89,141 research outputs found
Pepper-pot emittance measurement of laser-plasma wakefield accelerated electrons
No full textThe transverse emittance is an important parameter governing the brightness of an electron beam. Here we present the first pepper-pot measurement of the transverse emittance for a mono-energetic electron beam from a laser-plasma wakefield accelerator, carried out on the Advanced Laser-Plasma High Energy Accelerators towards X-Rays (ALPHA-X) beam line. Mono-energetic electrons are passed through an array of 52 mu m diameter holes in a tungsten mask. The pepper-pot results set an upper limit for the normalised emittance at 5.5 +/- 1 pi mm mrad for an 82 MeV beam
Chirped pulse Raman amplification in plasma: high gain measurements
No full textHigh power short pulse lasers are usually based on chirped pulse amplification (CPA), where a frequency chirped and temporarily stretched ``seed'' pulse is amplified by a broad-bandwidth solid state medium, which is usually pumped by a monochromatic ``pump'' laser. Here, we demonstrate the feasibility of using chirped pulse Raman amplification (CPRA) as a means of amplifying short pulses in plasma. In this scheme, a short seed pulse is amplified by a stretched and chirped pump pulse through Raman backscattering in a plasma channel. Unlike conventional CPA, each spectral component of the seed is amplified at different longitudinal positions determined by the resonance of the seed, pump and plasma wave, which excites a density echelon that acts as a "chirped'" mirror and simultaneously backscatters and compresses the pump. Experimental evidence shows that it has potential as an ultra-broad bandwidth linear amplifier which dispenses with the need for large compressor gratings
Effects of energy absorption on Raman amplification in plasma
No full textStimulated Raman backscattering in plasma has been suggested as a way to amplify short laser pulses to intensities not limited by damage thresholds as in chirped pulse amplification using conventional media. Energy is transferred between two transverse electromagnetic waves, pump and probe, through the parametric interaction with a longitudinal Langmuir wave that is ponderomotively excited by their beat wave. The increase of the plasma temperature due to collisional absorption of the pump wave modifies the dispersion of the Langmuir wave: firstly, its resonance frequency rises (Bohm-Gross shift), and secondly, Landau damping sets in. The frequency shift acts in a similar way to a chirp of the pump frequency, or a density ramp: different spectral components of the probe satisfy the resonance condition at different times. This limits their growth, while increasing the bandwidth of the amplifier, thus leading to superradiant amplification. Landau damping may shorten the probe pulse, but reduces the amplification efficiency. We investigate these effects analytically and using numerical simulations in order to assess their importance in experimental demonstrations, and the possibility of applications
Temporal evolution of density and field amplitudes in Raman amplification including relativistic and ponderomotive effects
No full textThe nonlinear regime of Raman amplification has been studied including the combined effects of relativistic and ponderomotive nonlinearities. The study is important for interaction of mildly relativistic pump and probe laser pulses. Nonlinear coupled temporal evolution of fields and density in Raman amplification is analyzed. It is shown that the saturation amplitude and time of the probe pulse in nonlinear regime depends upon the intensity of the electromagnetic waves and the density of the medium. Further in the nonlinear regime the probe laser pulse gain is severely affected by changes in both the electromagnetic wave amplitude and the plasma density
An ultrashort pulse ultra-violet radiation undulator source driven by a laser plasma wakefield accelerator
No full textNarrow band undulator radiation tuneable over the wavelength range of 150–260 nm has been produced by short electron bunches from a 2 mm long laser plasma wakefield accelerator based on a 20 TW femtosecond laser system. The number of photons measured is up to 9 × 106 per shot for a 100 period undulator, with a mean peak brilliance of 1 × 1018 photons/s/mrad2/mm2/0.1% bandwidth. Simulations estimate that the driving electron bunch r.m.s. duration is as short as 3 fs when the electron beam has energy of 120–130 MeV with the radiation pulse duration in the range of 50–100 fs
Photon frequency up-shifting by an amplified plasma density wake due to two co-propagating laser pulses
No full textAn analytical study of significant photon acceleration (frequency up-shift) in a plasma density wake produced by two laser pulses in the mildly relativistic and linearized regime is presented. The wake amplitude is amplified and its phase controlled using two coaxially, co-propagating laser pulses, which are considered to be identical but separated by a fixed time. A third probe pulse, with a variable delay, is considered as ``test particle'' or quasi-photon propagating through the amplified density wake, which experiences significant photon acceleration because of the local temporal and spatial variation of the permittivity. The evolution of the ``photon'' is studied using Hamiltonian theory. The significant frequency up-shift is much larger than that produced by the wake of a single relativistic laser pulse in the highly relativistic nonlinear wake regime. Our study demonstrates that the inter-pulse separation between the ``controlling'' pulse and the ``driver'' pulse, producing the amplified density wake, can provide an additional degree of freedom for tuning the maximum up-shift of the probe photon frequency
Linear and non-linear waves in Born-Infeld plasma
No full textThe notion of a cold Born-Infeld plasma is reviewed and the dispersion relation for a right-handed circularly polarized electromagnetic plane wave is given. The maximum amplitude (wave-breaking limit) of large amplitude longitudinal plane waves is summarized
A infraestrutura cognitiva da linguagem
No full textMichael Tomasello, um dos mais influentes pesquisadores dedicados ao estudo da evolução e da aquisição da linguagem, em sua palestra online para a Abralin ao vivo, apresenta sua tese de que a capacidade comunicativa humana, em amplo sentido, desenvolve-se antes da linguagem e é fundamental para explicar sua origem. Isto é, a capacidade comunicativa humana, que possui como infraestrutura uma gama de capacidades cognitivas gerais — como capacidades cooperativas, capacidade de inferir intenções, capacidade de decifrar gestos contextualmente, o que demanda o reconhecimento de common ground e atenção conjunta — são as condições básicas que permitem o desenvolvimento de uma linguagem humana complexa. Além disso, o autor demonstra como gestos (e.g. apontar) são usados de forma sofisticada por crianças, mas não por primatas não humanos. Desta forma, os gestos, apoiados na infraestrutura comunicativa, poderiam dar origem às convenções gramaticais19215Michael Tomasello, one of the most influential scholars dedicated to the study of language evolution and acquisition, in his online lecture to Abralin ao vivo, presents his thesis that human communication capacity is prior to language development and is fundamental to explain our linguistic ability. That is, a capacity for human communication, which emcompasses a variety of general cognitive skills —such as cooperation, inference of intentions, deciphering gestures contextually, which requires the recognition of com-mon ground and joint attention —are the conditions which can create or support a complex human language. In addition, the author demonstrates how gestures (e.g., pointing) are used in a sophisticated way by children, but not by non-human primates. Thus, the author argues that gestures, sup-ported by the human communicative infrastructure, can give rise to grammatical convention
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