FCC-ee optics tuning

The Challenge

R. Tomas for the FCC-ee tuning team

FCC-ee tuning team

CERN e-group FCCee_tuning-team: Ilya AGAPOV, Esmaeil AHMADI, Felix CARLIER, Antoine CHANCE, Tessa CHARLES, Barbara DALENA, Riccardo DE MARIA, Andrea FRANCHI, Cristobal GARCIA, Michael HOFER, Patrick HUNCHAK, Jacqueline KEINTZEL, Simone LIUZZO, Lukas MALINA, Katsunobu OIDE, Tobias PERSSON, Tatiana PIELONI, Tor RAUBENHEIMER, Guillaume SIMON, Rogelio TOMAS, Fani VALCHKOVA-GEORGIEVA, Leon VAN RIESEN-HAUPT, Simon WHITE, Yi WU, Frank ZIMMERMANN + Anyone is welcome!

Meetings so far: 22 Apr, 22 Mar, 17 Mar, 10 Feb, 17 Nov and 10 Nov.

Tuning & alignment mini-workshop and the workshop summary by Frank Z.

Next FCC-ee tuning meeting: June 9th (newcomers, process FCC week outcome)

SuperKEKB luminosity record 4.1x10³⁴ cm²/s @ 𝜷*=1mm

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Peak luminosity about factor 4 below previous projection.

𝜷* about factor 3 above design.

Optics drifts in SuperKEKB and LHC

H. Sugimoto in 1^st skb-itf-opt subgroup meeting: https://kds.kek.jp/event/39396/

Tune drifts measured in SKEKB due to unwanted SC quads changes over days.

Optics drifts measured in LHC over the last 20 days, under investigation.

⇒ Need very fast and efficient tuning for FCC-ee!

FCC-ee optics (repository V22, thanks Michael!), z-Z 45.6 GeV

100m

10 km

FCC-ee optics, t-top 182.5 GeV

7 km

FCC-ee optics, z-Z arc

170 m

180 deg (2 cells)

sextupole

19.3m

22.6m

FCC-ee optics, t-T arc

sextupole

180 deg (2 cells)

84 m

180 deg (2 cells)

Example girder

T. Charles, March 2022

H. Mainaud, 10 Feb: “The actual value of tolerances will not be the cost driver, being ‘size’ the main driver.”

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Factor 2 lower tolerances should be considered (at least).

Only non-linear error so far is arc sextupole strengths.

Status of optics tuning, tt lattice

Excellent progress with the horizontal emittance (source was too short orbit corrs.)

Design 𝝐_x= 1.45 nm

Leon van R.-H. DA after tuning

no radiation, 10 seeds.

(Design DAdp=0~15𝛔x)

D. Shatilov also experienced poor DA for tuned lattices asking for a factor 5 improved alignment !!!

T. Charles

DA is still expected to be poor.

IP 𝜷 function not yet well corrected.

Realistic tuning: Realistic lattice and magnets

• Need to split dipoles to keep a maximum length of about 12m (gap of 30cm).
• Need space for orbit and skew quadrupole correctors:
• Many locations without sextupoles
• Sextupole design is at the edge:
• Need longer sext. if combined with quad. and dip.
• Magnetic design with combined ele. being started
• Need to define girders locations

Jeremie Bauche, March 17th, and this morning

B~0.6T

Impact of reducing dipole filling factor

for tt lattice

Splitting dipoles to about ~12m length with 30cm gaps increases H emittance by 3% (linear behaviour around 30cm).

Leon V. R.-H., FCC-ee tuning miniworkshop. See updates on this at 5pm today.

Realistic tuning: Orbit correctors (OC) length

Need a design for orbit correction system in FCC-ee.

Using one seed from Tessa’s tuning simulations (assuming 1 OC/quad) optimal corrector length is about 25cm.

Alternatives in next slide

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Rad. from shorter main dipoles

Rad. from OCs

Actual value of emittance not representative as it is just for one tuned seed.

Alternatives to orbit and skew quad. corrector magnets

• Main dipoles trim coils could act as H orbit correctors
• Superconducting SSS: nested coils

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• Mounting quads and sextupoles on movers to correct via feed-down and tilts:
• Issue with the positions of the two apertures being coupled
• Use dipole trims and tilt for extra degrees of freedom?

M. Koratzinos

Cost - Performance ratio? (needed studies multiply…)

Dipoles

• Need to converge on tapering design (how many FODO per tapering unit?)
• Need to consider b2 of 4 units (sign changing arc-by-arc). Compensation in main quads? (see next slide)
• b3 of 2 units: Causes 20 units of chroma. At 2.5mm b3 gives another b2 unit. Impact on DA and tuning? Compensation with arc sextupoles not obvious.

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Jeremie Bauche, March 17th

Impact of the dipole b2 error (quadrupolar)

b2= 1.6 units @ 10mm gives a 1% beta-beating after rematching tunes with main quads.

Cristobal Garcia, EPFL/CERN

Alternatively a full arc and IR rematching (distinguishing between inner and outer arcs) should be done to tolerate more than 1.6 units of b2.

→Impact on sextupole pairs?

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and b3 + misalignment can give close to another unit of b2…

Quadrupole

• Shift of quadrupole centers outwards by 0.4 between Z and T operation
• Independent powering of apertures (for tapering or tuning) challenging: induced quadrupole offsets (0.2mm) and large b3 (10 units) →Mitigations under investigation
• Tolerance studies needed, considering compensating quad center shift with dipoles/correctors and quad b3 with sextupoles

Jeremie Bauche, March 17th, and this morning

→Study SC SSS too?

Systematic quad. offset of 0.15 mm yields 1.5mm CO!

Esmaeil Ahmadi, Iranian light Source Facility.

Same orbit from MAD-X and from ELEGANT.

Closed orbit correction being explored with Tessa’s approach and also new approach in ELEGANT.

MAD-X

ELEGANT

BPMs fit in the quadrupoles (M. Wendt, April 22)

BPM fits in this blue box

Optics measurements in FCC-ee

• Current tuning simulations assume ideal optics measurements
• Large energy loss in FCC-ee, fast damping or chromaticity may induce systematic errors in all techniques: single kick, AC dipole, NOECO and LOCO-like

J. Keintzel, single kick, March 22

L. Malina, AC dipole in PETRA III

Solution to the use of MAFF filters!!!

A.Franchi, ESRF meas.,

Sextupolar error identification.

NOECO technique

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First simulations for FCC-ee!

March 22

For LOCO-like: see S. Liuzzo’s presentation today

Single kick feasibility at Z and Top

While the single kick technique is appropriate for the Z energy it is not feasible at the Top for the too fast damping.

Need to study AC dipole and ORM or LOCO at the Top energy!

(teams at DESY, ESRF & CERN already at it!)

J. Keintzel’s poster

Z

Top

Effective models with errors for pol./luminosity studies?

Imperfections and corrections will drive machine design and performance!

Various attempts to provide ‘effective models’, however these models need validation…

Further studies needed!

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Yi Wu, Félix Carlier, Tatiana Pieloni (EPFL), Feb. 10

Codes (see dedicated session at 16:00)

• Codes are critical for these studies and we are far from ‘unificiation’
• Tuning codes:
• Tessa’s tuning: Python + MADX
• ESRF: MATLAB
• DESY + ESRF: Migrate AT to pyAT ?
• Beam dynamics codes (with popular uses):
• SAD: Lattice design, tracking, emit, DA, etc.
• MADX & MADX-PTC: Tracking, errors, DA, tapering, emit, Normal Form, basic polarization
• BMAD and SITROS: Polarization, etc.
• Xsuite: Tracking, collimation, etc.
• MAD-NG: Coming-up with a fast Normal Form
• (Main efforts: CERN: MAD-X/NG & Xsuite, EPFL: Xsuite)
• ELEGANT: Orbit, tracking, IPM (Iranian light source)

Contributions to codes are also extremely welcome and important

(very few volunteers so far…)

Missing studies

• Massive amount of tuning studies with alternative configurations and improved lattice realism needed (lengths, tapering schemes, multipoles, BPM errors, ground motion, solenoid imperfections, etc.), for alternative designs too: SC SSS, movers?
• Improvements to tuning efficiency / speed (both in code and machine)
• Studies for all lattices: Z, W, H, t
• Measurements simulations (single kick, AC dipole, (AC-)ORM or LOCO, K-mod, etc.)
• Local corrections of IR parameters (IP knobs, K-mod, waist-shift, etc.)
• Tolerance on multipoles / offsets with dedicated correction approaches.
• Calculation of DA and MA after tuning (and polarization eventually)
• Sextupole knobs or schemes for DA / MA optimization
• Effective models
• Repository for tuned lattices / tuning codes inputs / effective models
• Spin tuning
• Commissioning process (beta* squeeze, etc…)
• …

Summary & outlook

Time is now, people are here!!! but actually we need more people!

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• Learn from SuperKEKB and try to help
• Optics tuning is critical for the success of FCC-ee
• Optics tuning aspects need to be addressed at early lattice design stages!
• Correctors, BPMs, Girders, etc.
• Alignment technology highly challenged by ring size and tight tolerances
• Magnet field quality needs review (also tolerances from tuning studies)
• Alternative design options on the table: SC SSS, movers, etc.
• Optics measurement techniques to be demonstrated

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