利用速度群聚效應壓縮電子束時，在壓縮前期電子束的空間電荷作用會對電子束的品質有很大的影響，此時需要一個外加螺線圈磁鐵，產生軸對稱的磁場來克服橫向的空間電荷效應，但因為實際的儀器架設有許多部份避開，如冷卻水管、電流輸入端…等，因此螺線圈的外包鐵殼無法做到理想的圓柱對稱，進而影響到磁場分布。此外，螺線圈安裝上的誤差，同樣會使電子束感受到不對稱的磁場。本文中首先利用模擬軟體優化有外加螺線圈磁鐵後直線加速器出口處電子束的品質，並探討螺線圈磁鐵偏移所造成的影響。實驗的部分，主要量測螺線圈磁鐵提供不同的磁場下，電子束橫向的大小與束散度的變化，驗證螺線圈磁鐵的效果。

Space-charge effect is not negligible during the early stage of beam acceleration in a photoinjector rf linac that is operated for generation of short electron pulses by velocity bunching. A solenoid with iron shield can be used to provide the required axis-symmetric magnetic field to balance the radial space-charge force. However, the iron shield cannot be perfectly symmetric because openings are reserved for feeding water pipes and electrical cables to the coils. In addition, alignment errors of the solenoid may also spoil the symmetry of the system. In this study, simulation are carried out to optimize electron beam quality at linac exit and investigate how does the non-axisymmetric solenoid field of different origins influence beam properties, such as beam size, transverse emittance during the rf bunch compression. Beam size and transverse emittance are measured to check the adequacy of the linac solenoid magnet design.

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