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由直线加速器提供的用于自由电子激光（ＦＥＬ）的电子束团，在其传输过程中常会遇到传输管道孔径变化的情况．本文在电荷密度均匀分布假定下，应用直线加速器中空间电荷束团的有限长柱模型，推导得到了束流传输管道孔径变化引起的束团场能变化及场能差公式．数值计算结果表明，它与国外以往所采用的以连续束流来代表直线加速器中电荷束团而得到的结果，有很大的差别，并且是更符合直线加速器中的电荷束团的实际情况，因而是更精确和可靠的．
Electron bunches for free-electron lasers (FELs) provided by linear accelerators often experience changes in the transmission pipe aperture during their transmission. In this paper, under the assumption of uniform charge density distribution, the finite long-column model of space charge bundle in a linear accelerator is used to derive the beam field energy variation and the field energy difference formula due to the change of the beam transmission pipe aperture. The numerical results show that it is very different from the results obtained by using the continuous beam to represent the charge bundle in the linear accelerator and is more in line with the actual situation of the charge bundle in the linear accelerator. It is therefore more accurate and reliable.