Factory Accelerator Physics Issues in the BEPCII G. Xu for BEPCII AP Group SLAC, Oct , 2003, USA Factory Contents Introduction and issues tree Beam-Beam interaction Collimators system design IR design Lattice and dynamic aperture Impedance Collective effects Summary Factory Geometric Survey of BEPCII Factory Design Goals and Main Parameters Factory Issues tree E=1~2.1 Lattice design and dynamic aperture z ~ y * =1.5 Impedance y =0.04 Beam-beam interaction k b =93, I b =9.8 k b =93, I b =9.8 Collective effects L=110 33 Background and collimator system design y* = 1.5, R=1.5% IR design Factory Using code BBC (Beam-Beam interaction with a Crossing angle) developed by K.Hirata. BBC is a weak strong simulation code in 6-dimensional phase space including the effect of crossing angle. The effect of a finite bunch length was taken into account by dividing a strong bunch into 5 slices longitudinally. The weak bunch is represented by 50 randomly generated macro particles, with Gaussian distribution in 6- dimensional phase space. The simulation was done for more than 5 radiation times. Beam-Beam Interaction Factory Beam-beam tune scan Luminosity survey with a crossing angle of c =11mrad 2 Factory xx yy Luminosity survey with a crossing angle of c =11mrad 2 Factory The high luminosity region is around x = 0.53, y = These tune values are chosen as designed working points. The crossing angle of 11 mrad induces some reduction of the luminosity, as well as the region of the high luminosity. y =0.58 x =0.53 Factory Crossing angle effect Crossing angle effect Factory Finite bunch length effects (L vs. y * / z ) Factory Summary of beam-beam effects To choose the horizontal tune close (above) to half integer is a good choice to get the higher luminosity. The luminosity reduction factor due to hour glass effects and crossing angle is about 80%. The designed value of y=0.04 is reasonable and reachable for c =11mrad 2. Some further simulation should be done, including the coherent beam beam effects by strong-strong simulations. In BEPCII, there are only 3(for bunch spacing 2.4m) parasitic crossing points. One is the north IP which is locally separated by vertical correctors. The other two is near to the main IP. Due to the large crossing angle at IP, the distance between two beams is at least 15 x. So all of these parasitic beam-beam effects can be omitted. Factory Collimator System Design Collimator system design carried by both detector and accelerator people. Detector people do the background calculation for the circulating beam including SR light, Coulumb scattering, bremsstrahlung and Touschek effects. Accelerator people do the simulation of injection beam, injection efficiency is the main consideration, and the damage effects on the detector electronics parts is another important thing. Collimators should locate between injection point and IP as possible, but BEPCII have not sufficient space install all collimators in this range. Factory Quad Name near Collimator Distance. from IP(m) H/VAperture H/V (mm) 1R3OQ037.9H/V71.5/37.0moveable 2R3OQ0411H/V72.3/20.8moveable 3R3OQ0827.1H53.0/fix 4R3OQ H42.5/fix 5R2OQ V/24.7moveable 6R2OQ H47.4/moveable Collimators position in the positron ring(upstream IP) Factory Injection simulation with collimators: on momentum particles after 100 turns Factory Injection simulation with collimators: off momentum(0.4%) particles after 100 turns, the blue part will lose after 100 turns Factory Summary of Collimators System Design BEPCII is so tight that the location of collimators can not be arranged according to the beta function and phase advance. The results of the simulation show that, with current designed collimators, the background level during collision and the dose during injection in IR is acceptable, the injection efficiency is reasonable (about 60%~70%). The reason is that the particles lost in IR have already been collimated in transport line. The mechanical design for collimators is under progress. Factory IR Layout (Collision Mode) IR lattice consists of SCQ(RED), anti-solenoids(BLUE), ISPB, Q1a, Q1b. It has a doublet construction, the separated Q1a and Q1b function a focusing quadrupole. ISPB make the beams separate further. Anti- solenoids decouple the detector solenoid effects. Factory IR Layout (SR Mode) For SR mode SCB(GREEN) will switch on, the beam will circulate in the outer ring. Factory Detector solenoid compensation Since there is no enough space for arranging skew quadrupoles to compensate the detector solenoid, BEPCII uses a set of anti-solenoids to decouple the detector solenoid effects. The coupled vertical trajectory due to horizontal off-axis is shown in the right plot. The trajectory can be locally compensated near IR using vertical correctors. After compensation, the maximum vertical orbit will be 3 mm (at 1.89GeV). Factory The lattice design should meet the requirement of BEPCII as a dual-purpose machine for both HEP and SR researches, which makes BEPCII a three-ring machine: e - -ring, e + -ring and SR-ring, which e - -ring and e + -ring is symmetrical each other. It will provide the colliding beams of the center-mass between GeV optimized at 1.89 2 GeV with 1 cm -2 s -1. Design energy for SR is 2.5 GeV and current of 250 mA, the existing SR beam- lines should keep at their present position; RF frequency is chosen as 499.8Mhz(=2856 7/40) that can satisfy the requirement of two-bunch injection. The harmonic number of colliding and SR rings are 396 and 402 respectively, and the circumference are m and m respectively, the distance between the outer and inner rings is 1.18 m. There is no skew quadrupoles in arc, BEPCII will use the vertical orbit at sextupoles to control the coupling. The lattice and dynamic aperture Factory Two tune regions are studied: x / y = 6.5/7.5 and x / y = 6.5/5.5. Both meet the following requirements. Natural emittance ~ 140nm Momentum compact factor p ~ 0.02 x * = 1m y * = 1.5cm D x * = 0 x * _inj > 20m, D x_inj =0 x _kicker > 6 m, x _kickers=0.5 x _ rfc, y _rfc < 15m and D x _rfc=0 x _arc, y _arc < 25m and D x _arc1.5cm. Bellows: with RF-finger shielding BPM: PEPII style button ( 1.5cm, w=1mm) Pumping port: longitudinally narrow (w I th ~36mA Bunch lengthening ~ 5% Bunch lengthening calculated with |Z/n| eff A scheme with negative momentum compaction factor to reduce bunch length is being studied. Factory Coupled Bunch Instability Coupled Bunch Instability Due to HOM of SCC M = 99, I b = 9.8mA, the most unstable mode Growth time (ms) Longitudinala = 1 1 = 12.8 2 = 13.7 3 = 13.9 a = 2 1 = 304 2 = 323 3 = 338 Transversea = 0 1 = 26.6 2 = 29.0 3 = 30.0 a = 1 1 = 1076 2 = 1165 3 = 1229 For 93 bunches, similar results obtained with multi-bunch simulation. Factory For x / y = 6.53/7.58, =4.3ms At y =7.9, the most unstable mode has growth time of 1.5ms, which require the feedback system of damping time of 1ms. Growth time vs. unstable mode Resistive Wall Instability Growth time vs. transverse tune y (ms) mode Factory Ion Effects Ion trapping Possible method to eliminate ion trapping Partially filling the beam in the RF buckets. Install cleaning electrode. With ~6 bunches absent, the ions is not trapped. Detailed simulation is being done. Factory Ions created during a single revolution of the beam could potential causes instability: Growth rate: Coherent ion freq. =2.8 10 7 s -1, =1.38 10 7 s -1 e =3ms => FBII should be damped with feedback system. shorter bunch trains may be helpful. Fast Beam-Ion Instability (FBII) Factory )Coupled Bunch Instability With the saturated EC density: Rough estimation of ECI 2) Single Bunch Instability Electron Cloud Instability =N b r e |W y | y /16Q s, >1 unstable. Factory ECI Parameters of a few Storage rings BEPCIIKEKBPEPII Beam energy(GeV) Bunch population N b (10 10 ) Bunch spacing L sep (m) Rms bunch length z (m) Rms bunch sizes x,y (mm) 1.18, , ,0.2 Chamber half dimensions h x,y (mm)60,274745, 25 Slippage factor (10 -3 ) Synchrotron tune Q s Circumference C(km) Average beta function(m) CB (ms) TMCI threshold e [10 12 m -3 ] e,CB : the same level as B-factories. Similar specification on feedback to cure the CB. EC density threshold higher than B-factories TMCI in BEPCII due to EC may not be stronger. Factory To control ECI To guarantee the beam performance against ECI, precaution methods successfully adopted in PEPII and KEKB is considered in BEPCII design. Antechamber TiN coating of the inner surface Solenoid winding (as backup) Clearing electrode ( R&D) Simulation study being done Factory Consideration on the antechamber geometry 1)h=15mm, 99.5% out 2)L>5* h, 5*h from beam duct Photon reflection rate reduced: