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72. Sound velocities in shock-synthesized stishovite to 72 GPa

Author(s):
Berryman, Eleanor J.; Winey, J. M.; Gupta, Yogendra M.; Duffy, Thomas S.
Abstract:
Stishovite (rutile-type SiO2) is the archetype of dense silicates and may occur in post-garnet eclogitic rocks at lower-mantle conditions. Sound velocities in stishovite are fundamental to understanding its mechanical and thermodynamic behavior at high pressure and temperature. Here, we use plate-impact experiments combined with velocity interferometry to determine the stress, density, and longitudinal sound speed in stishovite formed during shock compression of fused silica at 44 GPa and above. The measured sound speeds range from 12.3(8) km/s at 43.8(8) GPa to 9.8(4) km/s at 72.7(11) GPa. The decrease observed at 64 GPa reacts a decrease in the shear modulus of stishovite, likely due to the onset of melting. By 72 GPa, the measured sound speed agrees with the theoretical bulk sound speed indicating loss of all shear stiffness due to complete melting. Our sound velocity results provide direct evidence for shock-induced melting, in agreement with previous pyrometry data.
Type:
Dataset
Issue Date:
2019

73. Full-wave simulations of ICRF heating regimes in toroidal plasmas with non-Maxwellian distribution functions

Author(s):
Bertelli, N; Valeo, E.J.; Green, D.L.; Gorelenkova, M.; Phillips, C.K.; Podesta, M.; Lee, J.P.; Wright, J.C.; Jaeger, E.
Abstract:
At the power levels required for significant heating and current drive in magnetically-confined toroidal plasma, modification of the particle distribution function from a Maxwellian shape is likely [T. H. Stix, Nucl. Fusion, 15 737 (1975)], with consequent changes in wave propagation and in the location and amount of absorption. In order to study these effects computationally, both the finite-Larmor-radius and the high-harmonic fast wave (HHFW), versions of the full-wave, hot-plasma toroidal simulation code TORIC [M. Brambilla, Plasma Phys. Control. Fusion 41, 1 (1999) and M. Brambilla, Plasma Phys. Control. Fusion 44, 2423 (2002)], have been extended to allow the prescription of arbitrary velocity distributions of the form f(v||, v_perp, psi , theta). For hydrogen (H) minority heating of a deuterium (D) plasma with anisotropic Maxwellian H distributions, the fractional H absorption varies significantly with changes in parallel temperature but is essentially independent of perpendicular temperature. On the other hand, for HHFW regime with anisotropic Maxwellian fast ion distribution, the fractional beam ion absorption varies mainly with changes in the perpendicular temperature. The evaluation of the wave-field and power absorption, through the full wave solver, with the ion distribution function provided by either aMonte-Carlo particle and Fokker-Planck codes is also examined for Alcator C-Mod and NSTX plasmas. Non-Maxwellian effects generally tends to increase the absorption with respect to the equivalent Maxwellian distribution.
Type:
Dataset
Issue Date:
May 2017

74. Resonance in Fast-Wave Amplitude in the Periphery of Cylindrical Plasmas and Application to Edge Losses of Wave Heating Power in Tokamaks

Author(s):
Perkins, R.J.; Hosea, J.C.; Bertelli, N.; Taylor, G.; Wilson, J.R.
Abstract:
Heating magnetically confined plasmas using waves in the ion-cyclotron range of frequencies typically requires coupling these waves over a steep density gradient. This process has produced an unexpected and deleterious phenomenon on the National Spherical Torus eXperiment (NSTX): a prompt loss of wave power along magnetic field lines in front of the antenna to the divertor. Understanding this loss may be key to achieving effective heating and expanding the operational space of NSTX-Upgrade. Here, we propose that a new type of mode, which conducts a significant fraction of the total wave power in the low-density peripheral plasma, is driving these losses. We demonstrate the existence of such modes, which are distinct from surface modes and coaxial modes, in a cylindrical cold-plasma model when a half wavelength structure fits into the region outside the core plasma. The latter condition generalizes the previous hypothesis regarding the occurence of the edge losses and may explain why full-wave simulations predict these losses in some cases but not others. If valid, this condition implies that outer gap control is a potential strategy for mitigating the losses in NSTX-Upgrade in addition to raising the magnetic field or influencing the edge density.
Type:
Dataset
Issue Date:
July 2016

75. Two-dimensional full-wave simulations of waves in space and tokamak plasmas

Author(s):
Kim, E.-W.; Bertelli, N.; Johnson, J.R.; Valeo, E.; Hosea, J.; Perkins, R.
Abstract:
We illustrate the capabilities of a recently developed two-dimensional full wave code (FW2D) in space and tokamak plasmas by adopting various values of density, magnetic field configuration and strength as well as boundary shape. As example, we first showed fast compressional wave propagation in the inner magnetosphere is dramatically modified by a plasmaspheric plume at Earth's magnetosphere. The results show that wave energy is trapped in the plume showing a leaky eigenmode-like structure with plume, which is similar to the detected magnetosonic waves. We also performed simulations of high harmonic fast waves in the scrape-off layer (SOL) plasmas of the National Spherical Torus eXperiment (NSTX)/NSTX-Upgrade. Comparison the results with previous full-wave simulations show that although the FW2D code uses a cold plasma approximation, the electric field and the fraction of the power losses in the SOL plasmas show excellent consistency and agreement with the previous full wave simulations performed by the AORSA code.
Type:
Dataset
Issue Date:
October 2018

76. Distinct cytoskeletal proteins define zones of enhanced cell wall synthesis in Helicobacter pylori

Author(s):
Taylor, Jenny A.; Bratton, Benjamin P.; Sichel, Sophie R.; Blair, Kris M.; Jacobs, Holly M.; DeMeester, Kristen E.; Kuru, Erkin; Gray, Joe; Biboy, Jacob; VanNieuwenhze, Michael S.; Vollmer, Waldemar; Grimes, Catherine L.; Shaevitz, Joshua W.; Salama, Nina R.
Abstract:
Helical cell shape is necessary for efficient stomach colonization by Helicobacter pylori, but the molecular mechanisms for generating helical shape remain unclear. We show that the helical centerline pitch and radius of wild-type H. pylori cells dictate surface curvatures of considerably higher positive and negative Gaussian curvatures than those present in straight- or curved-rod bacteria. Quantitative 3D microscopy analysis of short pulses with either N-acetylmuramic acid or D-alanine metabolic probes showed that cell wall growth is enhanced at both sidewall curvature extremes. Immunofluorescence revealed MreB is most abundant at negative Gaussian curvature, while the bactofilin CcmA is most abundant at positive Gaussian curvature. Strains expressing CcmA variants with altered polymerization properties lose helical shape and associated positive Gaussian curvatures. We thus propose a model where CcmA and MreB promote PG synthesis at positive and negative Gaussian curvatures, respectively, and that this patterning is one mechanism necessary for maintaining helical shape.
Type:
Dataset and Image
Issue Date:
April 2019

77. Attention and awareness in the dorsal attention network

Author(s):
Wilterson, Andrew; Nastase, Samuel; Bio, Branden; Guterstam, Arvid; Graziano, Michael
Abstract:
The attention schema theory (AST) posits a specific relationship between subjective awareness and attention, in which awareness is the control model that the brain uses to aid in the endogenous control of attention. We proposed that the right temporoparietal junction (TPJ) is involved in that interaction between awareness and attention. In previous experiments, we developed a behavioral paradigm in human subjects to manipulate awareness and attention. The paradigm involved a visual cue that could be used to guide a shift of attention to a target stimulus. In task 1, subjects were aware of the visual cue, and their endogenous control mechanism was able to use the cue to help control attention. In task 2, subjects were unaware of the visual cue, and their endogenous control mechanism was no longer able to use it to control attention, even though the cue still had a measurable effect on other aspects of behavior. Here we tested the two tasks while scanning brain activity in human volunteers. We predicted that the right TPJ would be active in relation to the cue in task 1, but not in task 2. This prediction was confirmed. The right TPJ was active in relation to the cue in task 1; it was not measurably active in task 2; the difference was significant. In our interpretation, the right TPJ is involved in a complex interaction in which awareness aids in the control of attention.
Type:
Dataset
Issue Date:
2020

78. Integrative Mechanisms of Social Attention

Author(s):
Bio, Branden; Graziano, Michael
Abstract:
Monitoring the attention of others is fundamental to social cognition. Most of the literature on the topic assumes that our social cognitive machinery is tuned specifically to the gaze direction of others as a proxy for attention. This standard assumption reduces attention to an externally visible parameter. Here we show that this assumption is wrong and a deeper, more meaningful representation is involved. We presented subjects with two cues about the attentional state of a face: direction of gaze and emotional expression. We tested whether people relied predominantly on one cue, the other, or both. If the traditional view is correct, then the gaze cue should dominate. Instead, people employed a variety of strategies, some relying on gaze, some on expression, and some on an integration of cues. We also assessed people’s social cognitive ability using two, independent, standard tests. If the traditional view is correct, then social cognitive ability, as assessed by the independent tests, should correlate with the degree to which people successfully use the gaze cue to judge the attention state of the face. Instead, social cognitive ability correlated best with the degree to which people successfully integrated the cues together, instead of with the use of any one specific cue. The results suggest a rethink of a fundamental component of social cognition: monitoring the attention of others involves constructing a deep model that is informed by a combination of cues. Attention is a rich process and monitoring the attention of others involves a similarly rich representation.
Type:
Dataset
Issue Date:
22 July 2021

79. Study of Stark Broadening of Krypton Helium-beta Lines and Estimation of Electron Density and Temperature in NIF Compressed Capsules

Author(s):
Hill, K. W.; Gao, L.; Kraus, B. F.; Bitter, M.; Efthimion, P. C.; Pablant, N. A.; Schneider, M. B.; Thorn, D. B.; Chen, H.; Kauffman, R. L.; Liedahl, D. A.; MacDonald, M. J.; MacPhee, A. G.; Scott, H. A.; Stoupin, S.; Doron, R.; Stambulchik, E.; Maron, Y.; Lahmann, B.
Abstract:
Numerical data used to draw the figures in the manuscript
Type:
Dataset
Issue Date:
6 June 2022

80. Initial Results from Boron Powder Injection Experiments in WEST Lower Single Null L-mode Plasmas

Author(s):
Bodner, Grant; Gallo, Alberto; Diallo, Ahmed; Lunsford, Robert; Moreau, Philippe; Nagy, Alex; Pellissier, Francis-Pierre; Guillemaut, Christophe; Gunn, James; Bourdelle, Clarisse; Desgranges, Corinne; Manas, Pierre; Bortolon, Alessandro; Klepper, Christopher; Tsitrone, Emanuelle; Unterberg, Ezekial; Vermare, Laure
Abstract:
Using a recently installed impurity powder dropper (IPD), boron powder (< 150 μm) was injected into lower single null (LSN) L-mode discharges in WEST. IPDs possibly enable real-time wall conditioning of the plasma-facing components and may help to facilitate H-mode access in the full-tungsten environment of WEST. The discharges in this experiment featured Ip = 0.5 MA, BT = 3.7 T, q95 = 4.3, tpulse = 12–30 s, ne,0 ~ 4×1019 m-2, and PLHCD ~ 4.5 MW. Estimates of the deuterium and impurity particle fluxes, derived from a combination of visible spectroscopy measurements and their corresponding S/XB coefficients, showed decreases of ~ 50% in O+, N+, and C+ populations during powder injection and a moderate reduction of these low-Z impurities (~ 50%) and W (~ 10%) in the discharges that followed powder injection. Along with the improved wall conditions, WEST discharges with B powder injection observed improved confinement, as the stored energy WMHD, neutron rate, and electron temperature Te increased significantly (10–25% for WMHD and 60–200% for the neutron rate) at constant input power. These increases in confinement scale up with the powder drop rate and are likely due to the suppression of ion temperature gradient (ITG) turbulence from changes in Zeff and/or modifications to the electron density profile.
Type:
Dataset
Issue Date:
2022