Development of the slow pumping & venting system

We usually encountered the degradation of the superconducting RF cavities on the cryomodule test even though the performance of these cavities was good on the vertical test. In reality, the degradation of Q-values of two cavities of cERL main-linac were observed after cryomodule assembly in KEK. Some dust and invisible particles might enter the cavity and generate field emission during the assembly work and vacuum work under pumping and/or venting of the cavity. Field emission is the most important cause of this degradation. Slow pumping system allow us not to enter the particulate into the cavity during pumping and/or venting of the cavity. In this paper, we show the development work of this slow pumping and venting system and its performance by using the vacuum particle sensor. (author)