accelerator plans at kek john w. flanagan, kek super b factory workshop honolulu 19 january 2004
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![Page 1: Accelerator Plans at KEK John W. Flanagan, KEK Super B Factory Workshop Honolulu 19 January 2004](https://reader036.vdocuments.us/reader036/viewer/2022062421/56649d3f5503460f94a1900e/html5/thumbnails/1.jpg)
Accelerator Plans at KEK
John W. Flanagan, KEK
Super B Factory Workshop
Honolulu
19 January 2004
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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
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SuperKEKB Machine Parameters
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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)
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Simulation: Crossing Angle Dependence
• Luminosity reduced with a crossing angle– Geometric effects
– Nonlinear diffusion -> beam size growth
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Simulation: Crab-Crossing• Crab-crossing restores full luminosity of a head-
on collision.
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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
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Lattice Design
Beam Optical Parameters of SuperKEKB:
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Non-interleaved 2.5-Pi Cell
Wide tunability ofhorizontal emittance,momentum compactionfactor. Principle nonlinearities insextupole pairs cancelledout to give large dynamicaperture
Lattice
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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
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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.
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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.
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Magnet System
• Outside of the IR, will largely reuse present KEKB magnets, with some modifications and upgrades for new vacuum system, crab cavities.
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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.
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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
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RF Parameters
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Coupled-Bunch Instabilities due to RF Cavities
•Longitudinal bunch-by-bunch feedback system will be needed.•New HOM dampers developed for ARES and SCC
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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 w
ay around the ring.
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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.
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Vacuum System
• Prototype ante-chamber tested at KEKB
• Combined with solenoid field is very effective at reducing photoelectron build-up.
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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
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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 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.
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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
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Vacuum System
• Comb-type RF shield developed to replace RF fingers.
• Tests at KEKB very promising.
• Development continuing.
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Beam Instrumentation
• Beam Position Monitors
• Bunch-by-Bunch Feedback System
• Synchrotron Radiation Monitors– HER and LER SR Monitors– Damping Ring SR Monitor
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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.
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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.
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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.
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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.
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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).
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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.
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Energy Upgrade
Pulse beam kicker installedbefore positron target forquick switching betweenbeams (50 Hz).
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C-Band KlystronsPrototype C-band structureinstalled and tested at linacusing actual beam (2003).Measured field gradient of41 MV at 43 MW agrees withexpectation.
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Linac BPMs
• Upgrade read-out oscilloscopes with newer models capable of full 50-Hz read-out.
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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 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.
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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.
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Damping Ring Parameters
RF: Use KEKB ARES cavity (509 MHz)
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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
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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.
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Summary
• LoI is in draft stage.
• SuperKEKB at L=~5e35/cm^2/s can be built.
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Machine Parameters
• Luminosity:
• Beam-beam parameters:
• Energy transparency:
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Beam-beam blowup
Evolution of luminosity and beam size inweak-strong (PIC) and exact solution
(Gaussian) models