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Femtosecond MeV electron beam generated by laser-plasma accelerators (LPA) is a promising source for ultrafast electron diffraction (UED) application. Compared to conventional UED instruments which limit temporal resolution to a few tens of fs, plasma electron accelerator-based UED is possible to make sub-10 fs temporal resolution because of no intrinsic time jitter between pump-probe pulses and ultrashort electron bunch length. Some groups have shown that a few MeV electron beam can be produced by using a few mJ laser pulse as it has shorter pulse duration (single- or few-cycle). In this regime, the laser pulse is tightly focused onto gas target, and thus electrons in relatively high density plasma (1020 cm-3) are self-injected and accelerated. However, the electron beam quality like energy spread and emittance should be still improved for applications. Here, we introduce plan of two laser pulses-based plasma electron acceleration research for UED application at Pohang Accelerator Laboratory (PAL). A laser pulse is separated to two pulses that one is used to drive plasma wakefield and the other one is delivered to induce electron injection in a plasma bubble. Since the driving pulse intensity is retained under threshold of self-injection to suppress electron injection, the electron injection occurs in a localized region the injection pulse is focused, resulting in the high quality electron generation. In addition, researches conducting for better electron beam quality are presented in this presentation.
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