Recently, there have been some theoretical studies on the use of two-color intense laser pulses for the laser wakefield acceleration (LWFA) research. Here, we experimentally demonstrate a laser-plasma accelerator driven by relativistic copropagating two-color laser pulses (CTLP) in pure helium and in helium-nitrogen mixed gas targets where we observed significant enhancements in the energy spectra of the electron beams (Sci. Adv. 5. eaav7940, 2019). Such enhancement has been further confirmed in a real-application, where electrons from the CTLP-driven LWFA scheme are used in a bremsstrahlung-based positron beam generation configuration, which led to a significant boost in the positron beam energy as well. Numerical simulations suggested that the trailing second-harmonic relativistic laser pulse can sustain the acceleration structure (i.e., the plasma wave) for much longer distances after the preceding fundamental laser pulse is depleted in the plasma medium. Therefore, our experimental work confirms the advantage and robustness of the CTLP-driven LWFA scheme over the standard LWFA driven by a single-pulse of equivalent power. This paves the way towards a significant down-sizing of laser-plasma electron accelerators making their use in scientific and technological applications extremely attractive and affordable.
The photon yield obtained during the betatron motion of electrons in the ion cavity is proportional to the number of such oscillations (or wiggling). In the case of high repetition rate laser systems the pulse energy is very limited, thus the laser pulse has to be tightly focused in order to achieve high peak intensity in the focus. In this way narrow gas jets are used, which results in short acceleration length of electrons, thus in low number of betatron oscillations. In order to increase the number of x-ray photons a mixture of clusters and gas target can be used, where the space-charge of nano-meter size droplets forces the electrons to oscillate at much higher frequency leading to the emission of more photons. In this work we present the interaction of such cluster targets with linearly polarized laser pulses and describe some possible positive effects when circular polarization is used.
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