High-quality, stable electron beams are produced from self-injected laser wakefield acceleration using the interaction of moderate 3 TW, 45 fs duration Ti:sapphire laser pulses with high density (>5 × 10 19 cm −3 ) helium gas jet plasma. The electron beam has virtually background-free quasimonoenergetic distribution with energy 35.6 þ3.9 −2.5 MeV, charge 3.8 þ2.8 −1.2 pC, divergence and pointing variation ∼10 mrad. The stable and high quality of the electron beam opens an easy way for applications of the laser wakefield accelerator in the future, particularly due to the widespread availability of sub-10 TW class lasers with a number of laser plasma laboratories around the world.
Kα X-ray sources generated from the interaction of ultra-short laser pulses with solids are compact and low-cost source of ultra-short quasi-monochromatic X-rays compared with synchrotron radiation source. Development of collimated ultra-short Kα X-ray source by the interaction of 45 fs Ti:sapphire laser pulse with Cu wire target is presented in this paper. A study of the Kα source with laser parameters such as energy and pulse duration was carried out. The observed Kα X-ray photon flux was ~2.7 × 108 photons/shot at the laser intensity of ~2.8 × 1017 W cm−2. A model was developed to analyze the observed results. The Kα radiation was coupled to a polycapillary collimator to generate a collimated low divergence (0.8 mrad) X-ray beam. Such sources are useful for time-resolved X-ray diffraction and imaging studies.
Development and characterization of a wire target based kHz rep rate Cu Kα x-ray source using a Ti:sapphire laser system and its use in time resolved x-ray diffraction (TXRD) of the InSb (111) sample are presented. The observed Kα x-ray photon flux is ∼3.2 × 109 photons sr−1 s−1 at a laser intensity of ∼3.5 × 1016 W cm−2. TXRD signal from the InSb (111) crystal pumped by an ultrashort Ti:sapphire laser pulse (fluence ∼ 13 mJ cm−2) shows a lattice expansion due to heating on a multipicosecond time scale. The crystal gradually cools down and recovers at ∼1.5 ns after the laser excitation. The observed strain variation in the crystal matches well with the simulated results. The study of full recovery of the sample will be helpful for the development of InSb based devices.
Ultra-short laser-pulse-induced strain propagation in a Ge crystal is studied in the [111] and [100] directions using time-resolved X-ray diffraction (TXRD). The strain propagation velocity is derived by analysis of the TXRD signal from the strained crystal planes. Numerical integration of the Takagi–Taupin equations is performed using open source code, which provides a very simple approach to estimate the strain propagation velocity. The present method will be particularly useful for relatively broad spectral bandwidths and weak X-ray sources, where temporal oscillations in the diffracted X-ray intensity at the relevant phonon frequencies would not be visible. The two Bragg reflections of the Ge sample, viz. 111 and 400, give information on the propagation of strain for two different depths, as the X-ray extinction depths are different for these two reflections. The strain induced by femtosecond laser excitation has a propagation velocity comparable to the longitudinal acoustic velocity. The strain propagation velocity increases with increasing laser excitation fluence. This fluence dependence of the strain propagation velocity can be attributed to crystal heating by ambipolar carrier diffusion. Ge is a promising candidate for silicon-based optoelectronics, and this study will enhance the understanding of heat transport by carrier diffusion in Ge induced by ultra-fast laser pulses, which will assist in the design of optoelectronic devices.
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