X-ray imaging of a small electron beam in a low-emittance synchrotron light source

We have developed the X-ray beam imager (XBI) at the SPring-8 storage ring to observe transverse profiles of a small electron beam in a low-emittance synchrotron light source. The XBI is based on a single Fresnel zone plate (FZP) and an X-ray zooming tube (XZT). The X-ray image of the electron beam moving in a bending magnet is obtained by the FZP, and it is enlarged by the XZT. Monochromatic X-rays are selected by a double crystal monochromator to avoid the effect of chromatic aberration of the FZP. The XBI has achieved a 1σ spatial resolution of approximately 4 μm, and a time resolution of 1 ms. Its field of view, which is free from vignetting, is larger than 1.5 mm in diameter on the coordinates of the electron beam. With the XBI, we have successfully measured the profiles of the electron beam of the SPring-8 and have found a vertical emittance lower than 10 pm rad at 10 mA circulating current. By utilizing the fast time resolution, we were able to study the transient behaviors of the effective beam profiles after beam injections in top-up operation at the SPring-8. In order to demonstrate the full two-dimensional imaging capability of the XBI, we also observed the dependence of the electron beam profile on the horizontal and the vertical betatron tunes.