Dephasing length optimization by controlling plasma density in laser wakefield accelerators

摘要

Summary form only given. The energy gain of an electron beam in the laser-driven plasma wave can be limited by dephasing between the accelerated electron bunch and the plasma wave. The dephasing length is defined as the length the electrons travel before it phase slip by one-half period of plasma wave. Since the velocity of the accelerated electrons is higher than the phase velocity of plasma wave, the electrons outrun the plasma wave and moves into decelerating region of the plasma wave phase space. In order to enlarge the dephasing length, the plasma wavelength should be increased by optimizing the plasma density profile. We report the measurements of dephasing length using plasma density distribution with linear, up-ramp and parabolic profile. The accelerated electron energy gain is calculated in different plasma density profiles. We observe that the plasma wavelength decreases when the plasma wave propagates from low to high-density layer. After density transition, the accelerated electron phase synchronizes with the phase of plasma wave. This increases the electron acceleration length and improves the beam quality. This may be crucial to determine the electron beam energy in LWFA.