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Physical designing of the pre-chopper in CSNS LEBT is carried out,which includes the deflecting voltage,the length and the width of the deflecting plates,and the gap between the deflecting plates.The most outstanding feature of the design is that both the gap and the width vary with the beam envelope size.So both the requried deflecting voltage and the loaded capacitance are lowered.In order to avoid destruction of the space charge neutralization by the pre-chopper in the whole LEBT,an electron-trapping electrode is arranged to confine the electrostatic field of the pre-chopper to the local area.To examine the reliability of the pre-chopping design in CSNS LEBT,a similar pre-chopping design in ADS RFQ LEBT is set up and an experiment on the pre-chopper is prepared.3-dimensional simulations are carried out to determine the loaded capacitance and the applied voltage of the electron-trapping electrode.