摘要:Energetic particle beams are important tool in many application, from basic science to healthcare, also a crucial element to produce high quality X-ray radiation. Conventional accelerators are relative large, from  10 m to   several km, due to  material-breakdown-limited  acceleration gradient and energy loss via synchrotron  radiation.    Size and cost of maintenance of conventional accelerator  limit  the extent of application for daily life. Wakefield accelerator driven by either high energy or high energy particle is a promising way to build a table-top accelerator. For example, GeV-scale electron bunch driven by laser pulses has been demonstrated by  using  cm long gas targets . As a contrast, it needs almost 100 m of conventional accelerator to reach this energy. In this talk, I will first introduce fundamental physic  of wakefield accelerator and related experiment. In the end of discussion, a physical phenomenon called "collective deceleration"  associated to the  unique properties of laser-driven electron bunches will be studied by experiment and PIC simulation.