Generation of short electron bunches by a laser pulse crossing a sharp boundary of inhomogeneous plasma
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Generation of Short Electron Bunches by a Laser Pulse Crossing a Sharp Boundary of Inhomogeneous Plasma S. V. Kuznetsov Joint Institute for High Temperatures, Russian Academy of Sciences, Moscow, 125412 Russia e-mail: [email protected] Received August 7, 2015
Abstract—The formation of short electron bunches during the passage of a laser pulse of relativistic intensity through a sharp boundary of semi-bounded plasma has been analytically studied. It is shown in one-dimensional geometry that one physical mechanism that is responsible for the generation of electron bunches is their self-injection into the wake field of a laser pulse, which occurs due to the mixing of electrons during the action of the laser pulse on plasma. Simple analytic relationships are obtained that can be used for estimating the length and charge of an electron bunch and the spread of electron energies in the bunch. The results of the analytical investigation are confirmed by data from numerical simulations. DOI: 10.1134/S1063776116070190
1. INTRODUCTION Laser-plasma-induced acceleration of electrons in rarefied plasma has been extensively studied in the past decade. This research interest is related to the fact that, according to theoretical estimations, the field strength in an accelerating plasma wake wave can exceed 300 GV m–1, which is several orders of magnitude greater than the strength of the accelerating electric field (~0.01 GV m–1) in modern accelerators of traditional types [1]. Investigations of the laser-plasma acceleration of electron bunches in various laboratories (see review [2] summarizing the experimental results) confirmed the validity of the premises underlying the idea of laser-plasma acceleration and, despite some technical difficulties and theoretical problems, demonstrated a gradual increase in the average energy of electrons in a bunch up to approximately 1 GeV. The best result was obtained at Berkeley [3], where a laser pulse of 300 TW peak power (40 fs duration, 0.815 μm wavelength) in a gas-filled capillary discharge accelerated bunched electrons up to an energy of 4.2 GeV in a 9-cm long waveguide. Electron bunches of this energy are of interest in many practical applications. At the same time, questions related to the quality of accelerated electron bunches, including their monoenergeticity, duration, emittance, and charge, are still not completely clear. As an example, approximately 6% of the relative energy spread of electrons in accelerated bunches in the aforementioned experiment [3] was also among the best achieved results. However, the degree of electron beam nonmonoenergeticity that is desirable for practical applications must not exceed
1% and in some cases should even fall within tenths of a percent [4, 5]. Evidently, the quality characteristics (monoenergeticity and emittance) of accelerated electron bunches are determined to a considerable degree by the method that is used to inject electrons into the accelerating wake field and by the initial parameters of the injected bunch. In p
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