Focal spot characteristics of Thomson scattering x-ray sources

With the recent rapid development of x-ray imaging using Thomson scattering x-ray sources, there is an increasing demand for knowledge of the focal spot characteristics of this type of light source. This is because imaging quality is closely related to focal spot characteristics. In this paper, an analytical expression of the focal spot size and beam parameters at arbitrary interaction angles is derived. Based on this, the focal spot can be optimized in a straightforward manner by adjusting the laser and electron focusing at a fixed interaction geometry. As the beam focusing increases, a smaller focal spot and a higher photon yield are obtained, while the bandwidth of scattered x rays deteriorates, leading to a compromise of the above three physical parameters. Considering the beam position and time fluctuations, the focal spot will be broadened and the photon yield will be reduced. Using typical beam parameters at the Tsinghua Thomson scattering x-ray source, it is shown that a micrometer-sized focal spot can be easily achieved. The results will help one to develop the x-ray imaging field, especially in micro-computed tomography applications based on Thomson light sources.