KEK Report 95-7 August 1995 A KEK--95-7 JP9603334 KEKB B-Factory Design Report June, 1995 NATIONAL LABORATORY FOR HIGH ENERGY PHYSICS o 1 U 1 % © National Laboratory for High Energy Physics, 1995 KEK Reports are available from: Technical Information & Library National Laboratory for High Energy Physics 1-1 Oho, Tsukuba-shi Ibaraki-ken, 305 JAPAN Phone: 0298-64-5136 Telex: 3652-534 (Domestic) (0)3652-534 (International) Fax: 0298-64-4604 Cable: KEK OHO E-mail: [email protected] (Internet Address) KEKB B-Factory Design Report June, 1995 National Laboratory for High Energy Physics Tsukuba, Ibaraki 305 Japan Table of Contents Overview 1 1 Introduction 1 2 Basic design 2 3 Hardware systems 6 4 Schedule 10 1 Physics requirements 1-1 1.1 Energy asymmetry 1-1 1.2 Luminosity 1-2 1.3 Energy range 1-3 1.4 Beam energy spread 1-4 1.5 Beam background 1-4 2 Machine Parameters 2-1 2.1 Luminosity, tune shift, and beam intensity 2-1 2.2 Crossing angle 2-2 2.3 Bunch length 2-5 2.4 Bunch spacing 2-6 2.5 Emittance 2-7 2.6 Momentum spread and synchrotron tune 2-7 2.7 Other issues 2-8 3 Beam-Beam Interactions 3-1 3.1 Introduction 3-1 3.2 Simulation of beam collision 3-2 3.3 Simulation with linear lattice functions 3-3 3.4 Simulation with the lattice which includes nonlinearity and errors 3-10 3.5 Quasi strong-strong simulation 3-13 3.6 Bunch tails excited by beam-beam interactions 3-15 3.7 Tail, luminosity and longitudinal tilt 3-17 3.8 Summary 3-19 3.9 Comments on parasitic crossing effects 3-21 4 RF Parameters 4-1 4.1 Introduction 4-1 4.2 Requirements 4-2 4.3 Cavity-related instabilities 4-3 4.4 RF parameters 4-7 4.5 Summary 4-13 5 Impedance and Collective Effects 5-1 5.1 Impedance 5-1 5.2 Single-bunch collective effects 5-11 5.3 Coupled-bunch instabilities 5-12 5.4 Beam blow-up due to transient ion trapping in the electron ring 5-17 5.5 Instabilities due to beam-photoelectron interactions 5-23 6 Lattice Design 6-1 6.1 Dynamic aperture 6-1 6.2 Development of beam optics design 6-3 6.3 Optics design of the interaction region 6-13 6.4 Optics design of other straight sections 6-16 6.5 Requirements on the magnet quality 6-17 6.6 Effects of machine errors and tuning procedures 6-18 6.7 Conclusions 6-22 7 Interaction Region 7-1 7.1 Beam-line layout 7-1 7.2 Detector boundary conditions 7-4 7.3 Beam line aperture considerations 7-9 7.4 Superconducting magnets for IR 7-10 7.5 Special quadrupole magnets for IR 7-19 7.6 Installation and magnet support for IR 7-26 7.7 Beam background 7-28 8 RF System 8-1 8.1 Normal conducting cavity 8-1 8.2 Superconducting cavity 8-32 8.3 Low-level RF issues 8-43 8.4 Crab cavity 8-52 9 Magnet System 9-1 9.1 Magnets in the arc and straight sections 9-1 9.2 Magnet power supplies and cabling 9-15 9.3 Installation and alignment 9-18 10 Vacuum System 10-1 10.1 LER Vacuum system 10-1 10.2 HER Vacuum system 10-11 11 Beam Instrumentation 11-1 11.1 Beam position monitor 11-1 11.2 Optical monitor 11-9 11.3 Laser wire monitor 11-19 11.4 Bunch feedback system 11-23 12 Injection 12-1 12.1 Linac " 12-1 12.2 Average Luminosity . 12-24 12.3 Beam transport 12-27 13 Accelerator Control System 13-1 13.1 System requirements 13-1 13.2 System architecture 13-5 Overview 1 Introduction KEKB is an asymmetric electron-positron collider at 8x3.5 GeV, which aims to provide electron-positron collisions at a center-of-mass energy of 10.58 GeV. Its mission is to support high energy physics research programs on CP-violation and other topcis in B-meson decays. Its luminosity goal is 1034cm~2s~1. With approval by the Japanese government as a five-year project, the construction of KEKB formally began in April of 1994. The two rings of KEKB (the low-energy ring LER for positrons at 3.5 GeV, and the high-energy ring HER for electrons at 8 GeV) will be built in the existing TRISTAN tunnel, which has a circumference of 3 km. Maximum use will be made of the infrastructure of TRISTAN. Taking advantage of the large tunnel width, the two rings of KEKB will be built side by side. Since vertical bending of the beam trajectory tends to increase the vertical beam emittance, its use is minimized. Figure 1 illustrates the layout of the two rings. KEKB has only one interaction point (IP) in the Tsukuba experimental hall, where the electron and positron beams collide at a finite angle of ±11 mrad. The BELLE detector will be installed in this interaction region. The straight section at Fuji will be used for injecting beams from the linac, and also for installing RF cavities of the LER. The RF cavities of the HER will be installed in the straight sections of Nikko and Oho. These straight sections are also reserved for wigglers for the LER. They will reduce the longitudinal damping time of the LER from 43 ms to 23 ms, i.e. the same damping time as the HER. In order to make the circumference of the two rings precisely equal, a cross-over will be built at the Fuji area. The HER and LER are located at the outer and inner sides inside the tunnel in the Tsukuba-Oho-Fuji part. The relative positions of the two rings are reversed in the Fuji-Nikko-Tsukuba part of the tunnel. To facilitate full-energy injection into the KEKB rings, and thus to eliminate the need for accelerating high-current beams in the rings, the existing 2.5 GeV electron linac will be upgraded to 8 GeV. The upgrade program includes the following modifications to the injector configuration: combine the main linac with the positron production 1 TSUKUBA OHO (TRISTAN I Accumulation Ring) 1 Figure 1: Configuration of the KEKB accelerator system. linac, increase the number of accelerating structures, replace the klystrons with higher- power types, and compress the RF pulse power by using a SLED scheme. With this upgrade the energy of electrons impinging on the positron production target will be increased from 250 MeV to 4 GeV. This will increase the available positron intensity by 16. With this improvement the injection time of positrons to the LER from zero to the full current is expected to be 900 s. A new bypass tunnel, which is 130 m long, will be excavated for building transport lines between the linac and KEKB. This new beam transport functionally separates the TRISTAN accumulation ring (AR) from KEKB. This will be an advantage to both KEKB and AR programs, since the upgrade work of either accelerator can be carried out without significantly affecting the other. The tunnel will be built in JFY 1996 and 1997. 2 Basic Design A Beam Parameters Table 1 summarizes the main parameters of KEKB. The HER and LER have the same circumference, beam emittance, and beta-function values at IP (/?*). The large current, small /?*, and finite-angle crossing of beams are salient features of KEKB. Table 1: Main Parameters of KEKB Ring LER HER Energy E 3.5 8.0 GeV Circumference C 3016.26 m Luminosity C lxlO34 cm"^"1 Crossing angle 9X ±11 mrad Tune shifts Zx/ty 0.039/0.052 Beta function at IP /3*//3* 0.33/0.01 m Beam current I 2.6 1.1 A Natural bunch length <yz 0.4 cm 4 4 Energy spread <ys 7.1xlO- 6.7xlO- Bunch spacing % 0.59 m Particles/bunch N 3.3xl010 1.4xlO10 8 10 Emittance ex/ey 1.8xl0- /3.6xl0- m Synchrotron tune Vs 0.01 ~ 0.02 Betatron tune VxIVy 45.52/45.08 47.52/43.08 4 4 Momentum ap lxlO" ~ 2xlO~ compaction factor Energy loss/turn Uo 0.81f/1.5tt 3.5 MeV RF voltage 5 ~ 10 10 ~ 20 MV vc RF frequency IRF 508.887 MHz Harmonic number h 5120 Longitudinal re 43f/23ft 23 ms damping time Total beam power Pb 2.7f/4.5ft 4.0 MW Radiation power PSR 2.1f/4.0tt 3.8 MW HOM power PROM 0.57 0.15 MW Bending radius P 16.3 104.5 m Length of bending 0.915 5.86 m magnet f: without wigglers, ff: with wigglers B Noninterleaved Chromaticity Correction and 2.5?r Lattice For efficient operation it is highly desirable to be able to inject beams into KEKB without having to modify the lattice optics from the collision time. This calls for a design which maximizes the dynamic aperture. A large dynamic aperture is also preferred for increasing the beam lifetime. For this goal, a noninterleaved sextupole chromaticity correction scheme has been adopted for the arc sections. In this scheme sextupole magnets are paired into a large number of families. Two sextupole magnets in each pair are placed ir apart in both the horizontal and vertical phases. No other sextupole magnets are installed within a pair. This arrangement cancels out geometric aberrations of the sextupole magnets through a "—/" transformation between each member of the sextupole pair.