Simulation of Wakefield Effect in Ilc Ir Chamber*

To achieve super high luminosity, high current beams with very short bunch length are needed, which carry high intensity EM fields. For ILC, two bunch trains with bunch length of 300μm and bunch charge of 3.2nC are needed to collide at the IR to achieve the ILC luminosity goals. When the 300μm bunches pass through the IR chamber, wakefields will be excited, which will cause HOM power flowing through the IR chamber beam pipe to the final doublets due to the high frequency characteristic of the induced wakefields. Since superconducting technology is adopted for the final doublets of ILC BDS, whose operation stability might be affected by the HOM power produced at the IR chamber, quench might happen. In this paper, we did some analytical estimation and numerical simulation on the wakefield effects in ILC IR chamber. INTRODUCTION In order to answer what the universe is made of and provide new insights into its working principle, energy regimes beyond the reach of today’s accelerators need to be investigated. The International Linear Collider (ILC) is a new cosmic doorway to realize this. The two main linacs of ILC accelerate very short (~300μm) and high peak current (3.2nC/bunch) bunches into the Beam Delivery System (BDS) on the way to the interaction point [1]. The ILC BDS is responsible for transporting the e/e beams from the exit of the high energy linacs, focusing them to the sizes required to meet the ILC luminosity goals [1][2] (σx*=639nm, σy*=5.7nm for norminal), bringing them into collision, and transporting the spent beams to the main beam dumps. In order to realize this, superconducting final doublet is adopted and placed on each beam line just before the IR chamber. When charged particle beams traverse through nonsmooth or resistive beam pipe walls, wakefield will be excited. If the wakefield frequency is below the beam pipe’s cut-off frequency, it will be trapped, otherwise it will propagate out and cause HOM heating of the surrounding components. For ILC IR chamber, one choice of the geometry is shown in Fig. 1 (here we simplify it to 2D to facilitate the analysis) [3]. The radius of the ingoing and outgoing beam pipe is very small (~10mm), whose cut-off frequency is about 18GHz for TE modes and 12GHz for TM modes, so part of the wakefield will stay in the chamber after the beam’s passage. On the other hand, due to the very short bunch length of ILC, the induced field spectrum will go to higher frequency (far beyond beam pipe cutoff frequency), so most of the wakefield will propagate out of the chamber, which may cause quench of the SC final doublets. We evaluated the geometric wakefield effects in IR chamber analytically and numerically, but it is limited to 2-D analysis based on the capability of simulation tools. The numerical analysis was done with simulation codes ABCI[4] and MAFIA[5]. Figure 1: Geometry of ILC IR chamber (Geo-1). ANALYTICAL ESTIMATION To do the analytical estimation, we split the whole IR chamber into 4 regions, shown in Fig. 1. Every part can be looked as one shallow cavity, the impedance of which can be roughly estimated by