Accelerator Plans at KEK

John W. Flanagan, KEKSuper B Factory Workshop

Honolulu19 January 2004

LoI: Accelerator Design for a Super B Factory at KEK

• Machine Parameters• Beam-Beam Interactions• Lattice Design• Interaction Region• Magnet System• Impedance and Collective Effects• RF System• Vacuum System• Beam Instrumentation• Injector Linac• Damping Ring• Construction Scenario

SuperKEKB Machine Parameters

Beam-Beam Interactions• Simulation Methods

– Particle distribution• Gaussian: bunch shape fixed• Particle-in-Cell (PIC): arbitrary bunch shape possible

– Should be more accurate, though numerical noise may be a problem.

• Coherent dipole motion causes growth in beam size and reduction of luminosity in PIC model. (Not seen in Gaussian model).– Beam-beam limit (zero crossing angle)– Tune difference may help smear out coherent motion.

Improvement in luminosity withdifferent tunes (~KEKB)

Simulation: Crossing Angle Dependence

• Luminosity reduced with a crossing angle– Geometric effects– Nonlinear diffusion ->

beam size growth

Simulation: Crab-Crossing• Crab-crossing restores full luminosity of a head-

on collision.

Simulation: Other Parameters

• Lower horizontal beta function improves luminosity.• Lower emittance does not.• Best current ratio: 10A (LER) / 4.4 A (HER)

– Energy transparency ratio

Lattice Design

Beam Optical Parameters of SuperKEKB:

Non-interleaved 2.5-Pi Cell

Wide tunability ofhorizontal emittance,momentum compactionfactor. Principle nonlinearities insextupole pairs cancelledout to give large dynamicaperture

Lattice

Lattice

• IR region: main difference from KEKB is greater overlap of solenoid field on final-focus quadrupoles. No major issue found.

• Transverse dynamic apertures:– LER ok– HER under study

• Refine modelling of IR fields

LER dynamic apertureRed: injected beam

Interaction Region

•Crossing angle: +/- 15 mrad is working assumption.•Horizontal beta function at IP and horizontal emittance chosen based on beam-beam simulations to maximize the expected luminosity.

Interaction Region• Move final focus quadrupole

s closer to IP for lower beta functions at IP.

• Preserve current machine-detector boundary.

• Rotate LER 8 mrad.• QCS and solenoid compensat

ion magnets overlap in SuperKEKB.

• Issues:– QC1 normal or superconductin

g?– Dynamic aperture => need dam

ping ring for positrons, at least.

Magnet System

• Outside of the IR, will largely reuse present KEKB magnets, with some modifications and upgrades for new vacuum system, crab cavities.

Impedance and Collective Effects• Resistive Wall Instability

– Growth rates (800-1000 s^-1) lower than damping rate of feedback system (5000 s^1).

• Closed Orbit Instability due to long-range resistive wake (Danilov)– Thresholds (12.3/12.2 mA for LER/HER) above design currents.

• Electron Cloud Instability (Positron Ring)– With ante-chambers and positrons in the HER, simulations show that 60G solenoid

field should clear the electrons. Uncertainties:• Distribution on walls and amounts of electrons.• Behavior of electrons inside lattice magnets.

• Ion Instability (Electron Ring)– Currently suppressed by feedback.– With electrons in LER, simulated initial growth rate faster than feedback damping r

ate, leading to dipole oscillation with amplitude of order of vertical beam size => possible loss of luminosity.

• Coherent Synchrotron Radiation– Rough numerical approximation of CSR in LER bends shows that beam pipe radius

is small enough to shield beam from energy loss at 6 mm bunch length, but at 3 mm bunch length the transient energy change has an amplitude of 1.5 keV (depending on location in bunch).

– Investigations just started.

RF System• ARES Cavity System

– Normal-conducting cavities with energy-storage cavities attached.– LER & HER

• Superconducting Cavity (SCC) System– High cavity voltage– HER only

Total number of RF units at KEKB and SuperKEKB.One unit = one klystron + 1 SCC or 1(2) ARES at SuperKEKB (KEKB)

ARES

SCC

RF Parameters

Coupled-Bunch Instabilities due to RF Cavities

•Longitudinal bunch-by-bunch feedback system will be needed.•New HOM dampers developed for ARES and SCC

Crab Crossing• Originally included as an opti

on for KEKB, but have managed to reach design luminosity without them.

• Simulations indicate that they will be needed to go from 1e35 to 5e35/cm^2/s.– New cavity being developed for

higher beam currents• Current plan is to start at KE

KB with a single crab cavity in Nikko– Beam will be crabbing all the w

ay around the ring.

Vacuum System

• Intense synchrotron radiation– 27.8 kW/m in LER, twice

as high as in KEKB– 21.6 kW/m in HER, 4

times as high as in KEKB• =>Ante-chamber structure

– Also motivated by need to reduce photo-electron clouds.

Vacuum System

• Prototype ante-chamber tested at KEKB

• Combined with solenoid field is very effective at reducing photoelectron build-up.

Vacuum System• HOM power losses

– Excessive heating– Minimize loss factors– Largest loss factors at

movable masks which protect detector from particle background

– Resistive wall and bellows are next.

• HOM absorbers to be installed near large impedance sources

T0 = revolution period (10 usec) = loss factorI = beam currentnb = number of bunches

Vacuum System

• HOM dampers have been developed for masks, to reduce heating of pump elements near masks.– Winged damper with SiC rod based o

n type developed for ARES.– Successfully cured pressure rise due t

o heating of pump elements at KEKB• Absorbs 25% of 20 kW generated

– HOM power of mask in SuperKEKB will reach 200 kW

• Efficiency and capacity of HOM damper need to be improved.

Vacuum System• Pumping scheme

– Pressure requirement: Average pressure of 5e-7 Pa to achieve a beam lifetime of 10 hours.

– 1e-7 around IP to minimize beam background in detector.

– <1e-6 locally in electron ring to keep ion trapping below level that can be handled by feedback.

– Adopt distributed pumping scheme, a strip-type NEG.

• To reduce number of high-current feedthroughs, U-shaped strip is used.

• Flange and Bellows– Helocoflex outside with copper

(MO?) RF bridge inside– Bellows heating requires better R

F shield

Vacuum System

• Comb-type RF shield developed to replace RF fingers.

• Tests at KEKB very promising.

• Development continuing.

Beam Instrumentation

• Beam Position Monitors• Bunch-by-Bunch Feedback System• Synchrotron Radiation Monitors

– HER and LER SR Monitors– Damping Ring SR Monitor

Beam Position Monitors

• Use same front-end electronics.

• New button electrodes– New connector design for

improved reliability.– 12 mm -> 6 mm diameter

• Signal power same as at present, at higher beam currents, to match dynamic range of existing front-end electronics.

Bunch-by-Bunch Feedback

• New BPMs for higher beam currents.• Transverse feedback similar to present design

– Detection frequency 2.0 -> 2.5 GHz.– Automated LO phase and DC offset tuning.– Transverse kicker needs work to handle higher currents

• Improved cooling, supports for kicker plates.• Longitudinal feedback to handle ARES HOM and 0/Pi mode instabilit

y– Use DANE-type (low-Q cavity) kicker.– QPSK modulation with center frequency 1145 MHz (2.25 x RF freq.)

• Digital FIR and memory board to be replaced by new GBoard under development at/with SLAC.– Low noise, high speed (1.5 GHz), with custom filtering functions possibl

e.– Extensive beam diagnostics.

SR Monitors

•Current extraction chamber (copper) may need increased cooling.•HOM leakage needs to be measured (500 W predicted at full current).

•May need absorbers•Direct mirror heating from SR irradiation should be minimized.

•Increase bend radius of weak bends

•Lowers total incident power.•Also increases visible light flux – desirable to help see effect of single crab cavity.

Second SR Monitor for Dynamic Beta Measurement

• Build a second SR source in each ring• Using known phase advance between

two locations, can measure the dynamic beta effect due to beam-beam collisions.– Correct beam size estimation at IP– More importantly, can monitor beam-

beam parameters directly, in real-time.– Useful for luminosity tuning.

• Second source: create a local bump near current source– Minimize disturbance to lattice– Can use existing optics huts.

Damping Ring SR Monitor

• Gated camera for imaging turn-by-turn bunch size damping.– Up to 4 bunches in ring at

one time, at two different stages of damping.

– Diffraction-limited resolution below 10% if optical line not too long (~10 m).

Injector Linac

• Intensity Upgrades– Electron: increase bunch current from pre-injec

tor– Positron: stronger focusing field in capture sect

ion after target• Energy Upgrade

– Replace S-band (2856 MHz) RF system with C-band (5712 MHz) system to double field gradient in downstream section of linac.

Energy Upgrade

Pulse beam kicker installedbefore positron target forquick switching betweenbeams (50 Hz).

C-Band KlystronsPrototype C-band structureinstalled and tested at linacusing actual beam (2003).Measured field gradient of41 MV at 43 MW agrees withexpectation.

Linac BPMs

• Upgrade read-out oscilloscopes with newer models capable of full 50-Hz read-out.

Damping Ring• Positron emittance needs to be damped, to pass re

duced aperture of C-Band section and to meet IR dynamic aperture restrictions.– Electron DR may be considered later to reduce injectio

n backgrounds in physics detector, but for now only positron DR considered.

• Damping ring located downstream of positron target, before C-Band accelerating section.

Damping Ring

• Energy Compression System (ECS) in Linac-To-Ring (LTR) line, to meet DR energy acceptance requirements.

• Bunch Compression System (BCS) in Ring-To-Linac (RTL) line to accommodate short bunch length needed by C-Band accelerating structures.

Damping Ring Parameters

RF: Use KEKB ARES cavity (509 MHz)

Damping Ring Lattice

• FODO cell has large dynamic aperture, but large momentum compaction factor increases required accelerating voltage.

• Reversing one of the bends reduces the momentum compaction factor.

• Adopt reverse/forward ratio of ~1/3

Dynamic apertureGreen = injected beam, red = 4000 turns max deviation (thick = ideal machine, thin = machine errors included)

FODO cell w/alternating bends

Construction Scenario

• The upgrade of KEKB to SuperKEKB is proposed for around 2007.

• R&D and production of various components will be done in the first four years in parallel with the physics experiment at KEKB.

• The installation will be done during a one year shutdown in 2007, and then the commissioning of SuperKEKB will begin.

Summary

• LoI is in draft stage.

• SuperKEKB at L=~5e35/cm^2/s can be built.

Machine Parameters

• Luminosity:

• Beam-beam parameters:

• Energy transparency:

Beam-beam blowup

Evolution of luminosity and beam size inweak-strong (PIC) and exact solution

(Gaussian) models