Beam halo and beam loss studies at the KEK Compact ERL

The beam halo studies are performed at KEK Compact ERL (cERL) systematically since machine commissioning in spring 2015. The beam loss in the recirculating loop of the accelerator was observed during the machine study. Wherein we found that the beam loss can be avoided making use of a collimation system of cERL. Therefore we established a beam halo formation study. Beam halo measurement in spring 2016 commissioning demonstrated a presence of the vertical beam halos at multiple locations of the beam line except the region near the electron gun. We guess that the transverse beam halo could occur from the longitudinal bunch tail arising at the photocathode. The halo formation process should include all the mechanisms transferring the longitudinal bunch tail into the transverse plane. They could be rf field kicks, due to injector line elements misalignments and an effect of the steering on the beam trajectory. In the present study we compare the results of the simulation including effects described above with the measurement results to explain the beam halo and to avoid the beam loss during the machine operation. (author)