LCABD Collaboration: Work Package 5: Crab Cavity

在国际线性对撞机（ILC）束流传输系统的操作参数方面，最灵活的设计通常是电子和正电子束流线之间的交叉角大于约14 mrad，这样电子和正电子束就不会穿过彼此的最终聚焦四极偶极，从而有助于在碰撞后提取废束流。基线选项有两个交叉角分别为20mrad和2mrad的交互点。另一种选择有一个14毫角的交叉角。电子束和正电子束在相互作用点之间的夹角将导致光度损失，除非通过适当的旋转加以纠正。这些束的旋转将使用两个蟹腔系统进行。螃蟹腔是一个横向偏转射频腔与90度相移操作。螃蟹腔系统的开发是ILC大交叉角方案的关键研发要求。英国在初步研究中发挥了主导作用，以确定最适合ILC波束发射系统（BDS）当前布局的系统类型，并将满足全球设计努力（GDE）规范。目前的一个项目已经确定，最适合ILC蟹状腔要求的是开发一种CKM 3.9 GHz超导腔的衍生物。已经建立了腔体运行的射频相位容差。螃蟹腔系统的开发现在需要优化超导射频偶极子腔，使其高阶模式（HOM），低阶模式（LOM）和同阶模式（SOM）组件得到充分的阻尼，以消除与ILC光束的任何不必要的尾场相互作用。将这种能量从腔体和低温模块耦合出来需要开发合适的耦合器和吸收剂解决方案。SRF空腔/低温模块中的传声器不稳定性也会降低RF系统的性能。为了减轻这种影响，需要进行详细的机械和热分析。螃蟹腔系统的振幅稳定性必须优于万分之一，电子和正电子螃蟹腔系统在3.9 GHz时的相对相位误差不大于0.07度。为了达到这一苛刻的规范，预计必须采用基于最先进数字处理技术的先进控制技术，并需要进行研究以突破目前可实现的极限。高功率3.9 GHz射频系统必须在其幅度和相位稳定性以及随后与螃蟹LLRF控制系统的集成方面进行表征。整个系统需要制造，在光束线上测试和操作验证，为2010年GDE的技术设计报告做准备。

The most flexible designs of the international linear collider (ILC) beam delivery system in terms of operating parameters, typically have a crossing angle between the electron and positron beamlines greater than about 14 mrad so that electron and positron bunches do not pass through each other's final focusing quadrupole doublets thereby assisting the extraction of spent beams after collision. The baseline option has two interaction points with crossing angles of 20mrad and 2mrad. An alternative option has one crossing angle of 14 mrad. The angle between the electron and positron bunches at the interaction point will cause a luminosity loss unless corrected by an appropriate rotation. The rotation of these bunches is to be performed using two crab cavity systems. A crab cavity is a transverse deflecting RF cavity operated with a 90 degrees phase shift. The development of a crab cavity system is a critical R&D requirement for ILC large crossing angle schemes. The UK has taken a lead role in initial studies to define the type of system that will most readily fit current layouts for the ILC beam delivery system (BDS) and that will meet Global Design Effort (GDE) specifications. A current project has established that the best fit to the ILC crab cavity requirement is to develop a derivative of the CKM 3.9 GHz superconducting cavity. RF phase tolerances for the operation of the cavity have been established. The crab cavity system development now requires optimisation of the superconducting RF dipole cavity, such that its Higher Order Mode (HOM), Lower Order Mode (LOM) and Same Order Mode (SOM) components are sufficiently damped so as to eliminate any unwanted wakefield interaction with the ILC beams. Coupling out this power from the cavity and cryomodule requires development of appropriate coupler and absorber solutions. Microphonics instabilities in the SRF cavity/cryomodule can also deteriorate RF system performance. To mitigate such effects detailed mechanical and thermal analysisis required. Amplitude stability of the crab cavity systems must be better than one part in 10,000 and the relative phase error between the electron and positron crab cavity systems must not be more than 0.07 degrees at 3.9 GHz. To reach this demanding specification it is anticipated that advanced control techiques built on state of the art digital digital processing must be employed and research is needed to push the limit on what is currently achieveable. The high power 3.9 GHz RF system must be characterised in terms of its amplitude and phase stability and subsequent integration with the crab LLRF control system. The entire system needs to be fabricated, tested on a beamline and operation proven in preparation for the Technical Design report of the GDE due in 2010.