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

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Accelerator plans at kek

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

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

SuperKEKB Machine Parameters


Beam beam interactions

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

Simulation: Crossing Angle Dependence

  • Luminosity reduced with a crossing angle

    • Geometric effects

    • Nonlinear diffusion -> beam size growth


Simulation crab crossing

Simulation: Crab-Crossing

  • Crab-crossing restores full luminosity of a head-on collision.


Simulation other parameters

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

Lattice Design

Beam Optical Parameters of SuperKEKB:


Non interleaved 2 5 pi cell

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

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

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 region1

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

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

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

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

RF Parameters


Coupled bunch instabilities due to rf cavities

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

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

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 system1

Vacuum System

  • Prototype ante-chamber tested at KEKB

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


Vacuum system2

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 system3

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 system4

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 system5

Vacuum System

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

  • Tests at KEKB very promising.

  • Development continuing.


Beam instrumentation

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

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

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

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

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

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

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

Energy Upgrade

Pulse beam kicker installed

before positron target for

quick switching between

beams (50 Hz).


C band klystrons

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

Linac BPMs

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


Damping ring

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 ring1

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

Damping Ring Parameters

RF: Use KEKB ARES cavity (509 MHz)


Damping ring lattice

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

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

Summary

  • LoI is in draft stage.

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


Machine parameters

Machine Parameters

  • Luminosity:

  • Beam-beam parameters:

  • Energy transparency:


Beam beam blowup

Beam-beam blowup

Evolution of luminosity and beam size in

weak-strong (PIC) and exact solution

(Gaussian) models


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