KEKB Design
1.
2.
3.
4.
5.
Parameters
Collider Ring
Interact Region
Magnet, RF, Instabilities
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
Layout of KEKB
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.
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% ------ (7  ,7  )
-transverse aperture > 1.2x10-5 m
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
Cause of limitation of Dynamic Aperture
• 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.
• 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 nonperfect 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 bends
Successive 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 local
Chromaticity correction is not implemented in HER.
Chromaticity correction with the 2.5 pi cell with
and without local chromaticity correction
Other Straight Sections
•
LER has four 12m long chicanes, to adjust bunchlength which is changed
due to wigglers.
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
Detector Boundry Condition
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.
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.
Compensation of Detector Solenoid Field
• 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  Bz dz  1.5 T m will result in a 30% beam lifetime
reduction because of the reduced dynamic aperture.
• Goal: net value of  Bz dz  0
Superconducting compensation solenoid magnet S-L
and S-R are implemented for this.
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
with
n
1  By
Kn 
B x n
RF System
A 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 HER
ARES (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.
Beam Instability
• 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
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