KEKB Design

1. Parameters2. Collider Ring 3. Interact Region4. Magnet, RF, Instabilities5. Recent progress with crab crossing

1. Basic Parameters

Parameter List in the Design Report (1995)

RF-related Parameters

2. Collider Rings

• Layout • Ring lattice design• Other straight sections

Wiggler (LER): reduce the longitudinal damping time from 43ms to 23ms, same as that for HER.Bypass: make the circumference of LER and HER same.

Layout of KEKB

Ring Lattice Design• Guideline

-realize beam parameters listed in the table- sufficiently large dynamic aperture for high injection efficiency and a long beam lifetime, particularly the Touschek lifetime in LER-wide range of tunability for beam parameters, especially horizontal emittance-reasonable tolerance for machine errors-small synchrotron tune (so as to find adequate working point in tune space)

• Dynamic Aperture requirement -momentum aperture +/- 0.5% ------ -transverse aperture > 1.2x10-5 m

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Comparison of the performances of the cell structures and chromaticity correction schemes for the LER

Dynamic aperture of the LER with five types of beam optics

• Transverse aperture limitation --nonlinearity of sextupole magnets used for chromaticity correction.

• Vertical aperture limitation --the kinetic terms of drift space around the IP

-- the fringe field of the final quad magnets at the edge facing IP.

Cause of limitation of Dynamic Aperture

• Momentum aperture limitation-- Beta functions at cavities have dependencies on momentum

change of energy at RF causes mismatch between beam and the betatron phase space ellipse excites synchro-betatron resonances exponential growth of betatron amplitude choose small synchrotron tune (small momentum

compaction)

--Chromaticity in the x-y coupling terms, because of non- perfect compensation of detector solenoid at IP

---Chromo-geometric aberration caused by a breakdown of the -I transformer for off-momentum particles

Noninterleaved 2.5 Cell

HER LER

5 pi/2 cells with 4 bend to form 2 dispersion bumps: keep dispersion small at bendsSuccessive SF(SD) pairs have a relative phase of 3pi/2

Dynamic Aperture of the HER with the 2.5 pi cell

Since the dynamic aperture requirement on the HER is less demanding, a localChromaticity correction is not implemented in HER.

Chromaticity correction with the 2.5 pi cell with and without local chromaticity correction

• LER has four 12m long chicanes, to adjust bunchlength which is changed due to wigglers.

Other Straight Sections

3. Interaction Region

• IR layout• IR Chromaticity correction• Magnets

Layout of the IR + Chromatic Correction Sections

IR Optics with Chromaticity Correction

Features of Local Chromaticity Correction

• It is practically difficult to install two sextuple pairs for correcting both the horizontal and vertical planes in the IP striaght section

• Sources of the horizontal chromaticity are not so strongly localized as the vertical .

• Thus, the design only places one sextupole pair for the vertical corrrection in the straight section on each side of IP

• The last sextupole pairs at the end of the arc are used for the horizontal correction

Feature of the Interaction Region

• Finite angle crossing -allows small bunch spacing

- minimize the bending of income beams (reduce SR background)• Superconducting final focusing magnet

systems (including compensation solenoids) -introduces flexibility of machine tuning

Layout of the beam line near the IP

Compensation of x-y coupling generated by the detector solenoid field =>use counter solenoid

Geometric Condition• The interaction point relative to the detector center is shifted

towards the left by 470 mm

This is for increasing the solid angle coverage in the forward direction, while taking into account of the Lorentz boost of the final state particles in the asymmetric collision.

• The accelerator components must fit within a cone-shaped space with an opening angle of 17 deg forward and 30 degree backward, clipped by the CDC inner radius.

• The high precision particle tracking using silicon micro strips requires a small vacuum chamber around the IP with an inner radius of 20mm for -80<z<80mm.

Detector Boundry Condition

Top View of Layout at IR

Detector solenoid : 1.5 Tesla

Magnet Condition• The solenoid field created by the detector magnet for

charged particle tracking is 1.5 T, extended in length of +/- 2.5m.

• Correction with skew quad magnets are exact only for on-energy particles

- the remaining chromatic coupling term will result in increased vertical emittance.

-Use of skew quad will cause extra chromaticity, its correction will reduce available dynamic aperture, and consequently the expected beam lifetime.

• Luminosity will be reduced if solenoid field compensation is not sufficient.

• It has been found that if the integral of the axial field Bz is cancelled to zero on the average, the reduction of the dynamic aperture and its effects on the beam lifetime are negligible.

• As an example of incomplete field compensation, a left-over field of will result in a 30% beam lifetime reduction because of the reduced dynamic aperture.

• Goal: net value of Superconducting compensation solenoid magnet S-L

and S-R are implemented for this.

m T 5.1 dzBz

0 dzBz

Compensation of Detector Solenoid Field

Management of Leak Field

• Magnetic fields from S-L, S-R, QCS-L and QCS-R will leak into the detector volume, and distorts its solenoid tracking field. Simulation shows that these effects are manageable.

• QCS-L is not in line with the detector axis and it may couple with the detector iron to create nonsymmetric multipole field and affect on the beam dynamics. It has been found their adverse effects are negligible.

• Efforts on reducing the detector leak field near QC1E-L and QC1E-R

5. Special Topics

• magnet requirement• RF system• Collective Effects

Requirement on the Magnet Quality

• Criteria: 2% reduction of the dynamic aperture integrated in both the momentum and transverse phase space

ny

n

n xB

BK

1with

RF SystemA straight-forward way to avoid coupled-bunch instabilities due to HOM is devise cavity where no HOMs are excited by the beam.

KEKB uses two kinds of HOM-free cavities:

Normal conducting cavities for LER and HERARES (Accelerators Resonantly coupled with Energy Storage) • three coupled cavities operated in the pi/2 mode• Features: large stored energy suitable for heavy beam loading no HOM excited by the beam

SRF cavities for HER• High energy storage and immune to beam loading• The diameters of the beam pipes are chose so that the frequencies of all modes, except for the

fundamental mone, become higher than the cut-off freq. of the pipes. HOMs propagate towards beam pipes and are eventually absorbed by ferrite dampers attached to the inner surfaces of the pipes.

• Electron Cloud -most serious instability which limits KEKB performance,

observed in LER-SR from the beam hits the inner wall of the vacuum chamber and produce photoelectrons.-The clouds then excite head-tail type oscillation within a bunch and the beam blow up-wound solenoids of total length of 800m

• Coupled Bunch Instability• Beam-beam Effects

Beam Instability

Solenoid Effect on Electron Cloud

Beam Instability (con’d)

• Coupled bunch instabilities (transv. & long.) -HOM of RF cavity

-accelerating mode of RF cavity-resistive wall of vacuum chambers (transverse)

• Solutions-damped cavity to reduce Q value for NCRF-use SRF for small detuning freq. of the fundamental mode-bunch-to-bunch feedback system

• TCBI may still a limitation to high current operation with short bunch spacing to achieve high luminosity

Growth rate of TCBI vs Beam Current

Luminosity vs. working point (from beam-beam simulation)

Effect of Solenoid Field Compensation on Luminosity

Recent Progress at KEKB---crab crossing

• Crab crossing: achieved L predicted by simulation• Prediction: L(with crab)=2 x L(without crab)• Reality: L(with crab)=1.2 x L(without crab)• Reason:

– Short beam lifetime at high current (horizontal physical aperture at crab due to beam-beam at tune

close to 0.5) – Machine errors not fully compensated by turning knobs– Chromaticity of the x-y coupling at IP could reduce luminosity

through the beam-beam interaction (adding skew sextuple increase luminosity by 15%)– Long. wake in Beam-beam simulation (CSR microwave instability in LER at I=1.0mA)

On May 6, 2009, KEKB broke the world luminosity record and achieved a luminosity of 1.96 x 1034/cm2/sec using the crab cavitie