BEPCⅡ真空内扭摆磁铁波纹管RF屏蔽结构的改进

RF Shielding Improvement of Bellows of in-Vacuum Wiggler for Second Phase Construction of Beijing Electron-Positron Collider

BEPCⅡ真空内扭摆磁铁的波纹管在电子束流的高流强运行中发生了损伤和真空泄漏。对损伤原因进行了分析,怀疑是波纹管RF屏蔽结构的弹簧片与接触套之间存在间隙,导致了高次模泄漏。如果波纹管没有压缩到设计的安装长度,弹簧片与接触套之间可能存在1 mm的间隙。利用MAFIA软件对这种情况下的尾场和阻抗进行计算,结果表明尾场收敛很慢,最大的尾场振幅为0.08 V/pC,阻抗在1.32 GHz存在窄带的共振峰。在该间隙处会发生高次模泄漏,有必要对屏蔽结构加以改进。对屏蔽结构的弹簧接触方式做了改进,将弹簧片改为环形的弹簧丝,最大调节量由4.0 mm提高到7.9 mm,且杜绝了间隙。300~500 mA的束流实验表明,屏蔽结构改进后,波纹管附近的真空度从2.66×10-7Pa提高到6.65×10-8Pa,波纹管的温度从40℃以上降低到25℃以下。

Here,we addressed one of the possible reasons responsible for damages and leaks of the bellows of the in vacuum wiggler in the second phase construction of Beijing Electron-Positron Collider（BEPCII） and its solution.The damage could be caused by high order modes（HOM）,and the leak may originate from a gap between the spring washer and the contact sleeve of the RF shielding structure in the bellows.If the bellows was not compressed to its designed installation length,mm wide gap may exist.After the wakefield and the impedance of the situation was simulated with MAFIA software package,we found that the wakefield converges at an extremely slow rate with a 0.08 V/pC maximum swing,and that the impedance has a narrow-band formant at 1.32 GHz.The fact that HOM easily leaks from the gap and damages the bellows requires improvement of the shielding structures.We replaced the spring washer with an annular spring,which increases the maximum adjustment from 4.0 mm to 7.9 mm,and puts an end to the gap.The experimental results with a 300～500 mA beam shows that the improvement reduced the pressure at the bellows from 2.66×10-7 Pa to 6.65×10-8 Pa,and that the temperature on the outside walls of the bellows dropped from above 40℃ to below 25℃.