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    6th Run II FiDeL meeting

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    Minutes of the 6th FiDeL RunII meeting

     

     

    Date: 2015-08-18

    Place: 927, and CCC

     

    Presents:

      Ezio Todesco, Frank Schmidt, Matteo Solfaroli Camillocci, plus Rogelio Tomas Garcia, Jorg Wenninger (in LBOC presentation)

     

    Review of the chromaticity control [M. Solfaroli Camillocci]

     

    Matteo makes a review of the chromaticity control.

     

    • The measurement is based on a modulation of the beam energy, and smoothing and a fit of the tune and a successive smoothing (slide 2). Post-processing is not trivial, and efforts are being done to have this algorithm on line (slide 13).
    • Recently one of the sextupole spool pieces circuits have been condemned (sector 78 beam 2) and the correction had been taken by the other 7 arcs. The implementation has been done in the past week and after some initial problems (a trim was unintentionally left in) it is now working correctly. Compensation is active during injection, during the ramp and at flattop.
    • During the scrubbing run, the validity of the chroma correction of decay at injection has been verified. Matteo tried some variations of the FiDeL coefficients, but the baseline used since more than one month is valid, with a time constant of 850 s, a coefficient d of 0.32 and an amplitude d at 1000 s of 0.26. Matteo points out the during the scrubbing run, conditions were rather extreme compared to usual operation (beam injected very rapidly, and requirement of good control over very long injection times).
    • Matteo has measured again the dependence of the chroma on the spool pieces, finding that 1 b3 unit of the dipoles gives -43 units in H and 35 in V (slide 9). These numbers are consistent with previous measurements in Run I. A cross-check with a MAD model, would provide an explication of the source of asymmetry between H and V. The cross-check of the conversion between b3 units and current in the spool pieces will be carried out.
    • Matteo also checked the chroma trim through the lattice sextupole, with successive stpes of 5 units of Q’, finding a good agreement (slide 10).
    • With present settings, we have a part of the chromaticity correction added as a trim to the lattice sextupoles (slide 11), with an increase of 10 units during the snapback (first 50 s) and a subsequent decrease of 40 units in the initial part of the ramp in H, and a decrease of 10 units with a successive increase of 50 units in V. Considering that the total amplitude of the snapback is 40 units, we have about 25% error in the snapback model. During the ramp the b3 change is 7 units, corresponding to a chromaticity change of 300 in H and 260 in V. The error of the modeling of the persistent current is 40/300=13% in H and 50/260=25% in V. A good fraction of this correction could be removed and placed on the spool pieces, with a fine tuning of the snapback and persistent current. To be probably done in the next restart (2016). After 200 s, there is no correction applied in H and 10 unit in V. The source of this error (about 3% of the whole chromaticity swing during the ramp) cannot be the b3 in the dipoles.
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