We found the " forward stimulated raman scattering " will be excited if the pulse length is greater than plasma wave length . the " forward stimulation raman scattering " decreases the phase velocity and the amplitude of the wake wave which will lead to the reduction of maximum kinetic energy of the electrons trapped 结论是：当脉冲长度接近等离子体波长时，稀薄等离子体将发生“前向受激raman散射” ，它使脉冲后沿拉长，导致尾流场的相速度变小，以至于被尾流场“捕获”的电子最大动能大大下降。
