The production of ultra-bright electron bunches using ionization injection triggered by two transversely colliding laser pulses inside a beam-driven plasma wake is examined via three-dimensional (3D) particle-in-cell (PIC) simulations. The relatively low intensity lasers are polarized along the wake axis and overlap with the wake for a very short time. The result is that the residual momentum of the ionized electrons in the transverse plane of the wake is much reduced and the injection is localized along the propagation axis of the wake. This minimizes both the initial thermal emittance and the emittance growth due to transverse phase mixing. 3D PIC simulations show that ultra-short (∼8 fs) high-current (0.4 kA) electron bunches with a normalized emittance of 8.5 and 6 nm in the two planes respectively and a brightness greater than 1.7 × 10 19 A · rad −2 · m −2 can be obtained for realistic parameters. The demonstration of the Linac Coherent Light Source (LCLS) as an X-ray free electron laser (X-FEL) [1] has given impetus to research on the fifth-generation light sources [2]. The goal is to make X-FELs smaller and cheaper while decreasing their wavelength and increasing their coherence and intensity. The FEL performance is partially determined by the brightness of the electron beam that traverses the undulator. The brightness is defined as B n = 2I/ǫ 2 n where I is the beam current and ǫ n is the normalized emittance of the beam. In order to make the length of the undulator needed to drive the SASE-FEL [3] into saturation, shorter, high current (∼kA), multi GeV electron beams with ǫ n ∼ 10nm will be needed. These emittances are an order of magnitude smaller than those from state-of-the-art photoinjector RF guns [4]. In this letter, we show the generation of ultrabright electron bunches using ionization injection triggered by two transversely overlapping laser pulses inside a beam-driven wake in plasma. In our scheme, the relatively low intensity lasers are polarized along the wake axis and overlap with the wake for a very short time. Particle-in-cell (PIC) simulations using OSIRIS [5] show that this geometry reduces the residual momentum of the ionized electrons in the transverse plane and localizes them along the propagation axis of the wake leading to an electron beam with a brightness greater than 10 19
The proposal of generating high quality electron bunches via ionization injection triggered by an counter propagating laser pulse inside a beam driven plasma wake is examined via two-dimensional particle-in-cell simulations. It is shown that electron bunches obtained using this technique can have extremely small slice energy spread, because each slice is mainly composed of electrons ionized at the same time. Another remarkable advantage is that the injection distance is changeable. A bunch with normalized emittance of 3.3 nm, slice energy spread of 15 keV and brightness of 7.2 × 10 18 A m −2 rad −2 is obtained with an optimal injection length which is achieved by adjusting the launch time of the drive beam or by changing the laser focal position. This makes the scheme a promising approach to generate high quality electron bunches for the fifth generation light source.
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