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PS magnetic model and BGI multipole
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Europe/Zurich
CERN
CERN
PS magnetic model and BGI multipole
23/07/2021 LNO meeting
The meeting was dedicated to the discussion of PS magnetic modeling and BGI multipole, with three presentations with slides.
The PS magnetic and optical models, Alexander Huschauer, slides
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Alex started with a more general recap : modeling and simulating the field distributions inside the PS main magnets (MU) is a a complex task.
The PS has combined function magnets, where a single main coil excites both dipole and quadrupole components.
Additional pole face windings are used to adjust tunes and chromaticities. 40 low energy quadrupoles are used to adjust tune below 3.5 GeV/c.
Beam dynamics studies use a beam effective model, adjusted to reproduce the measured tunes and chromaticities.
The collaboration with magnet experts started over 10 years ago to improve the knowledge and modeling the magnets.
Modeling using OPERA has considerably improved with time (from 2D to detailed 3D), and is important to understand MU multipolar component plus the impact of the pole face windings on tune, chromaticity and for further beam dynamics studies. Field distributions change with energy.
Remanent fields are more important at injection (2.49 GeV/c), and saturation effects become important over 4000 Amps in the ramp to extraction at 26 GeV/c. The magnet modeling shows where the higher order magnet components are localized (mostly entry/exit and at inner block junctions) and provides information on changes with energy by saturation effects.
The MAD model so far implements multipoles in the entry, exit and the central joint between the F and D sections, neglecting components in the core of the F and D sections. The resulting model gives relatively good tunes, with discrepancies in the order of 0.04 and poorer chromaticity agreement in the order of 2-3 units (with respect to measurements). Rogelio commented that multipolar components could be also added in the core of F/D sections to assess their impact (CPU time should not be an issue for optics matters). Other sources for discrepancies could be: angles between blocks ('wedge' that Hugo might be able to check in simulations), remnant fields and misalignments.
Actions:
-Alex, update slides with Brho and amps corresponding to the magnetic measurement shown (slides 18-19).
-Alex or other LNO member, Check impact of multipoles inside the F and D sections.
-Alex, Hugo, Check the measured MU a3 quantitatively in order to compare to BGI (Alex forwarded email from Anthony Beaumont after the meeting with the magnetic measurements info but with some unclear conventions that need clarification).
-Hugo, Evaluate if 'wedges' can be inserted between blocks and their impact on harmonics.
Measurements of multipolar components, Sasha Horney, slides
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Sasha reported on beam measurements of multipolar components of the smaller BGI magnets situated between the main MU magnets.
Issues with losses during PS commissioning were reported by Alex in the LNO meeting on 26 May https://indico.cern.ch/event/1041358 and related to the behavior of the BGI magnets. Skew sextupole components were measured and cross checked with MAD-X. Agreement in absolute value of the BGI skew sextupole component between magnetic simulations and beam measurements is found. Discrepancy with sign has to be understood. Sign conventions plus main field orientations are to be clarified.
The analysis of chromatic coupling is underway.
Moving window plots are obtained and show a very large tune oscillation with turn number. Periodic oscillations corresponding to ~ 10 kHz were observed, and will be followed up to see if this is a real effect possibly caused by switching power converters. FT analysis software had similar artifacts for decohered data, OMC3 implemented some mitigations.
Actions:
-All, Understand sign discrepancy between the beam-based MADX BGI skew sextupole value and the magnetic simulation.
-Shasha, Ewen, Measure ring chromatic coupling to compare to BGI a3.
-Alex, Ewen, Contact EPC colleagues to get estimates of the 10kHz PC modulations.
-Alex, Ewen, Shasha, perform experiments switching off circuits to try to identify source. Check orbit modulation.
-Mael, Ewen, redo tune analysis with OMC3.
Magnetic simulations of PS MU and BGI Hugo Bajas / TE-MSC-NCM
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Hugo reported about the BGI magnets stimulated by the issues observed in PS recommissing with a skew sextupolar component.
Detailed OPERA simulations using elaborated meshes with several million pieces were performed initially be Jaime Renedo and then by him, showing that the shielding plates of the BGI magnets have a significant impact on the sextupolar component. The cross talk between the BGI and the main PS MU magnets can now be studied in a combined model. The simulation shows that a modification of the plates (larger holes) should be possible to reduce by a factor 10 the BGI sextupolar component and still shield sufficiently from stray fields. More harmonic analysis is in progress. The goal would be to compare the BGI a3 to the MU a3 to exclude that the BGI would still dominate the skew sextupolar components of the ring after the fix.
Hugo will leave CERN in September. He is transferring his work to Dominique.
More discussions between the experts are planned for next week.
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