geology and Alteration at Northparkes Mines, NSW, Australia
'Society of Economic Geologists', 2019Co-Authors: Pacey A, Jj Wilkinson, Owens J, Priest D, Cooke Dr, Il MillarAbstract:The Late Ordovician-Early Silurian (~455-435 Ma) Northparkes system is a group of silica-saturated, alkalic porphyry deposits and prospects which developed within the Macquarie Island Arc. The system is host to a spectacular and diverse range of rocks and alteration-mineralization textures that facilitate a detailed understanding of its evolution, in particular into the nature and controls of porphyry-related propylitic alteration. The first intrusive phase at Northparkes is a pre- to early-mineralization pluton that underlies all the deposits and varies in composition from a biotite quartz monzonite (BQM) to alkali feldspar granite (AFG). Prior to total crystallization, this pluton was intruded by a more primitive quartz monzonite (QMZ) that marks the onset of a fertile fractionation series. Towards its upper levels, the QMZ is porphyritic and locally rich in Cu sulfides. Subsequently, a complex series of syn-mineralization quartz monzonite porphyries (QMP) were emplaced. The QMP intrusions have a distinct pipe-like morphology and are ubiquitously K-feldspar altered with a crystal-crowded porphyritic texture. The textures of the QMPs and common occurrence of porphyry-cemented contact breccias indicate they were forcibly emplaced and of relatively low viscosity. The QMPs are therefore interpreted as crystal-bearing, silicate melt-aqueous fluid slurries that represent the conduits through which deep-seated magmatic-derived ore fluid was discharged into the shallow crust (1-2 km depth). Each deposit is centred on a multiphase cluster of QMP intrusions that drove discrete hydrothermal systems. Initial fluid evolution was similar in all the deposits, with three major alteration facies developed as largely concentric zones around the QMP complexes. The innermost zone is host to Cu sulfide ore and dominated by K-feldspar alteration. This transitions Outwards through a shell of magnetite ± biotite alteration, with pyrite and minor chalcopyrite, to an outer halo of propylitic alteration. Generally, epidote, chlorite and pyrite are abundant in the most deposit-proximal propylitic zone, with a decrease in the abundance of pyrite, and then epidote, with increasing distance away from deposit centers. Propylitic alteration, particularly within relatively low permeability rocks, is fracture-controlled and a hierarchy of veins is observed. Veins of chlorite-quartz-pyrite ± calcite ± hematite ± epidote ± chalcopyrite (P1) appear to represent the principal fluid conduits. They are surrounded by pervasive and intense alteration halos with a distinct mineralogical zonation from vein-proximal chlorite-sericite (phengite) ± epidote ± pyrite, through hematite-sericite-chlorite ± epidote, ultimately to a vein-distal hematite-albite ± chlorite ± epidote assemblage. These P1 veins are surrounded by regions in which smaller epidote-chlorite ± calcite ± quartz ± pyrite veins (P2) are abundant, again with zoned alteration envelopes: vein-proximal chlorite-sericite (phengite) ± epidote ± pyrite grades out into an epidote-rich zone, which in turn transitions into vein-distal albite-hematite ± chlorite ± epidote. Areas of weakest propylitic alteration, distant from both P1 and P2 veins, are characterised by small epidote-only veinlets (P3) with albite-hematite halos. Mineralogical transitions across the propylitic zone are therefore repeated in the evolution from P1 to P3 veins, as well as in the halos around these veins. It is the overall vein abundance and overlap of associated alteration halos which controls the intensity and appearance of propylitic alteration in most rocks. Such scale-invariance and spatial relationships strongly suggests the transition from P1 to P3 veins reflects a broadly decreasing Outward Flux of (magmatic-derived?) fluid that passed through the fracture network and progressively reacted with country rocks. Further support for this hypothesis comes from cross cutting relationships and Rb-Sr dating of epidote (returning an age of 450 ± 11 Ma), which demonstrate the bulk of propylitic alteration was coeval with mineralization and potassic alteration. Late-stage fluid evolution at each deposit was unique. Much of the E48 orebody and locally the GRP314 deposit was overprinted by texturally-destructive, white sericite-albite-quartz-alunite ± chlorite alteration. In the E26 deposit, and in regions of the GRP314 deposit, a series of quartz-anhydrite ± pyrite ± Cu sulfide veins with distinctive, vein-proximal, sericite-dominant alteration halos cut the primary, deposit-concentric alteration facies. The vein-distal mineralogy of these alteration halos is controlled by their distance from deposit centers, changing from K-feldspar ± biotite in deposit-proximal veins to chlorite ± epidote-albite in deposit-distal veins. Late-mineralization QMPs at E26 and GRP314 also appear to be related to the generation of anhydrite-quartz ± sphalerite veins and a set of quartz-calcite-pyrite-sphalerite ± chalcopyrite ± galena veins. Post-mineralization magmatic activity produced relatively primitive and barren monzonite porphyries and younger alkali basalt dikes.© 2019 Economic Geology This document is the author’s final accepted version of the journal article. You are advised to consult the published version if you wish to cite from it
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The anatomy of an alkalic porphyry Cu-Au system: geology and alteration at Northparkes Mines, New South Wales, Australia
'Society of Economic Geologists', 2019Co-Authors: Pacey P, Jj Wilkinson, Owens J, Priest D, Cooke Dr, Il MillarAbstract:The Late Ordovician-early Silurian (∼455–435 Ma) Northparkes system is a group of silica-saturated, alkalic porphyry deposits and prospects that developed within the Macquarie island arc. The system is host to a spectacular and diverse range of rocks and alteration-mineralization textures that facilitate a detailed understanding of its evolution, in particular the nature and controls of porphyry-related propylitic alteration.The first intrusive phase at Northparkes is a pre- to early-mineralization pluton that underlies all the deposits and varies in composition from a biotite quartz monzonite to alkali feldspar granite. Prior to total crystallization, this pluton was intruded by a more primitive quartz monzonite that marks the onset of a fertile fractionation series. Toward its upper levels, the quartz monzonite is porphyritic and locally rich in Cu sulfides. Subsequently, a complex series of synmineralization quartz monzonite porphyries was emplaced. The quartz monzonite porphyry intrusions have a distinct pipe-like morphology and are ubiquitously K-feldspar–altered with a crystal-crowded porphyritic texture. The textures of the quartz monzonite porphyries and common occurrence of porphyry-cemented contact breccias indicate they were forcibly emplaced and of relatively low viscosity. The quartz monzonite porphyries are therefore interpreted as crystal-bearing, silicate melt-aqueous fluid slurries that represent the conduits through which deep-seated magmatic-derived ore fluid was discharged into the shallow crust (1–2 km depth).Each deposit is centered on a multiphase cluster of quartz monzonite porphyry intrusions that drove discrete hydrothermal systems. Initial fluid evolution was similar in all the deposits, with three major alteration facies developed as largely concentric zones around the quartz monzonite porphyry complexes. The innermost zone is host to Cu sulfide ore and dominated by K-feldspar alteration. This transitions Outward through a shell of magnetite ± biotite alteration, with pyrite and minor chalcopyrite, to an outer halo of propylitic alteration. Generally, epidote, chlorite, and pyrite are abundant in the most deposit-proximal propylitic zone, with a decrease in the abundance of pyrite, and then epidote, with increasing distance away from deposit centers.Propylitic alteration, particularly within relatively low permeability rocks, is fracture-controlled and a hierarchy of veins is observed. Veins of chlorite-quartz-pyrite ± calcite ± hematite ± epidote ± chalcopyrite (P1) appear to represent the principal fluid conduits. They are surrounded by pervasive and intense alteration halos with a distinct mineralogical zonation from vein-proximal chlorite-sericite (phengite) ± epidote ± pyrite, through hematite-sericite-chlorite ± epidote, ultimately to a vein-distal hematite-albite ± chlorite ± epidote assemblage. These P1 veins are surrounded by regions in which smaller epidote-chlorite ± calcite ± quartz ± pyrite veins (P2) are abundant, again with zoned alteration envelopes: vein-proximal chlorite-sericite (phengite) ± epidote ± pyrite grades out into an epidote-rich zone, which in turn transitions into vein-distal albite-hematite ± chlorite ± epidote. Areas of weakest propylitic alteration, distant from both P1 and P2 veins, are characterized by small epidote-only veinlets (P3) with albite-hematite halos. Mineralogical transitions across the propylitic zone are therefore repeated in the evolution from P1 to P3 veins, as well as in the halos around these veins. It is the overall vein abundance and overlap of associated alteration halos that controls the intensity and appearance of propylitic alteration in most rocks. Such scale invariance and spatial relationships strongly suggest the transition from P1 to P3 veins reflects a broadly decreasing Outward Flux of (magmatic-derived?) fluid that passed through the fracture network and progressively reacted with country ro cks. Further support for this hypothesis comes from crosscutting relationships and Rb-Sr dating of epidote (returning an age of 450 ± 11 Ma), which demonstrate the bulk of propylitic alteration was coeval with mineralization and potassic alteration.Late-stage fluid evolution at each deposit was unique. Much of the E48 orebody, and locally the GRP314 deposit, was overprinted by texturally destructive, white sericite-albite-quartz-alunite ± chlorite alteration. In the E26 deposit and in regions of the GRP314 deposit a series of quartz-anhydrite ± pyrite ± Cu sulfide veins with distinctive, vein-proximal, sericite-dominant alteration halos cuts the primary, deposit-concentric alteration facies. The vein-distal mineralogy of these alteration halos is controlled by their distance from deposit centers, changing from K-feldspar ± biotite in deposit-proximal veins to chlorite ± epidote-albite in depositdistal veins. Late-mineralization quartz monzonite porphyries at E26 and GRP314 also appear to be related to the generation of anhydrite-quartz ± sphalerite veins and a set of quartz-calcite-pyrite-sphalerite ± chalcopyrite ± galena veins. Postmineralization magmatic activity produced relatively primitive and barren monzonite porphyries and younger alkali basalt dikes
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The anatomy of an alkalic Porphyry Cu-Au system: geology and alteration at Northparkes Mines, New South Wales, Australia
'Society of Economic Geologists', 2019Co-Authors: Pacey A, Jj Wilkinson, Owens J, Priest D, Cooke Dr, Il MillarAbstract:The Late Ordovician-early Silurian (~455–435 Ma) Northparkes system is a group of silica-saturated, alkalic porphyry deposits and prospects that developed within the Macquarie island arc. The system is host to a spectacular and diverse range of rocks and alteration-mineralization textures that facilitate a detailed understanding of its evolution, in particular the nature and controls of porphyry-related propylitic alteration. The first intrusive phase at Northparkes is a pre- to early-mineralization pluton that underlies all the deposits and varies in composition from a biotite quartz monzonite to alkali feldspar granite. Prior to total crystallization, this pluton was intruded by a more primitive quartz monzonite that marks the onset of a fertile fractionation series. Toward its upper levels, the quartz monzonite is porphyritic and locally rich in Cu sulfides. Subsequently, a complex series of synmineralization quartz monzonite porphyries was emplaced. The quartz monzonite porphyry intrusions have a distinct pipe-like morphology and are ubiquitously K-feldspar–altered with a crystal-crowded porphyritic texture. The textures of the quartz monzonite porphyries and common occurrence of porphyry-cemented contact breccias indicate they were forcibly emplaced and of relatively low viscosity. The quartz monzonite porphyries are therefore interpreted as crystal-bearing, silicate melt-aqueous fluid slurries that represent the conduits through which deep-seated magmatic-derived ore fluid was discharged into the shallow crust (1–2 km depth). Each deposit is centered on a multiphase cluster of quartz monzonite porphyry intrusions that drove discrete hydrothermal systems. Initial fluid evolution was similar in all the deposits, with three major alteration facies developed as largely concentric zones around the quartz monzonite porphyry complexes. The innermost zone is host to Cu sulfide ore and dominated by K-feldspar alteration. This transitions Outward through a shell of magnetite ± biotite alteration, with pyrite and minor chalcopyrite, to an outer halo of propylitic alteration. Generally, epidote, chlorite, and pyrite are abundant in the most deposit-proximal propylitic zone, with a decrease in the abundance of pyrite, and then epidote, with increasing distance away from deposit centers. Propylitic alteration, particularly within relatively low permeability rocks, is fracture-controlled and a hierarchy of veins is observed. Veins of chlorite-quartz-pyrite ± calcite ± hematite ± epidote ± chalcopyrite (P1) appear to represent the principal fluid conduits. They are surrounded by pervasive and intense alteration halos with a distinct mineralogical zonation from vein-proximal chlorite-sericite (phengite) ± epidote ± pyrite, through hematite-sericite-chlorite ± epidote, ultimately to a vein-distal hematite-albite ± chlorite ± epidote assemblage. These P1 veins are surrounded by regions in which smaller epidote-chlorite ± calcite ± quartz ± pyrite veins (P2) are abundant, again with zoned alteration envelopes: vein-proximal chlorite-sericite (phengite) ± epidote ± pyrite grades out into an epidote-rich zone, which in turn transitions into vein-distal albite-hematite ± chlorite ± epidote. Areas of weakest propylitic alteration, distant from both P1 and P2 veins, are characterized by small epidote-only veinlets (P3) with albite-hematite halos. Mineralogical transitions across the propylitic zone are therefore repeated in the evolution from P1 to P3 veins, as well as in the halos around these veins. It is the overall vein abundance and overlap of associated alteration halos that controls the intensity and appearance of propylitic alteration in most rocks. Such scale invariance and spatial relationships strongly suggest the transition from P1 to P3 veins reflects a broadly decreasing Outward Flux of (magmatic-derived?) fluid that passed through the fracture network and progressively reacted with country rocks. Further support for this hypothesis comes from crosscutting relationships and Rb-Sr dating of epidote (returning an age of 450 ± 11 Ma), which demonstrate the bulk of propylitic alteration was coeval with mineralization and potassic alteration. Late-stage fluid evolution at each deposit was unique. Much of the E48 orebody, and locally the GRP314 deposit, was overprinted by texturally destructive, white sericite-albite-quartz-alunite ± chlorite alteration. In the E26 deposit and in regions of the GRP314 deposit a series of quartz-anhydrite ± pyrite ± Cu sulfide veins with distinctive, vein-proximal, sericite-dominant alteration halos cuts the primary, deposit-concentric alteration facies. The vein-distal mineralogy of these alteration halos is controlled by their distance from deposit centers, changing from K-feldspar ± biotite in deposit-proximal veins to chlorite ± epidote-albite in depositdistal veins. Late-mineralization quartz monzonite porphyries at E26 and GRP314 also appear to be related to the generation of anhydrite-quartz ± sphalerite veins and a set of quartz-calcite-pyrite-sphalerite ± chalcopyrite ± galena veins. Postmineralization magmatic activity produced relatively primitive and barren monzonite porphyries and younger alkali basalt dikes. GeoRef Subject alkali granites Australasia copper ores absolute age basalts Australia igneous rocks gold ores metal ores metamorphism fractures geochemistry granites mid-ocean ridge basalts faults ignimbrite intrusions quartz monzonite Northparkes Mine metasomatism mineral deposits, genesis New South Wales Australia Ordovician plutonic rocks Paleozoic Rb/Sr volcaniclastics metals rare earths pyroclastics volcanic rocks trachyte
P Helander - One of the best experts on this subject based on the ideXlab platform.
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impurity transport and bulk ion flow in a mixed collisionality stellarator plasma
Journal of Plasma Physics, 2017Co-Authors: Sarah Newton, P Helander, A Mollen, H M SmithAbstract:The accumulation of impurities in the core of magnetically confined plasmas, resulting from standard collisional transport mechanisms, is a known threat to their performance as fusion energy sources. Whilst the axisymmetric tokamak systems have been shown to benefit from the effect of temperature screening, that is an Outward Flux of impurities driven by the temperature gradient, impurity accumulation in stellarators was thought to be inevitable, driven robustly by the inward pointing electric field characteristic of hot fusion plasmas. We have shown in Helander et al. (Phys. Rev. Lett, vol. 118, 2017a, 155002) that such screening can in principle also appear in stellarators, in the experimentally relevant mixed collisionality regime, where a highly collisional impurity species is present in a low collisionality bulk plasma. Details of the analytic calculation are presented here, along with the effect of the impurity on the bulk ion flow, which will ultimately affect the bulk contribution to the bootstrap current.
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impurity transport and bulk ion flow in a mixed collisionality stellarator plasma
arXiv: Plasma Physics, 2017Co-Authors: Sarah Newton, P Helander, A Mollen, Hakan SmithAbstract:The accumulation of impurities in the core of magnetically confined plasmas, resulting from standard collisional transport mechanisms, is a known threat to their performance as fusion energy sources. Whilst the axisymmetric tokamak systems have been shown to benefit from the effect of temperature screening, that is an Outward Flux of impurities driven by the temperature gradient, impurity accumulation in stellarators was thought to be inevitable, driven robustly by the inward pointing electric field characteristic of hot fusion plasmas. We have shown in Helander et. al. (2017b) that such screening can in principle also appear in stellarators, in the experimentally relevant mixed collisionality regime, where a highly collisional impurity species is present in a low collisionality bulk plasma. Details of the analytic calculation are presented here, along with the effect of the impurity on the bulk ion flow, which will ultimately affect the bulk contribution to the bootstrap current.
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impurity transport in a mixed collisionality stellarator plasma
Physical Review Letters, 2017Co-Authors: P Helander, Sarah Newton, A Mollen, H M SmithAbstract:A potential threat to the performance of magnetically confined fusion plasmas is the problem of impurity accumulation, which causes the concentration of highly charged impurity ions to rise uncontrollably in the center of the plasma and spoil the energy confinement by excessive radiation. It has long been thought that the collisional transport of impurities in stellarators always leads to such an accumulation (if the electric field points inwards, which is usually the case), whereas tokamaks, being axisymmetric, can benefit from "temperature screening," i.e., an Outward Flux of impurities driven by the temperature gradient. Here it is shown, using analytical techniques supported by results from a new numerical code, that such screening can arise in stellarator plasmas, too, and indeed does so in one of the most relevant operating regimes, where the impurities are highly collisional while the bulk plasma is at low collisionality.
Martin E Pessah - One of the best experts on this subject based on the ideXlab platform.
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the signature of the magnetorotational instability in the reynolds and maxwell stress tensors in accretion discs
Monthly Notices of the Royal Astronomical Society, 2006Co-Authors: Martin E Pessah, Chikwan Chan, Dimitrios PsaltisAbstract:The magnetorotational instability is thought to be responsible for the generation of magnetohydrodynamic turbulence that leads to enhanced Outward angular momentum transport in accretion discs. Here, we present the first formal analytical proof showing that, during the exponential growth of the instability, the mean (averaged over the disc scaleheight) Reynolds stress is always positive, the mean Maxwell stress is always negative, and hence the mean total stress is positive and leads to a net Outward Flux of angular momentum. More importantly, we show that the ratio of the Maxwell to the Reynolds stresses during the late times of the exponential growth of the instability is determined only by the local shear and does not depend on the initial spectrum of perturbations or the strength of the seed magnetic field. Even though we derived this property of the stress tensors for the exponential growth of the instability in incompressible flows, numerical simulations of shearing boxes show that this characteristic is qualitatively preserved under more general conditions, even during the saturated turbulent state generated by the instability.
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the signature of the magnetorotational instability in the reynolds and maxwell stress tensors in accretion discs
arXiv: Astrophysics, 2006Co-Authors: Martin E Pessah, Chikwan Chan, Dimitrios PsaltisAbstract:The magnetorotational instability is thought to be responsible for the generation of magnetohydrodynamic turbulence that leads to enhanced Outward angular momentum transport in accretion discs. Here, we present the first formal analytical proof showing that, during the exponential growth of the instability, the mean (averaged over the disc scale-height) Reynolds stress is always positive, the mean Maxwell stress is always negative, and hence the mean total stress is positive and leads to a net Outward Flux of angular momentum. More importantly, we show that the ratio of the Maxwell to the Reynolds stresses during the late times of the exponential growth of the instability is determined only by the local shear and does not depend on the initial spectrum of perturbations or the strength of the seed magnetic. Even though we derived these properties of the stress tensors for the exponential growth of the instability in incompressible flows, numerical simulations of shearing boxes show that this characteristic is qualitatively preserved under more general conditions, even during the saturated turbulent state generated by the instability.