Trapping of electrons and acceleration of the electron bunch in a wake wave
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Trapping of Electrons and Acceleration of the Electron Bunch in a Wake Wave S. V. Kuznetsov Joint Institute for High Temperatures, Russian Academy of Sciences, Izhorskaya ul. 132, Moscow, 125412 Russia email: [email protected] Received September 6, 2013; in final form, February 17, 2014
Abstract—The process of electron trapping by a wake wave excited by a laser pulse in a plasma channel in the case where the electron bunches are injected into the vicinity of the maximum of the wakefield potential at a velocity lower than the wave phase velocity is considered. The mechanism for the formation of a compact electron bunch in the trapping region when only the electrons of the injected bunch that are trapped in the focusing phase mainly undergo the subsequent acceleration in the wakefield is analyzed. The influence of the spatial dimensions of the injected bunch and its energy spread on the length of the trapped electron bunch and the fraction of trapped electrons is studied analytically and numerically. For electron bunches with dif ferent ratios of their spatial dimensions to the characteristic dimensions of the wake wave, the influence of the injection energy on the parameters of the highenergy electron bunch trapped and accelerated in the wake field is studied. DOI: 10.1134/S1063780X14080054
1. INTRODUCTION Experiments on monoenergetic laser–plasma acceleration of electron bunches carried out in the past decade in several laboratories (a review summa rizing the experimental results in this field is given in [1]) have demonstrated an increase in the average energy of electrons of the accelerated bunch to ~1 GeV. The best result was achieved in [2], where a petawatt laser pulse (with a duration of 150 fs and wavelength of 1.057 μm) propagating in a gasfilled capillary accelerated the electron bunch to an energy of 2 GeV over a distance on the order of 1 cm, the rel ative energy spread in the accelerated bunch being ~5–10%. The low energy spread in the accelerated electron bunch is very important for practical applications. The required energy spread should not exceed 1%; in some cases, it should be several tenths of a percent [3, 4]. Obviously, such important parameters of the acceler ated electron bunch as its energy spread and emittance are mainly determined by the method used to inject electrons into the accelerating wakefield, as well as by the initial parameters of the injected bunch. At present, the most widespread injection method is electron selfinjection into the wakefield generated by a laser pulse interacting with plasma in the so called bubble regime [2, 5, 6], in which a round cavity practically free of electrons forms behind the laser pulse. An advantage of this scheme of electron injec tion into the wakefield is a simple design of the laser– plasma accelerator, which does not need a special external injector or some additional methods for initi
ating the trapping of plasma electrons by the wake wave. However, the process of electron selfinjection in the bubble regime is essential
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