Mitigation of coherent-OTR light effect for the beam profile monitor of SACLA

A screen monitor (SCM) system for SACLA has been developed and was installed in order to obtain a direct image of a transverse beam profile. The taken image has a spatial resolution of about 10 μm, which is required to investigate electron-beam properties, such as a beam emittance. The SCM takes an image of the beam profile by a CCD camera with a customized optical system through an OTR radiation from a stainless steel screen. At the beginning of SACLA operation, strong coherent OTR (COTR), which made an incorrect beam profile, was observed. In COTR light, light intensity and an imaging with a speckle widely fluctuate in every shot. In order to suppress the COTR on the SCM, the stainless steel target was replaced to a Ce:YAG scintillation target. Since the COTR was still generated from the Ce:YAG target, a spatial mask was employed. The mask was mounted on the center of an optical axis of the SCM, because the COTR light is emitted forward within ∼1/γ radian from the screen to the CCD, while scintillation light does not have angular dependence. Hence the mask obstructs the COTR light to the CCD. Clear beam profiles with a diameter of a few tens of micro-meter are observed by means of the SCMs with this simple improvement. This fact indirectly testifies the SCM has a spatial resolution of about 10 μm. (author)