切伦科夫辐射“双成像法”测量电子束发射度

A New Measurement of Electron Beam Emittance with Cerenkov Radiation "Double Imaging" Method

利用切伦科夫辐射 ,OTR或荧光靶等光学诊断方法进行发射度测量 ,国内外绝大部分实验是用CCD相机观测电子束打靶产生的光斑 ,变化四极透镜的磁场梯度 ,应用“三梯度法”计算出发射度 .文中提出了一种新的“双成像法”测量方法 ,使切伦科夫辐射光通过一长焦距的消色差薄透镜 ,分别在焦平面和像平面获取图像 .通过图像处理 ,前者可分析出电子束散角分布 ,后者可分析出电子束径向分布 ,从而直接得到均方根发射度 .该方法对束流相空间和电荷密度分布无需假设 ,无需借助“三梯度法” ,较其他常规测量方法具有实验装置更简便、测量精度更高和适用性更广等优点 .文中给出了该测量方法对北京大学DC SC光阴极注入器的发射度测量进行计算机模拟实验的结果和分析 .

A new way in electron beam emittance measurement with Cerenkov radiation 'double imaging' method is proposed in this paper. In the standard emittance measurements with optical diagnostics such as Cerenkov radiation, OTR (Optical Transition Radiation), fluorescence screen or BMP (beam profile monitor) etc., the emittance is indirectly calculated through quadrupole-scanning technique, with the prior ssumption that the beam phase space density distribution is ellipse, which will certainly induce systematic error when the beam profile is quite irregular or the space-charge effects can not be omitted. In proposed method, the Cerenkov radiation pass through a 1-meter focal-length thin convex lens, and a CCD camera is used to capture two images of Cerenkov radiation at the focal plane and at the image plane of the lens respectively. Then, with image processing technique, we acquire the angular divergence information of the electron beam from the image of the focal plane and the radial distribution information from the image of the image plane, which we call Cerenkov radiation 'double imaging' method. Therefore, the emittance can be directly attained according to the definition of the RMS emittance. By this method, we can measure the actual phase space distributions without making any prior assumptions about the density distributions. Compared with other general measurements, this 'double imaging' method has advantages of simpler equipment, higher precision and wider application. This paper also presents the computer simulation results of emittance measurement on the DC-SC (DC-Superconducting) photocathode injector of PKU-SCAF (Peking University Superconducting Accelerator Facility) being built at Peking University.