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LNO section meeting with focus on injectors
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Europe/Zurich
6/2-008 (CERN)
Vittorio, Optics and magnetic modelling of the PS-MU and beam-based measurements
A detailed introduction of the PS magnet system is given along with the status of the magnetic and MADX models of the PS unit. MADX has CFMs and multipoles to include all the magnetic components. In the past the strengths were mostly effectively computed. Vittorio aims at developing a new magnetic modeling from simulations to feed the MADX model. Initial magnetic model provided to Vittorio had wrong boundary conditions. Also, magnetic simulations took too long, days. The number of cells from the finite element model was too large in some regions. CPU time was reduced to 8h. The magnet is divided into various relevant sections for a closer analysis with appropriate boundary conditions.
For optics measurements the bare machine is studied without powering the pole face windings and figure of 8 loop (only main coil). This requires the set-up of new magnetic cycles. Highest energy that could be reached only with main coils is 23.11 GeV. Furthermore, for the survival of the beam the excitation had to be fired (negative chromaticity instability, about -6 units, cured by larger emittance?). Differences are seen between the 10GeV and 18GeV cycles. In the 23GeV cycle there clear correlations of tune changes with the speed of the magnetic change, likely due to eddy currents.
A strong dependency is found on the cycle preceding the measurement cycle, which changes a lot depending on the day. This explained the discrepancies at 7 and 14Gev. Making the measurements with the same magnetic history shows a much better agreement between measurement dedicated plateus or continuous measurements in the long 23GeV cycle.
Comparisons betwen magnetic model and measurements shows agreement within 0.07. It was noticed that the beam central position is changed by 30mm during the ramp! Correction of mean orbit changed tunes by abut 0.05 tunes and improves the comparison between continuous and plateuas measurements. It also improves agreement simulations and measurements with largest deviation of about 0.05. Adding quadrupolar fringe fields further improves model. Magnetic simulations show different saturation at larger energies of the quad and dipolar fields.
Next steps: curved magnets, harmonics, improved MADX model to add multipoles in the air gaps.
Wietse, Zero dispersion optics in the PS
Different optics are set-up to have zero dispersion at different measurement devices. Measured Dx at BPM and wire scanner slightly differ while MADX model shows same value. Probably due to BPM calibration, which is common, or also wire calibration could affect. Measured and model beta functions show very good agreement close to 1%. The zero dispersion optics has a different emittance measurement (thanks to the improve accuracy in the zero dispersion optics, about 215%) but also has a much better error bar by almost a factor 4. There is a slight discrepancy in the emittance versus knob measurements that could come from space charge. This is really a clear improvement of the emittance measurements, hopefully to become operational.
Felix, optics measurements in LEIR and ELENA
Last year a beta-beating in LEIR of almost 30% was observed at injection energy. In 2023 excitation method has been improved and k-modulation measurements are added. First measurements at top energy of beta-beating show 40%, so larger than at injection, but further analysis is needed to clarify measurement resolution.
K-modulation measurements are automated to measure though the cycle. At injection max beating is about 20%. At top energy deviations increase to slightly above 25% beating.
For ELENA 2023 was used for the preparation work on BPM synchronization, etc. Various excitations techniques were used. The best one seems to be to excite the tune for about 50 turns and then use the free oscillation. From phase advance there are clear tune jumps, meaning that the acquisitions are still offset by one turn. Measured beta-beating reaches 70% in the H plane! Cross checks with K-mod should be performed. At extraction (lower) energy the BPM signal deteriorates because the bunches are split. Immediate fix would be to increase the sample rate at the cost of number of turns.
For the long-term avoid bunch splitting in ELENA should be explored. K-mod in ELENA also to be performed.
Ewen, Updates on OMC studies in PSB
In the PSB 2 spare BPMs were installed in new locations only in ring 1. BPM resolution seems to be slightly worse for the new BPMs but still OK. Synchronization issues appear when changing RF user. Maximum measured beta-beating is 10%.
Kmod measurement improved in 2023 in a quadrupole near to the new BPMs. Slight discrepancy between K-mod and Ac dipole of about 5% to be analyzed taking into account systematic errors. Some lack of reproducibility between different days in K-mod. To be understood. The current of the quadrupole could be increased in the future for a better measurement.
AC dipole measurements done close to the 3Qy resonance. Some lack of reproducibility is being analyzed. For large number of turn large sidebands around the AC dipole tune close to 5e-4, which is about 500 Hz (that could be just 10x50Hz). To be further analyzed also at flattop.
Using BBQ as extra BPMs in PSB is being explored. The issue is that the BBQ does not use the same RF signal. This can be fixed in the future.
In PS studies if ACDipole can be used for for optics measurements at the transition energy. Various configurations of the gamma-transition jump optics knobs were used.
Syrmatenia, Improving LHC aperture models
The goal is to converge into the single aperture model of the laytout database. Various elements have been reviewed:
-TCLIA location had to be changed in LS2. This generated some losses that were mitigated by changing the lower jaw opening by 5-6 mm. The problem was that TCLIA was shifted paralled to Beam 1 and not to B1-B2 center. This wrong positioning will only be fixed only in LS3, but maybe we should discuss if this should be advanced to avoid new risks in LS3.
-RF cavities have no aperture in LDB. Some issues with names and lengths in the drawings (more than 15 years old). There were no final drawings after installation. Information will be extracted from new 3D models. Aperture definitions are being introduced in the LDB based on best knowledge for now.
-TDIS aperture model is now in the LDB.
-VVGSY and transitions aperture added
-TDIS transitions added with a SVG image. SVG editor is now added in the ACC models repository.
-TDIS jaws also fixed to be 4 or 6mm. Alternatively parking position could be used in LDB, this would make it consistent with other collimators.
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