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This paper analyzes school segregation in Chile in two dimensions: socioeconomic segregation and academic segregation; a matter of great interest given the radical market oriented organization of the Chilean education. The analysis is situated in the context of the growing concern about the educational equity and proposes a conceptualization that links school segregation to the study of the peer-effect in education (i.e. the effect caused by school peers). Additionally, the paper reviews and discusses the available empirical evidence about the magnitude, causes and effects of the socioeconomic and academic segregation in Chile. Finally, the paper identifies some of the challenges that school segregation poses to the scientific research and proposes some research branches for additional studies, in order to advance in the academic knowledge of this relevant but poorly studied issue. &nbsp, Este trabajo analiza la segregación de la población escolar en Chile en dos dimensiones: la segregación socioeconómica y la segregación académica, materia de enorme interés dado la fuerte configuración de mercado del sistema educacional chileno. El texto sitúa el análisis de la segregación en el marco de la creciente preocupación por la equidad educacional y propone una conceptualización que vincula la segregación escolar con la investigación acerca del "efecto de los compañeros" en educación(peer effects).A continuación, se revisa y discute la evidencia empírica disponible acerca de la magnitud, causas y efectos de la segregación socioeconómica y la segregación académica de la educación chilena. El trabajo concluye identificando algunos de los desafíos que este campo de estudio presenta a la investigación científica y proponiendo líneas de investigación futura que permitan avanzar en el conocimiento de un tema relevante pero comparativamente poco estudiado. &nbsp, Analisa-se a segregação da população escolar no Chile em duas dimensões: a segregação socioeconõmica e a acadêmica, tema de grande interesse, dada a forte configuração do mercado do sistema educacional chileno. Situa-se a análise da segregação no contexto da crescente preocupação com a equidade educacional e propõe-se uma conceituação que liga a segregação escolar á pesquisa sobre o "efeito de pares" na educação (peer effects). Revisa-se e situa-se a evidência empírica sobre a magnitude, causas e efeitos da segregação socioeconômica e da segregação acadêmica na educação chilena. Conclui-se identificando alguns dos desafios que este campo de estudo apresenta á pesquisa científica, propondo linhas para futura pesquisa que permita avançar no conhecimento de um tema relevante, porém pouco estudado. &nbsp

The transport of relativistic electrons generated in the interaction of petawatt class lasers with solid targets has been studied through measurements of the second harmonic optical emission from their rear surface. The high degree of polarization of the emission indicates that it is predominantly optical transition radiation (TR). A halo that surrounds the main region of emission is also polarized and is attributed to the effect of electron recirculation. The variation of the polarization state and intensity of radiation with the angle of observation indicates that the emission of TR is highly directional and provides evidence for the presence of μm-size filaments. A brief discussion on the possible causes of such a fine electron beam structure is given. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.

High-energy-density flows through dense matter are needed for effective progress in the production of laser-driven intense sources of energetic particles and radiation, in driving matter to extreme temperatures creating state regimes relevant for planetary or stellar science as yet inaccessible at the laboratory scale, or in achieving high-gain laser-driven thermonuclear fusion. When interacting at the surface of dense (opaque) targets, intense lasers accelerate relativistic electron beams which transport a significant fraction of the laser energy into the target depth. However, the overall laser-to-target coupling efficiency is impaired by the large divergence of the electron beam, intrinsic to the laser-plasma interaction. By imposing a longitudinal 600T laser-driven magnetic-field, our experimental results show guided >10MA-current of MeV-electrons in solid matter. Due to the applied magnetic field, the transported energy-density and the peak background electron temperature at the 60micron-thick targets rear surface rise by factors 5, resulting from unprecedentedly efficient guiding of relativistic electron currents., 11 pages, 7 figures

Recent progress in generating high-energy (>50 MeV) protons from intense laser–matter interactions (1018–1021Wcm2; refs 1–7) has opened up new areas of research, with applications in radiography8, oncology9, astrophysics10, medical imaging11, high-energy-density physics12–14, and ion-proton beam fast ignition15–19. With the discovery of proton focusing with curved surfaces20,21, rapid advances in these areas will be driven by improved focusing technologies. Here we report on the first investigation of the generation and focusing of a proton beam using a cone-shaped target.We clearly show that the focusing is strongly affected by the electric fields in the beam in both open and enclosed (cone) geometries, bending the trajectories near the axis. Also in the cone geometry, a sheath electric field effectively ‘channels’ the proton beam through the cone tip, substantially improving the beam focusing properties. These results agree well with particle simulations and provide the physics basis for many future applications.

Transport modeling of idealized, cone-guided fast ignition targets indicates the severe challenge posed by fast-electron source divergence. The hybrid particle-in-cell [PIC] code Zuma is run in tandem with the radiation-hydrodynamics code Hydra to model fast-electron propagation, fuel heating, and thermonuclear burn. The fast electron source is based on a 3D explicit-PIC laser-plasma simulation with the PSC code. This shows a quasi two-temperature energy spectrum, and a divergent angle spectrum (average velocity-space polar angle of 52 degrees). Transport simulations with the PIC-based divergence do not ignite for > 1 MJ of fast-electron energy, for a modest 70 micron standoff distance from fast-electron injection to the dense fuel. However, artificially collimating the source gives an ignition energy of 132 kJ. To mitigate the divergence, we consider imposed axial magnetic fields. Uniform fields ~50 MG are sufficient to recover the artificially collimated ignition energy. Experiments at the Omega laser facility have generated fields of this magnitude by imploding a capsule in seed fields of 50-100 kG. Such imploded fields are however more compressed in the transport region than in the laser absorption region. When fast electrons encounter increasing field strength, magnetic mirroring can reflect a substantial fraction of them and reduce coupling to the fuel. A hollow magnetic pipe, which peaks at a finite radius, is presented as one field configuration which circumvents mirroring., 16 pages, 17 figures, submitted to Phys. Plasmas

Simulations of ignition-scale fast ignition targets have been performed with the new integrated Zuma-Hydra PIC-hydrodynamic capability. We consider an idealized spherical DT fuel assembly with a carbon cone, and an artificially-collimated fast electron source. We study the role of E and B fields and the fast electron energy spectrum. For mono-energetic 1.5 MeV fast electrons, without E and B fields, the energy needed for ignition is E_f^{ig} = 30 kJ. This is about 3.5x the minimal deposited ignition energy of 8.7 kJ for our fuel density of 450 g/cm^3. Including E and B fields with the resistive Ohm's law E = \eta J_b gives E_f^{ig} = 20 kJ, while using the full Ohm's law gives E_f^{ig} > 40 kJ. This is due to magnetic self-guiding in the former case, and \nabla n \times \nabla T magnetic fields in the latter. Using a realistic, quasi two-temperature energy spectrum derived from PIC laser-plasma simulations increases E_f^{ig} to (102, 81, 162) kJ for (no E/B, E = \eta J_b, full Ohm's law). This stems from the electrons being too energetic to fully stop in the optimal hot spot depth., Comment: Minor revisions in response to referee comments

We present complete experimental characterisation of the temporal shape of an intense ultrashort 200-TW laser pulse driving a laser wakefield. The phase of the pulse was uniquely measured using (second order) frequency resolved optical gating (FROG). The pulses are asymmetrically compressed, and exhibit a positive chirp consistent with the expected asymmetric self-phase modulation due to photon acceleration/deceleration in a relativistic plasma wave. The measured pulse duration decreases linearly with increasing length and density of the plasma, in quantitative agreement with the intensity dependent group velocity variation in the plasma wave.

The transport of relativistic electrons generated in the interaction of petawatt class lasers with solid targets has been studied through measurements of the second harmonic optical emission from their rear surface. The high degree of polarization of the emission indicates that it is predominantly optical transition radiation (TR). A halo that surrounds the main region of emission is also polarized and is attributed to the effect of electron recirculation. The variation of the polarization state and intensity of radiation with the angle of observation indicates that the emission of TR is highly directional and provides evidence for the presence of μm-size filaments. A brief discussion on the possible causes of such a fine electron beam structure is given. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.

Each successive generation of x-ray machines has opened up new frontiers in science, such as the first radiographs and the determination of the structure of DNA. State-of-the-art x-ray sources can now produce coherent high brightness keV x-rays and promise a new revolution in imaging complex systems on nanometre and femtosecond scales. Despite the demand, only a few dedicated synchrotron facilities exist worldwide, partially due the size and cost of conventional (accelerator) technology. Here we demonstrate the use of a recently developed compact laser-plasma accelerator to produce a well-collimated, spatially-coherent, intrinsically ultrafast source of hard x-rays. This method reduces the size of the synchrotron source from the tens of metres to centimetre scale, accelerating and wiggling a high electron charge simultaneously. This leads to a narrow-energy spread electron beam and x-ray source that is >1000 times brighter than previously reported plasma wiggler and thus has the potential to facilitate a myriad of uses across the whole spectrum of light-source applications., 5 pages, 4 figures

Este trabajo analiza la segregación de la población escolar en Chile en dos dimensiones: la segregación socioeconómica y la segregación académica, materia de enorme interés dado la fuerte configuración de mercado del sistema educacional chileno. El texto sitúa el análisis de la segregación en el marco de la creciente preocupación por la equidad educacional y propone una conceptualización que vincula la segregación escolar con la investigación acerca del "efecto de los compañeros" en educación (peer effects). A continuación, se revisa y discute la evidencia empírica disponible acerca de la magnitud, causas y efectos de la segregación socioeconómica y la segregación académica de la educación chilena. El trabajo concluye identificando algunos de los desafíos que este campo de estudio presenta a la investigación científica y proponiendo líneas de investigación futura que permitan avanzar en el conocimiento de un tema relevante pero comparativamente poco estudiado.