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Accelerator Plans at KEK. John W. Flanagan, KEK Super B Factory Workshop Honolulu 19 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

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Accelerator Plans at KEK

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Accelerator Plans at KEK

John W. Flanagan, KEK

Super B Factory Workshop

Honolulu

19 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 with

different 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:


Lattice

Non-interleaved 2.5-Pi Cell

Wide tunability of

horizontal emittance,

momentum compaction

factor.

Principle nonlinearities in

sextupole pairs cancelled

out to give large dynamic

aperture


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 aperture

Red: 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 quadrupoles closer to IP for lower beta functions at IP.

  • Preserve current machine-detector boundary.

  • Rotate LER 8 mrad.

  • QCS and solenoid compensation magnets overlap in SuperKEKB.

  • Issues:

    • QC1 normal or superconducting?

    • Dynamic aperture => need damping 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 rate, 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

ARES

SCC

Total number of RF units at KEKB and SuperKEKB.

One unit = one klystron + 1 SCC or 1(2) ARES at SuperKEKB (KEKB)


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 option 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 KEKB with a single crab cavity in Nikko

    • Beam will be crabbing all the way 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

T0 = revolution period (10 usec)

k = loss factor

I = beam current

nb = number of bunches

  • 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


Vacuum System

  • HOM dampers have been developed for masks, to reduce heating of pump elements near masks.

    • Winged damper with SiC rod based on type developed for ARES.

    • Successfully cured pressure rise due to 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 RF 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 instability

    • Use DAfNE-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 possible.

    • 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-injector

    • Positron: stronger focusing field in capture section 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 installed

before positron target for

quick switching between

beams (50 Hz).


C-Band Klystrons

Prototype C-band structure

installed and tested at linac

using actual beam (2003).

Measured field gradient of

41 MV at 43 MW agrees with

expectation.


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 reduced aperture of C-Band section and to meet IR dynamic aperture restrictions.

    • Electron DR may be considered later to reduce injection 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

FODO cell w/alternating bends

Dynamic aperture

Green = injected beam, red = 4000 turns max deviation

(thick = ideal machine, thin = machine errors included)


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 in

weak-strong (PIC) and exact solution

(Gaussian) models


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