There are two FACET tunnel access modes (Permitted & Guarded. There is no controlled access in FACET):
1). Guarded Access with Radiation Physics Escort - this is designed for short (less than 30 minutes) accesses for one or two experimenters. This access mode minimizes the beam time lost because by guarding the entrance there is no need for a search after the access.
2). Permitted Access - this is for when longer access is needed or more thanone or two experimenters need to go into the tunnel. This access mobe is used for PAMM. If such an access is required during run time, the experiment may lose as much as a shift of beam so the decision to go in is not done lightly.
More information on the these access modes, how they are done and the impact is found here.