LCABD Collaboration: Work Package 5: Crab Cavity

国际线性对撞机（ILC）束输送系统在操作参数方面的最灵活设计通常具有大于约14 mrad的电子束线与正电子束线之间的交叉角，使得电子束团与正电子束团不穿过彼此的最终聚焦四极偶，从而有助于在碰撞之后提取用过的束。基线选项有两个相互作用点，交叉角为20 mrad和2 mrad。另一种选择是一个14 mrad的交叉角。电子束和正电子束在相互作用点之间的角度将导致亮度损失，除非通过适当的旋转来校正。这些束的旋转将使用两个蟹腔系统进行。蟹状腔是以90度相移操作的横向偏转RF腔。蟹状腔系统的开发是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束流传输系统（BDS）当前布局的系统类型，并满足全球设计工作量（GDE）规范。目前的一个项目已经确定，最适合ILC螃蟹腔的要求是开发CKM 3.9 GHz超导腔的衍生物。已经建立了用于腔的操作的RF相位容限。蟹状腔系统的开发现在需要优化超导RF偶极腔，使得其高阶模（HOM）、低阶模（LOM）和同阶模（SOM）分量被充分阻尼，以便消除与ILC束的任何不想要的韦克菲尔德相互作用。从腔和低温制冷机耦合出这种功率需要开发适当的耦合器和吸收器解决方案。SRF腔/低温振荡器中的微音不稳定性也会使RF系统性能恶化。为了减轻这种影响，需要进行详细的机械和热分析。蟹状腔系统的振幅稳定性必须优于万分之一，电子和正电子蟹状腔系统之间的相对相位误差在3.9 GHz时不得超过0.07度。为了达到这一要求的规格，预计先进的控制技术，建立在国家的最先进的数字数字处理必须采用和研究，需要推动什么是目前可替代的限制。高功率3.9 GHz RF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。整个系统需要在光束线上进行制造、测试和运行证明，为2010年到期的GDE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.