Prediction of vacuum deterioration caused by vacuum accident in the beamline

Vacuum deterioration at the most upstream of the front-end was predicted quantitatively under the assumption that a rupture of windows and subsequent inrush of air might occur in the SPring-8 beamline. A 10-m-long shock tube was employed to simulate an inrush of air into a region with rarefied gas dynamics. The shock Mach number (Ma) and pressure increase were measured with the fast-closing shutter (FCS) system both activated and de-activated. Actual front-end components as well as plain pipes having various lengths and another pipe with baffle plates inside were inserted into the shock tube by arranging various combinations in tandem. Although it could be observed that an increase in the number of repetitions of the cross-sectional change decreases Ma, the pressure increase essentially depends on the total internal volume of the shock tube. Besides, for the other internal structure, which has a gently tapered cross-sectional change, the pressure increase are suppressed as well as the decrease of Ma. The effective leak rate of the FCS at the inrush of air could be presented as a function of the total internal volume; thus, the number of gas molecules flowing into the most upstream of the front-end could be estimated.