STRUCTURE DESIGN OF THE BEIJING SPECTROMETERⅢBEAM PIPE

摘要

＜正＞The Beijing spectrometerⅢ（BESⅢ）beam pipe is in the center of the BESⅢ,which is the detector of the upgrade project of Beijing electron and positron collider（BEPCⅡ）.Electrons and positrons collide in the BESⅢbeam pipe.According to the demands of the BEPCⅡ,a key program of Chinese Academy of Sciences,the BESⅢbeam pipe is designed based on the finite elements analysis.The BESⅢbeam pipe is installed in the inner cylinder of the BESⅢdrift chamber.As a vacuum tube,the BESⅢbeam pipe is designed as 1000 mm in length,63 mm in inner diameter and 114 mm in outer diameter,respectively.The BESⅢbeam pipe consists of a central beryllium pipe cooled by EDM-1,the oil No.1 for electric discharge machining,and two extended copper pipes cooled by deionized water（DW）.The three parts are jointed by vacuum welding.Factors taken into account in the design are as follows.①The wall thickness of the central beryllium pipe should be designed as small as possible to reduce the multi-scattering and improve the particle momentum resolution.And the wall thickness of the extended copper pipe should be designed as large as possible to protect the detectors from the backgrounds.②The BESⅢbeam pipe must be sufficiently cooled to avoid the damage and prevents its influence to the BESⅢdrift chamber（DC）operation.The inner surface temperature of the DC inner cylinder must be maintained at 293±2 K.③The magnetic permeability of the materials used in the BESⅢbeam pipe must be less than 1.05 H/m to avoid large magnetic field distortions.④The static pressure of the vacuum chamber of the BESⅢbeam pipe must be less than 800μPa.The simulating results show that the designed structure oftbe BESⅢbeam pipe satisfies the requirements mentioned above.The structure design scheme is evaluated and adopted by the headquarters of BEPCⅡ.