Development of a laser wire beam profile monitor

超低发射度电子束的产生是实现第三代同步辐射光源（SOR）、自由电子激光（FEL）和高能电子直线对撞机的关键技术之一。本研究的主要目的是研制一种能直接测量电子束束流尺寸的高精度束流轮廓监测仪。具体来说，该监测器必须能够测量KEK ATF阻尼环中的电子束轮廓。该项目的重点是（1）实现满足所需性能的激光线靶，（2）将其安装在阻尼环中。首先，我们成功开发了束腰为15 μm、精细度为321的法布里-珀罗光学腔。在判断上述腔体足够好之后，我们将其安装在阻尼环中，并测量实际光束尺寸或发射度。结果表明，束流尺寸σ_B=9.8±1.0μm，与设计值一致。这证明了ATF阻尼环中的束流发射度是世界上各种机器中最低的。我们进一步改进了腔，并测量了发射度，以寻找可能的电流依赖性。该测量中的关键点是建立Toucheck效应（如果存在的话）。目前正在公布结果。综上所述，我们成功地研制出了具有所需性能的激光线束轮廓监测仪。

Production of ultra low emittance electron beams is one of the essential techniques in realizing third-generation synchrotron radiation light sources (SOR), free electron lasers (FEL) and/or high-energy electron linear colliders. The main purpose of the present research is to develop a precision beam profile monitor which can directory measure their electron beam size. Specifically, the monitor under development must be able to measure the electron beam profile in an ATF damping ring at KEK.Most important points in the project are (1) to realize a laser wire target that satisfies needed properties, and (2) to install it in the damping ring.First of all, we successfully developed a Fabry-Perot optical cavity with a beam waist of 15 μm and finess of 321. Having judged that the cavity above was good enough, we then installed it in the damping ring, and measured actual beam size or emittance. We found that the beam size was σ_b=9.8±1.0μm, consistent with the designed value. This has proved that the beam emittance in the ATF damping ring is lowest among the world's various machines. We further improved the cavity, and measured the emittance in search of possible current dependence. The crucial point in this measurement is to establish Toucheck effect, if present. Publication of the results is under way. In conclusion, we successfully developed the laser wire beam profile monitor with needed properties.