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FCC-ee tuning and MADX issues
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FCC-ee tuning and MADX issues
Rogelio reminds that the deadline for the submission of IPAC contributions on the group level is today. A group information meeting is planned next week, and proposals for presentations are welcome. The meeting today focuses on FCC-ee tuning and related MAD-X improvements.
Status of FCC-ee tuning studies with special focus on coupling, Tessa Charles, slides
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Tessa studies emittance tuning by orbit and coupling correction for the FCC-ee. She uses MAD-X
with random errors in position (x,y,s) for magnets and BPMs, and also in angle (theta, phi, psi) for magnets, and shows the emittance distributions obtained after correction for different random seeds.
After correction, she finds a median horizontal emittance of 2.3 nm and a vertical of 0.23 pm, with some tail to higher values in the vertical plan. The studies are done for single beams, i.e. without emittance blow up by beam-beam. She suggests to use for first estimates of beams sizes (as relevant for collimation) an increase in 10% in the horizontal and by factor 10 in the vertical plane compared to the ideal machine. She also shows the coupling matrix elements (R11-R22) obtained from MAD-X around the ring and observes peaks at the IPs. Tobias says this is related to the strongly varying beta functions around the IP and suggests to use normalized quantities. Helmut asks which energy spread was assumed and suggests to check if the increase in energy spread by Beamstrahlung effects emittances.
Outstanding points:
-BPMs roll errors are not added on top of the attached quadrupole roll. This would be more realistic as done for the position errors.
-BPM orthogonality error is not included, which would affect coupling correctability.
-Impact of energy deviations up to 2% to be considered.
-Possible solenoid imperfections to be considered
-Local corrections should be implemented to target the spikes, and hopefully progressing towards meeting horizontal emittance target.
Considering coupling errors for aperture, Michael Hofer, slides
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Michael reminds that the stored beam energy for the FCC-ee will be very high for an electron energy machine,
comparable to the HL-LHC in heavy ion mode, requiring a good protection strategy, and a realistic modeling that includes coupling. From the LHC, there is good experience in protecting apertures using the MAD-X aperture model. The aperture model already allows to take into account machine imperfections using BBEAT, COR and DPARX parameters.
In the following discussion, it was argued that the current LHC based aperture model, maybe with some upgrade, could also be adapted for the FCC-ee.
Helmut and Ilya reminded that electron machines are very different from the LHC: more dynamic with the need for large acceptances for good lifetime. Collimation should focus on removing the unstable particles outside the machine acceptance which maybe poorly described by twiss parameters and better assessed by simulations.
Ilya reminded that collimations should also be compatible with the top-up injection in FCC-ee.
Outstanding points:
-The aperture module might not need important upgrades urgently but rather general updates in MADX to better inform the user about coupling of the machine. Proposals:
* Compute DQmin in MADX (Twiss table?)
* Output angle of the x-y ellipse, maybe in the output of aperture module?
* Introduce H & V BBEAT parameters in aperture module (instead of just one), backwards compatible.
Emit with tapering in MAD-X, Tobias Persson and Leon Van Riesen-Haupt, slides
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The beam energy in FCC-ee varies significantly around the ring by the energy loss in synchrotron radiation and gain in RF. It is planned to keep the orbit centered in the magnets by adapting the dipole strenths to the local beam energy (tapering). MAD-X (from version 5.04.00 on) allows to adapt the bending strength without changing the ring-geometry (keeping the design bending angle). In addition, the phase (LAG in MAD-X) needs to be adapted.
The default MAD-X value of LAG=0 is longitudinally stable below transition. Above transition with negligible energy loss, the stable phase angle corresponds to LAG=0.5. MAD-X is now checking this by looking at eigenvalues and issuing a warning when they differ from 1 by more than 1.e-4. When the debug flag is set on,
the eigenvalues are printed. The optimal phase with energy loss can be obtained by matching of the LAG. Tobias shows an example, now included in the MAD-X tests in the MAD-X repository.
A slide prepared by Leon shows that EMIT now also works with tapering : the emittances obtained from EMIT with tapering now agree with design values and what is expected by scaling from low energy (1 GeV where the sawtooth is negligible) to FCC-ee energies.
Rogelio asks when the improved MAD-X will be released. Tobias says very soon (< 2 weeks), to be followed by describing tapering and LAG matching also in the documentation.
Roderik reported on a bizarre behaviour of MADX tracking with multiple particles and radiation. To be looked at.
Next meetings:
Next week, on May 5, a joint NDC-LNO meeting https://indico.cern.ch/event/1033415/ including a presentation of the LHC connection cryostat status, to be followed by a LNO meeting the week after
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