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HIAF-BRing四极磁铁钛合金内衬真空管道宽带阻抗研究

Research on Broadband Impedance of Titanium Alloy Linings-lined Vacuum Chambers for Quadrupole Magnet on HIAF-BRing

  • 摘要: 快循环同步加速器的二极磁铁和四极磁铁磁场变化过快可能会在常规真空管道中引发严重的涡流效应,针对这一问题国内外提出了多种新型真空管道方案,例如陶瓷管道、外侧加筋超薄壁不锈钢管道以及陶瓷内衬超薄壁不锈钢管道等等。最近中国科学院近代物理研究所(IMP)的HIAF工程团队提出了用钛合金内衬代替陶瓷内衬的方案,形成的钛合金内衬超薄壁不锈钢真空管道横向孔径更小,出气率更低,制造成本也更少。但钛合金内衬真空管道将成为HIAF装置的环形主加速器BRing上重要的束流耦合阻抗源。其中四极铁钛合金内衬真空管道引入的阻抗及其影响,以及相应阻抗减小设计的有效性尚待评估。本工作首先在模拟软件CST studio suite中使用两种不同的方法分别对各种规格的四极铁钛合金内衬真空管道的阻抗进行了模拟,两种方法给出了互相接近的结果。然后在此基础上计算了换用钛合金内衬真空管道后BRing的全环横向阻抗。进一步地,本工作结合全环阻抗数据利用Vlasov solver计算了换用钛合金内衬真空管道后BRing上78Kr19+束在不同流强下的模式频率移动。发现二极磁铁和四极磁铁钛合金内衬真空管道采用阻抗减小设计前,BRing上78Kr19+束发生横向模耦合不稳定性的流强阈值为 2.7\times10^11 ppp,低于BRing上78Kr19+束的设计流强( 3\times10^11 ppp)。采用阻抗减小设计后,78Kr19+束发生横向模耦合不稳定性的流强阈值会提升到 4.4\times10^11 ppp。这表明二极铁和四极铁钛合金内衬真空管道的阻抗减小设计是有效的。仅从横向宽带束流耦合阻抗的角度来看,BRing二极磁铁和四极磁铁上用钛合金内衬真空管道替代陶瓷内衬真空管道是可行的。

     

    Abstract: Rapid change of the magnetic field of the dipole and quadrupole magnets on a rapid-cycling synchrotron may induce serious eddy-current effect in the conventional vacuum chambers. In response to this issue, numerous new type vacuum chamber designs for these magnets have been proposed. These include ceramic chambers, thin-walled stainless steel chambers strengthened with transverse ribs, and ceramic rings-lined thin-walled stainless steel chambers. Recently, the HIAF engineering team at The Institute of Modern Physics, Chinese Academy of Sciences, suggested 3D-printed titanium alloy lining as an alternative to the ceramic ring lining. A vacuum chamber lined with 3D-printed titanium alloy linings offers several advantages over other designs. These include higher structural strength, smaller aperture, lower outgassing rate, and relatively lower cost. However, titanium alloy linings in vacuum chambers could become significant sources of coupling impedance on BRing, the circular main accelerator of the HIAF Facility. The influence of this impedance on beam instability, as well as the impact of potential impedance mitigation designs, still requires evaluation. This paper employs two distinct methods in the CST Studio Suite software to simulate the coupling impedance introduced by all types of titanium alloy linings-lined vacuum chambers of quadrupole magnets. The results from both methods closely agree with each other. Based on these works, we calculated the transverse dipolar impedance of the entire ring. Furthermore, using the Vlasov solver, we calculated the mode shift of 78Kr19+ bunch on BRing under varying beam intensity conditions.The results indicate that, before the implementation of impedance mitigation on the titanium alloy linings-lined vacuum chambers of the dipolar and quadrupole magnets, the beam intensity threshold of 78Kr19+ bunch on BRing, as given by transverse mode coupling instability, was 2.7×1011 ppp. This is below the designed beam intensity for 78Kr19+ bunch on BRing (3×1011 ppp). However, after the application of impedance mitigation design, this beam intensity threshold will shift to 4.4×1011 ppp. This suggests that the impedance mitigation on the titanium alloy linings-lined vacuum chambers of the dipolar and quadrupole magnets is effective. From the perspective of only the transverse broadband coupling impedance, it is viable to substitute the ceramic rings-lined thin-walled stainless steel vacuum chambers with the titanium alloy linings-lined thin-walled stainless steel vacuum chambers for dipolar and quadrupole magnets on BRing.

     

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