History of Sextupole Calibration Runs
and
Voltages Measured on 9/23/2003
Analysis of Data Taken 10/28/2002, 9/17/2003 and 9/28/2003

3 October 2003
JAC
The beam-based calibration of the 76 sextupole magnets proceeds by bumping the beam (t1.b1. positrons) in each sextupole and measuring the horizontal and vertical tune change for two settings of the sextupole strength. The program which does this is [cesr.dcs.sext]sex_multipole.exe. A complete run takes about 90 minutes, if there are no interruptions. The present version of the program does no sweeping of either the bump setting or the sextupole setting to avoid hysterisis effects. User-controlled parameters of the calibration run are the choice of which sextupoles to calibrate, the range over which the sextupole current is varied and a limit on the maximum tune excursion. The latter can be important to avoid crossing resonances during the calibration. For example, the conditions of 9/17/03 and 9/28/03 had a horizontal synchrobetatron resonance at 213 kHz. So before running the the horizontal tune was moved to 220 kHz and the tune change limited to 5 kHz. Restricting the current range of the sextupole can also help to avoid beam loss during the calibration.

Phase measurements can be used by the program which analyzes the raw calibration data ([cesr.dcs.sext]anal_sex_multipole.exe) to improve the accuracy of the calibration by providing more accurate calculation of the beta function (as opposed to assuming the design value.) The phase measurement must be taken just prior to the calibration run, and CESRV is then used to produce a digested file for use by the analysis program.

The three calibration runs presented here have the following characteristics:
  • 1) 10/28/2002
    Beam energy Upsilon 2S (5.012 GeV). CSR save set 86287. Range +-1000 cu.
  • 2) 9/17/2003
    1.9 GeV. Range +-500 cu. CSR save set 91896. The sextupole currents were those of the design lattice. Sextupole number 85 (14E) was was wired backward during this calibration, as shown by its calibration factor of approximately -1.
  • 3) 9/28/2003
    1.9 GeV. Range +-250 cu. CSR save set 92264. No phase measurement was available for this data. The sextupole currents were those optimized on positron injection. Despite the restricted range of sextupole current used for the calibration, the beam was lost quite a few times during this calibration run, resulting in missing calibration data for some sextupoles.
All of these tune measurements were done using the faster tune measurement algorithm (see 9/22/2003), which is known to be problematic at 1.9 GeV.

On 9/23/2003 the voltages across all the sextupoles were measured for a uniform set value of 1000 cu. The distribution of these values show that the power supply fluctuations are about 2.5% rms. Subsequent investigation of a few power supplies which exhibited the most anomalous voltage values showed that these did not exhibit similarly anomalous current values. The variation in the beam-based measurements of 10-20% show that accuracy of the measurement method itself dominates the results.

Suggestions for improving this calibration procedure are:
  • Use slower, more accurate tune measurement algorithm as implemented in XIMES on 9/22/2003 (done 10/3/2003 by DCS)
  • Sweep through bump settings to avoid hysterisis effects. Make sure the bump is zeroed after use.
  • Sweep through sextupole current settings to avoid hysterisis effects.
  • Include a correction for the tune change due to chromaticity, since the introduction of bumps changes the orbit and hence the path length. Such a correction may not be easy, since the chromaticity may not be well known or easily measurable.
  • Measure the sextupole strength by varying the RF and using the dispersion to create displacement in the sextupoles, rather than using the hysterisis-plagued bump method.


Creation date: 10/06/03.