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#3091

#3091. ¾ of shift looking at BPM and transients ¼ shift looking at sextupole alignment . Sextupole Alignment.

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#3091

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  1. #3091 • ¾ of shift looking at BPM and transients • ¼ shift looking at sextupole alignment

  2. Sextupole Alignment • Sextupole should alter the path length if aligned, just the path length vs energy curve. So you might be tempted to say if the sextupole doesn’t change the TOF, then it’s aligned, if the TOF changes, then you’re misaligned. • Arc R56 is sensitive to sextupole field if sextupole misaligned with beam • If, with the beam energy set to nominal on-energy and dipole set to nominal current, changing sextupole strength wouldn’t change the r56 of the arc (dl/dE), (and in fact dl/dE would change from 0 to 0 for the isochronous arc) • But we don’t really know if we’ve got the right energy or the right dipole strength. • We can, for a given dipole current, measure a l vs E curve and thus work out the R56 for any E • We can then turn on the sextupole to see and measure the l vs E curve, and work out the new R56 for any E. • Want to calculate the change in R56 from a change in sextupole strength. • Method • Calculate the path length vs energy curve using TOA on AR1-BPM-06 with sextupole OFF. From this you can calculate the R56 at a given energy. Or can calculate the energy Emin at which the R56 = 0 ( dl/dE = 0) • Calculate the path length vs energy curve using TOA on AR1-BPM-06 with sextupole ON. From this you can calculate the new R56 at Emin.

  3. Sextupole 1 nominal beam energy set using AR1-OTR1 Can see that ARC1 not set to the isochronous for the nominal beam energy Q1/4 = 2.19 Q2/3 = 1.14 nominal dipole in original set-up nominal dipole for p =26.5 MeV ‘isochronous’ energy • To get R56 = 0 (i.e. sextupole aligned) at p = 26.5 MeV, • would need to change AR1-DIP-01 from 42.95 to (using contours) ~ 42. 75 amps i.e. ~ 0.2 amp change. This equates to a change of beam position of ~ 3 mm at the sext. • conclusion is that with dipole and energy set for 30°, beam is offset ~ 3 mm in sextupole 1 compared to the reference orbit. ~42.75 0.2A ~42.65 30° angle, ref. traj. 0.2A

  4. Is the beam energy – LC1 GS relation correct? Taken at 14:00 just before TOF measurements start. Beam position on AR1-1 with LC1GS = 34.7 and AR1-DIP-01 = 42.86 A -> KE = 25.94 -> p = 26.45 AR1-DIP-01 not quite set right for p =26.5 This method trusts that the dipole calibration is perfectly correct and that x = 0 on the screen gives exactly 30° bend.

  5. Sextupole 2 Dipole-02 was nominally set to 42.76 which gives 30° for p = 26.50 MeV (different calibration to DIP-01) For 26.45 MeV, 42.65 A gives 30°, the reference trajectory And using contours, dipole value that gives R56 = 0 at 26.5 MeV is ~ 42.85. Thus a change of 0.20 A is required to centre beam in sext 2. Thus sext 2 is ~ 3.0 mm misaligned. 30° angle, ref. traj.

  6. Conclusions • There are issues in trusting the measurement of absolute sext offset (relies on dipole calibration and screen alignment, bunch-to-bunch energy variations/transients) • But perhaps the relative offset of the two sexts is more robust. Results do not indicate that SEXT-02 is significantly more misaligned than SEXT-01 where by ‘misaligned’ I mean offset with respect to the reference orbit.

  7. P.S. AR1-DIP-01 values in FEL set-ups in 2012 • To precisely set up for KE = 26.00 MeV or p = 26.50 MeV need AR1-DIP-01 = 42.95 A

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