Optics & Beam-Beam Issues
Akio Morita
KEKB Review 2020.07.15

P.00

Major Issues●

Global XY-Coupling Correction●IP Local Coupling●Injection Kicker Local Coupling●

LER High Emittance Optics●Crab Waist Optics●

IP Chromatic Coupling

P.01

Global XY-Coupling Correction (1)●One of key components for nano-beam collision:– Smaller εy is required for squeezing down β*y in order tokeep beam-beam parameter.

– It is minimizing L2-norm of vertical orbit crosstalk errors

induced by 6-kinds of horizontal steering single kicks.
●L2-norm minimizing are performed by using SVD and optics
model response against skew quadrupole correctors.

P.02

Global XY-Coupling Correction (2)●Strange behavior of XRM beam size during xy-coupling correction iterations:– XRM vertical beam size increases after xy-coupling correction.
●This behavior trends to increase after β*y squeezing down to 1mm.

– XRM beam size is reproduced by rolling back/forward the set of skew quadrupolecorrector parameters.
– In some case, the reduced vertical orbit crosstalks are observed by xy-couplingmeasurement after beam size increasing.
– In some case, recorrection with different SVD tolerance and damping factor (typicallyweaker correction) achieves vertical beam size shrink or same beam-size.
– Empirically, skew quadrupole corrector on SL (local chromaticity corrector sextupole)makes bad results.

P.03

Global XY-Coupling Correction (3)●Strange behavior of XRM beam size during xy-coupling correction iterations:– Where these behaviors come from?

Correction side effect to X-ray radiation point for XRM●Correction vector exceeding linear perturbation region●
Lack of XRM beam size monotonicity against xy-coupling correction iterations (Eitehr algotithm orobservation operator issue?)●Modeling Error (Modeling parameter mistakes, missing field components?)●– We HAVE TO establish stable xy-coupling correction scheme for sqeezing down todesign β*y.
●Such cut & try of xy-coupling needs extra time to startup accelerator after regular maintenance.

P.04

IP Local Coupling (1)●●For maximizing Lsp, R1~R4, ηy and η’y at IP are adjusted by using skew
quadrupole correctors so called “IPTilt knob”. (QK{A…} & QKSLY inTsukuba section and QKS{DF} on arc sextupole nearby Tsukubasection)
A few mm R2* knob is required to maximize L sp.
– Optics measurement CAN observe localized XY-coupling & V-dispersion

induced IPTilt knob, however, vertical beam-size measured by XRM is
minimized such IPTilt knob setting.
– Typical IPTilt knob setting during collision operation exceeds linear perturbation

region, and COULD make side effect to other optical function.

P.05

IP Local Coupling (2)●LER optics measurement result with/without IPTilt knob @ 2020/06/22:
Estimated β*y = 1.0mm

Estimated β*y = 1.3mm

Turn on IPTilt

without IPTilt

R1

4.306 mrad

R2

4.8 mm

R3-0.6 m-1

R40 mrad

EY-15 μm

EPY

40 mradwith IPTilt

P.06

IP Local Coupling (3)●XY-Coupling Correction with QC1 R2* Knob:– In order to reduce IPTilt R2* knob height, yet another R2* knob byusing skew quadrupole corrector on QC1* are applied before xycoupling correction.
QC1 R2* knob adjustment would be right correction if such big R2* error
came from rotation alignment error of QCS.
●
R2 perturbation of QC1 R2* knob is almost localized like the IP waist knob,
however, other R# coupling parameters are not localized.
●
In order to control global coupling, xy-coupling correction after QC1 R2*knob adjustment is required.
●

P.07

IP Local Coupling (4)●XY-Coupling Correction with QC1 R2* Knob:– QC1 R2* preset knob reduces IPTilt R2* knob height for maximizing Lsp, however,strong global xy-coupling correction reduces QC1 R2* preset knob effect.
We HAVE TO correct global xy-coupling, however, SHOULD NOT correct IR xy-coupling related R2*.
●
Systematic & stable correction scheme is not established at this moment.
●– What’s happened?

At the view point of vertical orbit crosstalk against horizontal single kicks, machine optics on optimum
IP coupling seems different with ideal model optics one.
●R2* error source may not be simple QCS rotation alignment error.
●If machine optics is systematically different with model optics, we SHOULD update optics modeling.
●– How to measure exact optical parameters?
– How to survey source of modeling error?

P.08

Injection Kicker Local Coupling (1)●From investigation of LER beam oscillation signal due to injection,correlation between vertical beam oscillation signal due to injection
and injection kicker field pattern is found. In the other hand,horizontal beam oscillation signal is not correlated with kicker pattern.
– Either kicker rotation alignment error or xy-coupling error between kicker pairis suggested.

P.09

Injection Kicker Local Coupling (2)●Try to correct vertical kicks from injection kicker system by installing
permanent skew quadrupole magnet between kicker pair.

Two Rotatable PMQ

Kicker bump shapeP.10

Injection Kicker Local Coupling (3)●●Significant improvement of both vertical beam oscillation signal during injection and luminosity
drop during injection oscillation decay is observed by adjusting PM skew quadrupole.
Injection background duration of Belle II detector is improved too.

w/o PM Skew QuadLuminosity Dropwith PM Skew Quad(~60mT)

with PM Skew Quad(~95mT)P.10

Injection Kicker Local Coupling (3)●●Significant improvement of both vertical beam oscillation signal during injection and luminosity
drop during injection oscillation decay is observed by adjusting PM skew quadrupole.
Injection background duration of Belle II detector is improved too.

Reduce background duration
by PM skewQ tuning.

S.Terui @ 2020.06.26

P.11

Injection Kicker Local Coupling (4)●During operation after PM skew quadrupole adjustment study, increasement ofvertical beam oscillation signal during LER injection are reported.
– Either PMQ rotation angle drift or optics/orbit drift is suggested.
– We start to consider replacement of these PM quadrupoles with electromagnetic skew

quadrupole magnet dedicated for online tuning.
●

How to survey and find & correct such localized optics error?
– This would be important things to achieve ultimate low emittance.

P.12

LER High Emittance Optics●

Merit – Improvement of Touschek lifetime.
– Maximum stored current WOULD be improved under insufficient injector power.
●●Current injector power is not enough to keep >1A e+ beam stably during β*y=1mm collision.

Side Effect – Increasing of effective bunch length at collision.
– HER design emittance is larger than LER one.
– Demerit is depending with β*y.

●We tried εx = 4nm LER optics instead of εx = 1.9nm optics.
– Significant demerit for Lsp is not observed.
– Should further emittance optimization be considered for specific operating point?
●For example, in the case that operatable lower bound of β*y is limited by QCS quench & detector background due tobeam loss at QC1 physical aperture.

P.12

LER High Emittance Optics●No significant difference
between 1.9nm and 4nm.

Merit – Improvement of Touschek lifetime.
– Maximum stored current WOULD be improved under insufficient injector power.
●●

Current injector power is not enough to keep >1A e+ beam stably during β*y=1mm collision.

Side Effect – Increasing of effective bunch length at collision.
– HER design emittance is larger than LER one.

εx = 1.9nm– Demerit is depending with β*y.

εx = 4nm●We tried εx = 4nm LER optics instead of εx = 1.9nm optics.
– Significant demerit for Lsp is not observed.
– Should further emittance optimization be considered for specific operating point?
●For example, in the case that operatable lower bound of β*y is limited by QCS quench & detector background due tobeam loss at QC1 physical aperture.

P.13

Crab Waist Optics (1)
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FCC-ee like crab waist is implemented into both rings by spliting
PS for SLY* vertical local chromaticity correction sextupole magnetpairs. (2 additional PS / ring)
Beam collision with crab waist is confirmed until β*y = 0.8mm.

Crab waist improves beam-beam blowup in high bunch current
product region (>0.3mA2).
It looks like no significant Lsp improvement in low bunch current
product region (~0.2mA2).

Lsp of β*y = 1mm collision

No significant Lsp improvement

P.13

Crab Waist Optics (1)Lsp is improved by CrabWaist

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FCC-ee like crab waist is implemented into both rings by spliting
PS for SLY* vertical local chromaticity correction sextupole magnet
pairs. (2 additional PS / ring)
Beam collision with crab waist is confirmed until β*y = 0.8mm.

Crab waist improves beam-beam blowup in high bunch current
product region (>0.3mA2).
It looks like no significant Lsp improvement in low bunch current
product region (~0.2mA2).

P.14

Crab Waist Optics (2)●Remarks for further operation with crab waist– Operation feasibility of FCC-ee like crab waist latticewith design β*y=0.3mm is not confirmed at this moment.

– In the past simulation study, design β*y=0.3mm latticewith additional dedicated sextupole pairs for crab waistshows significant degradation of dynamic aperture byturning on crab waist sextupole. (different design case)

P.15

IP Chromatic Coupling (1)●●Existence of big IP chromatic coupling error issuggested from beam-beam simulation in order
to explain the observed beam-beam blowup
bigger than beam-beam simulation expectation.
IP chromatic coupling measurement & correction
study are performed by using TbT-BPM system
and rotatable sextupole on LER.

P.16

IP Chromatic Coupling (2)●IP chromatic coupling correction studies:– 2020/06/05 at β* = (80mm, 1mm) optics

Correction parameter constraint: R1*’ = 2rad, R3*’ = 780m -1●
Rotate 4 SLY* sextupole magnets●
Chromatic coupling measurements are performed befor & after correction.
●– 2020/06/08 at β* = (80mm, 1mm) optics

Correction parameter constraint: R1*’ = 1.5rad●
Rotate 24 sextupole magnets●– 2020/06/24 at β* = (60mm, 0.8mm) optics●

Rotate 24 sextupole magnets

P.17

IP Chromatic Coupling (3)●IP chromatic coupling correction study @ 2020/06/05:Rotation Angle●●●

Bad S/N for extracting R1* chromaticity signal.
R3* chromaticity seems different before/after rotation.
Optics correction affects R3* chromaticity?

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IP Chromatic Coupling (4)●IP chromatic coupling correction study @ 2020/06/08:

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IP Chromatic Coupling (5)●IP chromatic coupling correction study @ 2020/06/24:

P.20

IP Chromatic Coupling (6)●After IP Chromatic Coupling Correction:– Significant Lsp improvement is not observed.
– Detector background is improved after many tuning

iterations including collision tuning (collision
feedback and IPTilt knob), injection tuning, beamcollimator tuning, etc. Contribution of IP chromatic
coupling correction is not clear.

P.21Summary●

Optics correction becomes difficult by squeezing β*y.
– We should improve measurement & correction scheme in order to achieve

stable operation with further squeezed β*y.

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Crab waist works fine, and it improves beam-beam blowup in highbunch current region.
One of LER injection background sources is identified and resolved.
LER IP chromatic coupling correction is tried, however, significant
improvement is confirmed.