1,721,140 research outputs found
Tunnelling time of a gaussian wave packet through two potential barriers
No full textThe resonant and non-resonant dynamics of a Gaussian quantum wave packet travelling through a double barrier system is studied as a function of the initial characteristics of the spectrum and of the parameters of the potential. The behaviour of the tunnelling time shows that there are situations where the Hartman effect occurs,while, when the resonances are dominant, and in particular for b> pi/Delta k(b being the inter-barrier distance and Delta k the spectrum width), the tunnelling time becomes very large and the Hartman effect does not take place. (C) Central European Science Journals. All rights reserved
Time asymptotic expansion of the tunneled wave function for a double-barrier potential
No full textA time-asymptotic analytical expression of the wave function transmitted by a double-barrier potential is presented. This analytical expression is validated by comparing it with the complete numerical solution of the Schroedinger equation. In the resonant regime, where the asymptotic expression is not valid, an approximation of the wave function, valid for all times, is given
One-dimensional free-electron laser equations without the slowly varying envelope approximation
Full text linkA set of one-dimensional equations has been deduced in the time domain from the Maxwell-Lorentz system with the aim of describing the free-electron laser radiation without using the slowly varying envelope approximation (SVEA). These equations are valid even in the case of arbitrarily short electron bunches and of current distributions with ripples on the scale of or shorter than the wavelength. Numerical examples are presented, showing that for long homogeneous bunches the new set of equations gives results in agreement with the SVEA free-electron laser theory and that the use of short or prebunched electron beams leads to a decrease of the emission lethargy. Furthermore, we demonstrate that in all cases in which the backward low frequency wave has negligible effects, these equations can be reduced to a form similar to the usual 1D SVEA equations but with a different definition of the bunching term
Comment on Competition between coherent emission and broadband spontaneous emission in the quantum free electron laser : Phys. Plasmas 20, 033106 (2013)
No full textWe point out that in the equation for the electron distribution evolution during Thomson/Compton or undulator radiation used in the paper: “Competition between coherent emission and broadband spontaneous emission in the quantum free electron laser” by G. R. M. Robb and R. Bonifacio [Phys. Plasmas 20, 033106 (2013)], the weight function should be the distribution of the number of emitted photons and not the photon energy distribution. Nevertheless, the considerations expressed in this comment do not alter the conclusions drawn in the paper in object
Ultrahigh brightness electron beams by plasma-based injectors for driving all-optical free-electron lasers
No full textWe studied the generation of low emittance high current monoenergetic beams from plasma waves driven by ultrashort laser pulses, in view of achieving beam brightness of interest for free-electron laser (FEL) applications. The aim is to show the feasibility of generating nC charged beams carrying peak currents much higher than those attainable with photoinjectors, together with comparable emittances and energy spread, compatibly with typical FEL requirements. We identified two regimes: the first is based on a laser wakefield acceleration plasma driving scheme on a gas jet modulated in areas of different densities with sharp density gradients. The second regime is the so-called bubble regime, leaving a full electron-free zone behind the driving laser pulse: with this technique peak currents in excess of 100 kA are achievable. We have focused on the first regime, because it seems more promising in terms of beam emittance. Simulations carried out using VORPAL show, in fact, that in the first regime, using a properly density modulated gas jet, it is possible to generate beams at energies of about 30 MeV with peak currents of 20 kA, slice transverse emittances as low as 0.3 mm mrad, and energy spread around 0.4%. These beams break the barrier of 10^{18} A/(mm mrad)^{2} in brightness, a value definitely above the ultimate performances of photoinjectors, therefore opening a new range of opportunities for FEL applications. A few examples of FELs driven by such kind of beams injected into laser undulators are finally shown. The system constituted by the electron beam under the effect of the electromagnetic undulator has been named AOFEL (for all optical free-electron laser)
FEL SASE and wave undulators
No full textWe investigate the working conditions of Free Electron Lasers operating in the SASE regime with wave undulators. We provide general scaling criteria and corroborate them with appropriate numerical simulations
Physics and applications of high brightness beams workshop, HBEB 2013 inverse Compton cross section revisited
Full text linkThe design of advanced machines working in the quantum regime (ELI-NP, IRIDE, e - γ and γ - γ colliders) requires to set the fundamentals needed to have an accurate prediction of the radiation qualities after the Compton scattering. Due to the high energy of the electron beam in the cases above mentioned, the quantum effects, referred as inverse Compton, which occur during the collision with the laser radiation, are not negligible. We present a rigorous method to obtain the inverse Compton cross section in the general case of not null initial momentum of the electrons from a pure QED calculation, avoiding the usual approaches based on the derivation of this cross section either from the Klein and Nishina formula and the Lorentz transformations or throught Feynman diagrams and Mandelstam invariants. In the derivation of the cross section from the transition amplitude we pay particular attention to the long time behavior of the system evolution. Proceeding in this way we obtain the transition probability in the time unit, which integrated over the solid angle of emission defines spectrum and number of the scattered photons
Two-colour X-gamma ray inverse Compton back-scattering source
Full text linkWe present a simple and new scheme for producing two-colour Thomson/Compton radiation with the possibility of controlling separately the polarization of the two different colours, based on the interaction of one single electron beam with two light pulses that can come from the same laser setup or from two different lasers and that collide with the electrons at different angle. One of the most interesting cases for medical applications is to provide two X-ray pulses across the iodine K-edge at 33.2 keV. The iodine is used as contrast medium in various imaging techniques and the availability of two spectral lines accross the K-edge allows one to produce subtraction images with a great increase in accuracy
Two-Color TeraHertz Radiation by a Multi-Pass FEL Oscillator
Full text linkIn this paper, we show that an electron beam produced by a super-conducting linac, driven in a sequence of two undulator modules of different periods, can generate two-color Terahertz radiation with wavelengths ranging from 100 μm to 2 μm. The generated pulses are synchronized, both MW-class, and highly coherent. Their specific properties and generation will be discussed in detail. Besides the single-spike pulse structure, usually observed in oscillators, we show that both the THz pump and probe can be modulated in a coherent comb of pulses, enabling periodic excitation and stroboscopic measurements
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