A vacuum protection system for the front end of SPring-8

Abstract

The design of a high-speed movable heat absorber (HA) to protect a fast closing valve (FCV) is presented and a unique safety system to protect the storage ring ultrahigh vacuum from a vacuum break accident in downstream beamlines is proposed. The speed of the incoming shock wave upon a vacuum failure can be measured by two sensors separated by a given distance. If the measured speed does not exceed the prescribed critical speed calculated from both the operating time of the HA and the distance between the sensors and the FCV, the storage ring vacuum can be maintained without dumping the electron beam. If the measured speed exceeds the critical speed, beam dumping will be requested. Fortunately the vacuum sensor can be installed at a distance 14 m from the FCV for the SPring-8 and the operation of the HA can be completed in 40 ms. The acceleration for moving at 40 ms is approximately 2 G, which is a practical value. Therefore the critical value of the shock wave speed is 350 m/s. In order to perform thermal analysis for the absorber plate, the fluorescent x rays and Compton scattered rays emitted from 3° inclined copper absorber can be calculated as 4.7% and 0.41% of the SPring-8 undulator power, respectively. Hence the highest temperature at the absorber surface exposed to intense undulator radiation of the total power of 2.06 kW was about 350 °C.