对低发射度储存环真空问题的认识

Vacuum Design Considerations of Low Emittance Storage Rings

低束流发射度环的lattice结构是决定真空系统设计指标和方案的重要因素之一。大量插入件使束流的库仑散射寿命成为限制气体散射寿命的主要因素。作者提出了适用于第三代光源的布泵新概念———布泵的终极目标是提高气体散射束流寿命 ,使环上各处的库仑散射作用截面尽可能一致和较小是提高库仑散射寿命的合理途径。为此 ,布泵时必须综合考虑影响各处库仑散射作用截面的三个因素 ,即各处的真空室内高度、压力和 β函数值。低发射度环磁铁系统限制了真空系统的理想空间 ,低发射度环允许采用小截面束流真空室 ,带有抽气前厅的新型双真空室应运而生。提高环能量接收度将增大束流的托歇克寿命和轫致辐射寿命 。

A novel vacuum pump arrangement was developed for the third generation light sources.Various physical considerations involving beam life time in vacuum design were discussed.The lattice structure of a low emittance storage ring is of primary importance in its vacuum system design because large number of inserted devices lead to considerable shortening of Coulomb scattering life time of the beam, τ cs ,which in turn restricts the life time of the gases scattering life time, τ g.A feasible way to solve the problem is to minimize Coulomb reaction scattering cross section and to ensure its uniform distribution inside the ring.Three major factors including vacuum chamber height,pressure and β function in a ring should be carefully balanced in vacuum pump arrangement because they may significantly affect Coulomb scattering cross section.Since the optimum space of the vacuum system was restricted severely by magnet system of a low emittance ring,a beam vacuum chamber with small cross section and which has no influence on τ q can be employed.The new vacuum chamber with an antechamber was constructed.A larger energy acceptance will relax the rather stringent requirements of vacuum system,and increase both τ t and τ bs .