Alternative Design of CEPC LINAC

Abstract

Circular Electron-Positron Collider (CEPC) is a 100 km ring e+ ecollider for a Higgs factory. The injector is composed of a Linac and a Booster. The baseline design of CEPC Linac is a normal conducting S-band linear accelerator with frequency in 2860 MHz, which can provide electron and positron beam at an energy up to 10 GeV and bunch charge up to 3 nC. To reduce the design difficulty of booster and booster magnet, an alternative design of the Linac with C-band accelerating structure at high energy part is proposed and the energy is up to 20 GeV. In this paper, the physics design of this scheme is presented. INTRODUCTION In September 2012, Chinese scientists proposed a Circular Electron Positron Collider (CEPC) in China at 240 GeV centre of mass for Higgs studies [1]. It could later be used to host a Super Proton Proton Collider (SppC) in the future as a machine for new physics and discovery. After that a great effort have been made in physics design [2]. The injector of CEPC is composed of a Linac and a full energy booster. The first part of the injector is a normal conducting S-band Linac with frequency in 2860 MHz and provide electron and positron beams at an energy up to 10 GeV [3, 4]. The main parameters of the CEPC linac are shown in Table 1 and layout is shown in Fig.1. The Linac is composed of electron source and bunching system (ESBS), the first accelerating section (FAS) where electron beam is accelerated to 4 GeV, positron source and pre-accelerating section (PSPAS) where positron beam is produced and accelerated to more than 200 MeV, the second accelerating section (SAS) where positron beam is accelerated to 4 GeV, the third accelerating section (TAS) where electron beam and positron beam are accelerated to 10 GeV, the electron bypass transport line (EBTL) where the electron beam is bypass the PSPAS and SAS section and one damping ring (DR) to reduce the emittance of positron beam. The electron Linac consists of ESBS, FAS, EBTL and TAS. The positron Linac consists of ESBS, FAS, PSPAS, SAS, DR and TAS. The horizontal distance between EBTL and SAS is 2.0 m. The Linac should be have potential to meet higher requirements and upgrade in the future, the designed bunch charge is larger than 3 nC both for electron and positron beam. Based on this consideration, the energy of electron beam for positron production is chosen as 4 GeV and the positron yield of positron source with some cut-off condition is 0.55 [5]. Table 1: Main Parameters of CEPC Linac Parameter Unit Value e-/e+ beam energy GeV 10 Repetition rate Hz 100 e-/e+ bunch population nC >1.5 Energy spread (e-/e+) <2×10-3 Emittance (e-/e+) nm <120 The CEPC booster [6] provides 120 GeV electron and positron beams to the CEPC collider and is in the same tunnel as the collider, which of circumference is 100 km. The electron beam is ramped from 10 GeV to 120 GeV in the booster. The magnetic field of dipole magnet is about 30 Gs at injection energy. It’s very challenging for magnet design and operation, on the other hand, the operating mode of magnet power supply is ramping and dynamic, so it’s a very critical issue. In order to reduce the design difficulty, increasing the energy of the Linac is a good way. Based on this idea, an alternative 20-GeV Linac scheme is proposed. High gradient accelerating structure is needed for high energy linac to reduce the length and cost. Considering the bunch charge is not very high and the emittance is small, the C-band accelerating structure is introduced to the alternative design. The C-band accelerating structure is used to replace S-band accelerating structure in the TAS and the energy of the Linac is increased to 20 GeV. The alternative design is presented and discussed in this paper. Figure 1: The layout of the CEPC Linac. ___________________________________________ *Work supported by National Natural Science Foundation of China (No. 11705214) and Youth Innovation Promotion Association CAS. † [email protected] 10th Int. Partile Accelerator Conf. IPAC2019, Melbourne, Australia JACoW Publishing ISBN: 978-3-95450-208-0 doi:10.18429/JACoW-IPAC2019-MOPTS065 MC1: Circular and Linear Colliders A08 Linear Accelerators MOPTS065 1005 Co nt en tf ro m th is w or k m ay be us ed un de rt he te rm so ft he CC BY 3. 0 lic en ce (© 20 19 ). A ny di str ib ut io n of th is w or k m us tm ai nt ai n at tri bu tio n to th e au th or (s ), tit le of th e w or k, pu bl ish er ,a nd D O I