Improvement of Screen Monitor with Suppression of Coherent-otr Effect for Sacla

The construction of SACLA (SPring-8 Angstrom Compact free electron LAser) was already completed and it is under operation. A screen monitor (SCM) system has been developed and was installed in order to obtain a direct image of a transverse beam profile with a spatial resolution of about 10 m, which is required to investigate electron-beam properties, such as a beam emittance. The SCM originally has a stainless steel target as an OTR radiator or a Ce:YAG crystal as a scintillation target. At the beginning of SACLA operation, strong coherent OTR (COTR), which provides incorrect beam profile image, was observed after full bunch compression to make a peak-current of over 1 kA. In order to suppress the COTR effect on the SCM, the stainless steel target was replaced to the 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 optical axis around the center of the SCM image, because the COTR light is emitted forward within ~1/γ radian, while the scintillation light almost has no angular dependence. Clear beam profiles with a diameter of a few tens of micrometre are observed by means of the SCM with this simple improvement.