drifting distance meaning in English

流程

With the increasing initial orientation angle and length - width ratio , the drifting distance , the fluctuation of the orientation angle , lateral drifting velocity and rotation velocity all increase , while the final settling velocity decreases with the increasing length - width ratio
初始取向角和长宽比增大，则粒子的横向漂移以及取向角、侧向漂移速度和转动角速度的振荡幅度都增大；同时随着长宽比的增大，粒子的沉降速度相应减小。
Numeric simulation is done based on 30mev linac structure and the corresponding parts " parameters . the simulate result shows that the design of beam line is reasonable . the beam properties change very small when the drifting distance before the first acceleration stage is changed in the certain ranges
根据30mevlinac结构和元件参数进行了整体的数值模拟，模拟结果表明： 30mevlinac中各个加速段的结构与性能较好，输出束流的发射度和能散度都较小，束线的设计也较合理。