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MIT Open Access Articles

Mostrando recursos 101 - 120 de 27.239

  1. Competition and Price Dispersion in the U.S. Airline Industry

    Borenstein, Severin; Rose, Nancy L
    We study dispersion in the prices an airline charges to different passengers on the same route. This variation in fares is substantial: the expected absolute difference in fares between two passengers on a route is 36 percent of the airline's average ticket price. The pattern of observed price dispersion cannot easily be explained by cost differences alone. Dispersion increases on routes with more competition or lower flight density, consistent with discrimination based on customers' willingness to switch to alternative airlines or flights. We argue that the data support models of price discrimination in monopolistically competitive markets.

  2. Superelasticity in micro-scale shape memory ceramic particles

    Du, Zehui; Zeng, Xiao Mei; Liu, Qing; Gan, Chee Lip; Schuh, Christopher A
    Shape memory ceramics that exhibit repeatable superelastic deformation are of considerable significance for possible energy damping and micro-actuation applications, and the present work aims to further establish the structural conditions required to avoid fracture in these brittle materials. Spray dried micro-scale superelastic ceramic particles with a variety of grain structures were produced, ranging from single crystal to oligocrystal to polycrystalline particles. Micro-compression experiments showed that whereas polycrystalline samples fracture upon loading, oligocrystal and single crystal particles can exhibit cyclic superelasticity, the latter particles achieving highly reproducible superelasticity to over one hundred cycles with particle compressions up to 3.8% and dissipated...

  3. Superelasticity in micro-scale shape memory ceramic particles

    Du, Zehui; Zeng, Xiao Mei; Liu, Qing; Gan, Chee Lip; Schuh, Christopher A
    Shape memory ceramics that exhibit repeatable superelastic deformation are of considerable significance for possible energy damping and micro-actuation applications, and the present work aims to further establish the structural conditions required to avoid fracture in these brittle materials. Spray dried micro-scale superelastic ceramic particles with a variety of grain structures were produced, ranging from single crystal to oligocrystal to polycrystalline particles. Micro-compression experiments showed that whereas polycrystalline samples fracture upon loading, oligocrystal and single crystal particles can exhibit cyclic superelasticity, the latter particles achieving highly reproducible superelasticity to over one hundred cycles with particle compressions up to 3.8% and dissipated...

  4. Cell-specific modulation of plasticity and cortical state by cholinergic inputs to the visual cortex

    Sugihara, Hiroki; Chen, Naiyan; Sur, Mriganka
    Acetylcholine (ACh) modulates diverse vital brain functions. Cholinergic neurons from the basal forebrain innervate a wide range of cortical areas, including the primary visual cortex (V1), and multiple cortical cell types have been found to be responsive to ACh. Here we review how different cell types contribute to different cortical functions modulated by ACh. We specifically focus on two major cortical functions: plasticity and cortical state. In layer II/III of V1, ACh acting on astrocytes and somatostatin-expressing inhibitory neurons plays critical roles in these functions. Cell type specificity of cholinergic modulation points towards the growing understanding that even diffuse neurotransmitter...

  5. Cell-specific modulation of plasticity and cortical state by cholinergic inputs to the visual cortex

    Sugihara, Hiroki; Chen, Naiyan; Sur, Mriganka
    Acetylcholine (ACh) modulates diverse vital brain functions. Cholinergic neurons from the basal forebrain innervate a wide range of cortical areas, including the primary visual cortex (V1), and multiple cortical cell types have been found to be responsive to ACh. Here we review how different cell types contribute to different cortical functions modulated by ACh. We specifically focus on two major cortical functions: plasticity and cortical state. In layer II/III of V1, ACh acting on astrocytes and somatostatin-expressing inhibitory neurons plays critical roles in these functions. Cell type specificity of cholinergic modulation points towards the growing understanding that even diffuse neurotransmitter...

  6. A light- and calcium-gated transcription factor for imaging and manipulating activated neurons

    Wildes, Craig P; Sanchez, Mateo I; Glober, Gordon F; Matthews, Gillian A; Tye, Kay M; Wang, Wenjing; Pattarabanjird, Tanyaporn; Ting, Alice Y
    Activity remodels neurons, altering their molecular, structural, and electrical characteristics. To enable the selective characterization and manipulation of these neurons, we present FLARE, an engineered transcription factor that drives expression of fluorescent proteins, opsins, and other genetically encoded tools only in the subset of neurons that experienced activity during a user-defined time window. FLARE senses the coincidence of elevated cytosolic calcium and externally applied blue light, which together produce translocation of a membrane-anchored transcription factor to the nucleus to drive expression of any transgene. In cultured rat neurons, FLARE gives a light-to-dark signal ratio of 120 and a high-to low-calcium...

  7. A light- and calcium-gated transcription factor for imaging and manipulating activated neurons

    Wildes, Craig P; Sanchez, Mateo I; Glober, Gordon F; Matthews, Gillian A; Tye, Kay M; Wang, Wenjing; Pattarabanjird, Tanyaporn; Ting, Alice Y
    Activity remodels neurons, altering their molecular, structural, and electrical characteristics. To enable the selective characterization and manipulation of these neurons, we present FLARE, an engineered transcription factor that drives expression of fluorescent proteins, opsins, and other genetically encoded tools only in the subset of neurons that experienced activity during a user-defined time window. FLARE senses the coincidence of elevated cytosolic calcium and externally applied blue light, which together produce translocation of a membrane-anchored transcription factor to the nucleus to drive expression of any transgene. In cultured rat neurons, FLARE gives a light-to-dark signal ratio of 120 and a high-to low-calcium...

  8. Control of plasma stored energy for burn control using DIII-D in-vessel coils

    Hawryluk, R. J.; Eidietis, N. W.; Grierson, B. A.; Hyatt, A. W.; Kolemen, E.; Logan, N. C.; Nazikian, R.; Paz-Soldan, C.; Solomon, W. M.; Wolfe, Stephen M
    A new approach has been experimentally demonstrated to control the stored energy by applying a non-axisymmetric magnetic field using the DIII-D in-vessel coils to modify the energy confinement time. In future burning plasma experiments as well as magnetic fusion energy power plants, various concepts have been proposed to control the fusion power. The fusion power in a power plant operating at high gain can be related to the plasma stored energy and hence, is a strong function of the energy confinement time. Thus, an actuator that modifies the confinement time can be used to adjust the fusion power. In relatively...

  9. Control of plasma stored energy for burn control using DIII-D in-vessel coils

    Hawryluk, R. J.; Eidietis, N. W.; Grierson, B. A.; Hyatt, A. W.; Kolemen, E.; Logan, N. C.; Nazikian, R.; Paz-Soldan, C.; Solomon, W. M.; Wolfe, Stephen M
    A new approach has been experimentally demonstrated to control the stored energy by applying a non-axisymmetric magnetic field using the DIII-D in-vessel coils to modify the energy confinement time. In future burning plasma experiments as well as magnetic fusion energy power plants, various concepts have been proposed to control the fusion power. The fusion power in a power plant operating at high gain can be related to the plasma stored energy and hence, is a strong function of the energy confinement time. Thus, an actuator that modifies the confinement time can be used to adjust the fusion power. In relatively...

  10. Present Status and Recent Developments of the Twisted Stacked-Tape Cable Conductor

    Takayasu, Makoto; Chiesa, Luisa; Allen, Nathaniel C.; Minervini, Joseph V
    The high magnetic field performance of a 40-tape twisted stacked-tape cable (TSTC) conductor made of 4-mm-width 0.1-mm-thick REBCO tapes was successfully tested. The critical current was 6.0 kA, with the n-value of 35, at 4.2 K with a background field of 17 T. No cyclic load effect was observed between 10 and 17 T with the maximum Lorentz load of 102 kN/m. Various issues, such as sample length, nonuniformity of termination resistances, and soldered joint of a coated tape cable, with regard to a TSTC conductor are discussed. Large-scale conductor designs of various scalable TSTC conductors are discussed, taking into...

  11. Present Status and Recent Developments of the Twisted Stacked-Tape Cable Conductor

    Takayasu, Makoto; Chiesa, Luisa; Allen, Nathaniel C.; Minervini, Joseph V
    The high magnetic field performance of a 40-tape twisted stacked-tape cable (TSTC) conductor made of 4-mm-width 0.1-mm-thick REBCO tapes was successfully tested. The critical current was 6.0 kA, with the n-value of 35, at 4.2 K with a background field of 17 T. No cyclic load effect was observed between 10 and 17 T with the maximum Lorentz load of 102 kN/m. Various issues, such as sample length, nonuniformity of termination resistances, and soldered joint of a coated tape cable, with regard to a TSTC conductor are discussed. Large-scale conductor designs of various scalable TSTC conductors are discussed, taking into...

  12. Non-resonant destabilization of (1/1) internal kink mode by suprathermal electron pressure

    Delgado-Aparicio, L.; Sugiyama, L.; Gates, D. A.; Gorelenkov, N.; Scott, S.; Bertelli, N.; Phillips, P. E.; Rowan, W. L.; Wilson, R.; Wukitch, S.; Shiraiwa, Shunichi; Irby, James Henderson; Granetz, Robert S; Parker, Ronald R; Baek, Seung Gyou; Faust, Ian Charles; Wallace, Gregory Marriner; Mumgaard, Robert Thomas; Gao, Chi; Greenwald, Martin J; Hubbard, Amanda E; Hughes Jr, Jerry; Marmar, Earl S; Rice, John E; Wolfe, Stephen M
    New experimental observations are reported on the structure and dynamics of short-lived periodic (1, 1) "fishbone"-like oscillations that appear during radio frequency heating and current-drive experiments in tokamak plasmas. For the first time, measurements can directly relate changes in the high energy electrons to the mode onset, saturation, and damping. In the relatively high collisionality of Alcator C-Mod with lower hybrid current drive, the instability appears to be destabilized by the non-resonant suprathermal electro n pressure - rather than by wave-particle resonance, rotates toroidally with the plasma and grows independently of the (1, 1) sawtooth crash driven by the thermal...

  13. Non-resonant destabilization of (1/1) internal kink mode by suprathermal electron pressure

    Delgado-Aparicio, L.; Sugiyama, L.; Gates, D. A.; Gorelenkov, N.; Scott, S.; Bertelli, N.; Phillips, P. E.; Rowan, W. L.; Wilson, R.; Wukitch, S.; Shiraiwa, Shunichi; Irby, James Henderson; Granetz, Robert S; Parker, Ronald R; Baek, Seung Gyou; Faust, Ian Charles; Wallace, Gregory Marriner; Mumgaard, Robert Thomas; Gao, Chi; Greenwald, Martin J; Hubbard, Amanda E; Hughes Jr, Jerry; Marmar, Earl S; Rice, John E; Wolfe, Stephen M
    New experimental observations are reported on the structure and dynamics of short-lived periodic (1, 1) "fishbone"-like oscillations that appear during radio frequency heating and current-drive experiments in tokamak plasmas. For the first time, measurements can directly relate changes in the high energy electrons to the mode onset, saturation, and damping. In the relatively high collisionality of Alcator C-Mod with lower hybrid current drive, the instability appears to be destabilized by the non-resonant suprathermal electro n pressure - rather than by wave-particle resonance, rotates toroidally with the plasma and grows independently of the (1, 1) sawtooth crash driven by the thermal...

  14. Towards an Emerging Understanding of Non-local Transport

    Ida, K.; Shi, Z.; Sun, H.J.; Inagaki, S.; Kamiya, K.; Tamura, N.; Diamond, P.H.; Dif-Pradalier, G.; Zou, X.L.; Itoh, K.; Sugita, S.; Gurcan, O.D.; Estrada, T.; Hidalgo, C.; Hahm, T.S.; Stroth, U.; Field, A.; Ding, X.T.; Sakamoto, Y.; Oldenburger, S.; Yoshinuma, M.; Kobayashi, T.; Jiang, M.; Hahn, S.H.; Jeon, Y.M.; Hong, S. H.; Dong, J.; Itoh, S.-I.; Rice, John E
    In this overview, recent progress on the experimental analysis and theoretical models for non-local transport (non-Fickian fluxes in real space) are overviewed. The non-locality in the heat and momentum transport observed in the plasma, the departures from linear fluxgradient proportionality and spontaneously and externally triggered non-local transport phenomena will be described in both L-mode and improved-mode plasmas. We will report on ongoing evaluation of "fast front" and "intrinsically nonlocal" models, and their success in comparisons with experimental data.

  15. Towards an Emerging Understanding of Non-local Transport

    Ida, K.; Shi, Z.; Sun, H.J.; Inagaki, S.; Kamiya, K.; Tamura, N.; Diamond, P.H.; Dif-Pradalier, G.; Zou, X.L.; Itoh, K.; Sugita, S.; Gurcan, O.D.; Estrada, T.; Hidalgo, C.; Hahm, T.S.; Stroth, U.; Field, A.; Ding, X.T.; Sakamoto, Y.; Oldenburger, S.; Yoshinuma, M.; Kobayashi, T.; Jiang, M.; Hahn, S.H.; Jeon, Y.M.; Hong, S. H.; Dong, J.; Itoh, S.-I.; Rice, John E
    In this overview, recent progress on the experimental analysis and theoretical models for non-local transport (non-Fickian fluxes in real space) are overviewed. The non-locality in the heat and momentum transport observed in the plasma, the departures from linear fluxgradient proportionality and spontaneously and externally triggered non-local transport phenomena will be described in both L-mode and improved-mode plasmas. We will report on ongoing evaluation of "fast front" and "intrinsically nonlocal" models, and their success in comparisons with experimental data.

  16. A block-tridiagonal solver with two-level parallelization for finite element-spectral codes

    Lee, Jungpyo; Wright, John C
    Two-level parallelization is introduced to solve a massive block-tridiagonal matrix system. One-level is used for distributing blocks whose size is as large as the number of block rows due to the spectral basis, and the other level is used for parallelizing in the block row dimension. The purpose of the added parallelization dimension is to retard the saturation of the scaling due to communication overhead and inefficiencies in the single-level parallelization only distributing blocks. As a technique for parallelizing the tridiagonal matrix, the combined method of "Partitioned Thomas method" and "Cyclic Odd-Even Reduction" is implemented in an MPI-Fortran90 based finite...

  17. A block-tridiagonal solver with two-level parallelization for finite element-spectral codes

    Lee, Jungpyo; Wright, John C
    Two-level parallelization is introduced to solve a massive block-tridiagonal matrix system. One-level is used for distributing blocks whose size is as large as the number of block rows due to the spectral basis, and the other level is used for parallelizing in the block row dimension. The purpose of the added parallelization dimension is to retard the saturation of the scaling due to communication overhead and inefficiencies in the single-level parallelization only distributing blocks. As a technique for parallelizing the tridiagonal matrix, the combined method of "Partitioned Thomas method" and "Cyclic Odd-Even Reduction" is implemented in an MPI-Fortran90 based finite...

  18. Three-dimensional simulation of H-mode plasmas with localized divertor impurity injection on Alcator C-Mod using the edge transport code EMC3-EIRENE[superscript a)]

    Lore, J. D.; Reinke, M. L.; Lipschultz, B.; Pitts, R. A.; Feng, Y.; Brunner, Daniel Frederic; Labombard, Brian; Terry, James L
    Experiments in Alcator C-Mod to assess the level of toroidal asymmetry in divertor conditions resulting from poloidally and toroidally localized extrinsic impurity gas seeding show a weak toroidal peaking (∼1.1) in divertor electron temperatures for high-power enhanced D-alpha H-mode plasmas. This is in contrast to similar experiments in Ohmically heated L-mode plasmas, which showed a clear toroidal modulation in the divertor electron temperature. Modeling of these experiments using the 3D edge transport code EMC3-EIRENE [Y. Feng et al., J. Nucl. Mater. 241, 930 (1997)] qualitatively reproduces these trends, and indicates that the different response in the simulations is due to...

  19. Three-dimensional simulation of H-mode plasmas with localized divertor impurity injection on Alcator C-Mod using the edge transport code EMC3-EIRENE[superscript a)]

    Lore, J. D.; Reinke, M. L.; Lipschultz, B.; Pitts, R. A.; Feng, Y.; Brunner, Daniel Frederic; Labombard, Brian; Terry, James L
    Experiments in Alcator C-Mod to assess the level of toroidal asymmetry in divertor conditions resulting from poloidally and toroidally localized extrinsic impurity gas seeding show a weak toroidal peaking (∼1.1) in divertor electron temperatures for high-power enhanced D-alpha H-mode plasmas. This is in contrast to similar experiments in Ohmically heated L-mode plasmas, which showed a clear toroidal modulation in the divertor electron temperature. Modeling of these experiments using the 3D edge transport code EMC3-EIRENE [Y. Feng et al., J. Nucl. Mater. 241, 930 (1997)] qualitatively reproduces these trends, and indicates that the different response in the simulations is due to...

  20. Full wave simulations of fast wave efficiency and power losses in the scrape-off layer of tokamak plasmas in mid/high harmonic and minority heating regimes

    Bertelli, N.; Jaeger, E.F.; Hosea, J.C.; Phillips, C.K.; Berry, L.; Gerhardt, S.P.; Green, D.; LeBlanc, B.; Perkins, R.J.; Qin, C.M.; Pinsker, R.I.; Prater, R.; Ryan, P.M.; Taylor, G.; Valeo, E.J.; Wilson, J.R.; Zhang, X.J.; Bonoli, Paul T; Wright, John C
    Several experiments on different machines and in different fast wave (FW) heating regimes, such as hydrogen minority heating and high harmonic fast waves (HHFW), have found strong interaction between radio-frequency (RF) waves and the scrape-off layer (SOL) region. This paper examines the propagation and the power loss in the SOL by using the full wave code AORSA, in which the edge plasma beyond the last closed flux surface (LCFS) is included in the solution domain and a collisional damping parameter is used as a proxy to represent the real, and most likely nonlinear, damping processes. 2D and 3D AORSA results...

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