| | Vacuum systems containing superconducting cavities
which have to be operated at high gradients need to
preserve the cleanliness of the superconducting cavity
surfaces. In addition to an adequate preparation of the
cavities and the neighbouring vacuum components special
care needs to be taken during pump down and venting.
Neither should be particles introduced into the vacuum
system, nor should particles already present within the
system be moved towards critical areas.
For the superconducting linear accelerators of FLASH
and the European XFEL at DESY a series of
measurements have been performed to study the
movement of particles in long tubes during pump down
and venting. For this purpose an in-situ vacuum particle
counter has been used. By reducing and varying the gas
flow during these processes, it is possible to perform
these actions without moving particles present inside such
systems. Based on these measurements a set-up using
various filters, flow controllers and a pressure gauge has
been developed to avoid introducing particles into the
vacuum system as well as moving existing particles. This
set-up allows automated pump-down and venting of
critical vacuum systems in a reliable and reproducible
way, being much faster than the procedures used so far. | |