The x-ray emission in laser-plasma accelerators can be a powerful tool to understand the physics of relativistic laser-plasma interaction. It is shown here that the mapping of betatron x-ray ...radiation can be obtained from the x-ray beam profile when an aperture mask is positioned just beyond the end of the emission region. The influence of the plasma density on the position and the longitudinal profile of the x-ray emission is investigated and compared to particle-in-cell simulations. The measurement of the x-ray emission position and length provides insight on the dynamics of the interaction, including the electron self-injection region, possible multiple injection, and the role of the electron beam driven wakefield.
We show that the control and the mapping of the x-ray emission reveals unique features of the laser-plasma accelerator physics, including strong correlations between electron and x-ray beams, and ...density-dependence of electron injection position.
Thomas has recently derived scaling laws for X-ray radiation from electrons accelerated in plasma bubbles, as well as a threshold for the self-injection of background electrons into the bubble A. G. ...R. Thomas, Phys. Plasmas 17, 056708 (2010). To obtain this threshold, the equations of motion for a test electron are studied within the frame of the bubble model, where the bubble is described by prescribed electromagnetic fields and has a perfectly spherical shape. The author affirms that any elliptical trajectory of the form x'^2/{\gamma}_p^2 + y'^2 = R^2 is solution of the equations of motion (in the bubble frame), within the approximation p'_y^2/p'_x^2 \ll 1. In addition, he highlights that his result is different from the work of Kostyukov et al. Phys. Rev. Lett. 103, 175003 (2009), and explains the error committed by Kostyukov-Nerush-Pukhov-Seredov (KNPS). In this comment, we show that numerically integrated trajectories, based on the same equations than the analytical work of Thomas, lead to a completely different result for the self-injection threshold, the result published by KNPS Phys. Rev. Lett. 103, 175003 (2009). We explain why the analytical analysis of Thomas fails and we provide a discussion based on numerical simulations which show exactly where the difference arises. We also show that the arguments of Thomas concerning the error of KNPS do not hold, and that their analysis is mathematically correct. Finally, we emphasize that if the KNPS threshold is found not to be verified in PIC (Particle In Cell) simulations or experiments, it is due to a deficiency of the model itself, and not to an error in the mathematical derivation.
The x-ray emission in laser-plasma accelerators can be a powerful tool to understand the physics of relativistic laser-plasma interaction. It is shown here that the mapping of betatron x-ray ...radiation can be obtained from the x-ray beam profile when an aperture mask is positioned just beyond the end of the emission region. The influence of the plasma density on the position and the longitudinal profile of the x-ray emission is investigated and compared to particle-in-cell simulations. The measurement of the x-ray emission position and length provides insight on the dynamics of the interaction, including the electron self-injection region, possible multiple injection, and the role of the electron beam driven wakefield.
Americans have long regarded the freedom of travel a central tenet of citizenship. Yet, in the United States, freedom of movement has historically been a right reserved for whites. In this book, ...Elizabeth Stordeur Pryor shows that African Americans fought obstructions to their mobility over 100 years before Rosa Parks refused to give up her seat on a Montgomery bus. These were "colored travelers," activists who relied on steamships, stagecoaches, and railroads to expand their networks and to fight slavery and racism. They refused to ride in "Jim Crow" railroad cars, fought for the right to hold a U.S. passport (and citizenship), and during their transatlantic voyages, demonstrated their radical abolitionism. By focusing on the myriad strategies of black protest, including the assertions of gendered freedom and citizenship, this book tells the story of how the basic act of traveling emerged as a front line in the battle for African American equal rights before the Civil War.Drawing on exhaustive research from U.S. and British newspapers, journals, narratives, and letters, as well as firsthand accounts of such figures as Frederick Douglass, Harriet Jacobs, and William Wells Brown, Pryor illustrates how, in the quest for citizenship, colored travelers constructed ideas about respectability and challenged racist ideologies that made black mobility a crime.
Astrocytes play crucial roles in regulating brain circuit formation and physiology. Recent technological advances have revealed unprecedented levels of astrocyte diversity encompassing molecular, ...morphological, and functional differences. This diversification is initiated during embryonic specification events and (in rodents) continues into the early postnatal period where it overlaps with peak synapse development and circuit refinement. In fact, several lines of evidence suggest astrocyte diversity both influences and is a consequence of molecular crosstalk among developing astrocytes and other cell types, notably neurons and their synapses. Neurological disease states exhibit additional layers of astrocyte heterogeneity, which could help shed light on these cells’ key pathological roles. This review highlights recent advances in clarifying astrocyte heterogeneity and molecular/cellular crosstalk and identifies key outstanding questions.
Astrocytes have complex roles in brain circuit formation and physiology. Recent studies have revealed that astrocytes display heterogeneous molecular, morphological, and functional signatures between and within brain regions.Patterning and specification of glial progenitors play an early role in creating heterogenous astrocytic subpopulations, upon which subsequent signaling events elicited through extrinsic factors enable the full diversity of astrocytes to be reached.The processes of astrocyte maturation and synaptogenesis are interlinked in time and space, enabling astrocytes to influence synapse formation, function, and elimination and neurons to regulate astrocytic properties.The astrocytic responses to aging, inflammation, and disease states are heterogeneous and can differentially alter synaptic function and pathological outcomes.
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