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Septum Stray field and COD measurements

Septum Stray field and COD measurements. David Kelliher 19/5/11 (original version 14/4/11). Contents. Initial measurement of septum stray field Comparison with set of COD measurements Results of Experimental effort to correct for effects of stray field.

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Septum Stray field and COD measurements

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  1. Septum Stray field and COD measurements David Kelliher 19/5/11 (original version 14/4/11)

  2. Contents • Initial measurement of septum stray field • Comparison with set of COD measurements • Results of Experimental effort to correct for effects of stray field

  3. Strategy to measure septum stray field • Yoel, James observed difference in beam behaviour with and without extraction septum • Measure COD in both cases (April 8). Difference in COD shows effect of stray field. • Calculate injection, extraction septum stray field by simulation. • COD measurements consistent with a 0.6 mTm septum stray field • This analysis described in detail by S. Machida in document named “machida20100413.pdf”

  4. (S. Machida)

  5. Comparison with set of COD measurements • Estimate stray field required to produce observed COD • Compare with COD measurements made at different momenta - gives an indication of variation in stray field across aperture

  6. COD dataset (unshifted quads)

  7. COD dataset (shifted quads) All quadrupoles moved outward by 2 mm. The closed orbit then moved outward by about that amount. *Shift Log claims 14MeV/c settings same as for 16MeV/c case.

  8. Tune calculation (by observation) • Calculate tune at each momentum • Integer part from COD pattern • Fractional part normally estimated from turn-by-turn oscillation

  9. Example: Tune @ 20.5 MeV/c COD over 42 cells Turn-by-turn coordinate in E10 Integer part ~6 Fractional part ~ 0.1 • Total tune 6 +/- 0.1 • Shape of “kink” around injection septum indicates fractional part is just below 1 => Tune is 5.9.

  10. Simulated COD • Zgoubi tracking code used • Introduce thin dipole element in centre of long drift • Set kick to -0.0005 T m (-0.5 mTm). Negative sign means an outward kick. • Compare generated COD with measurements – apply offset to simulated result for ease of comparison. • Fix momentum at 12.5 MeV/c • Quadrupole gradients in Zgoubi adjusted to match measured measuredtune at each momentum.

  11. Unshifted quadrupoles • Compare measured COD with simulated COD • Simulated stray field -5 mTm • Slides 9 - 14

  12. 11.1 MeV/c

  13. 12.5 MeV/c

  14. 15.5 MeV/c

  15. 18.0 MeV/c

  16. 18.3 MeV/c

  17. 20.5 MeV/c

  18. Shifted quadrupoles • Compare measured COD with simulated COD • Simulated stray field -5 mTm • Slides 19-24

  19. 12.0 MeV/c

  20. 14.0 MeV/c

  21. 16.0 MeV/c

  22. 18.0 MeV/c April 8 measurement April 16 measurement

  23. 20.3 MeV/c NAFF calculated horizontal tune = 6.006 Adjust tune to match COD, tune = 5.914

  24. Local correction of stray field • First study by Y. Giboudot presented 10/6/10 • Shift quadrupole pairs on either side of injection septum to cancel effect of stray field (using SVD) • Analysis showed single correction setting works over momentum range assuming a uniform stray field

  25. Correction at multipole momenta • Assume uniform septum stray field • When adjusting equivalent momentum, quadrupole gradients change. Need to scale quadrupole correction shift accordingly. • When accelerating, quad gradients constant. Kick due to stray field and quadrupole shift scale with rigidity • Neglecting variation in phase advance with momentum, one correction setting should work to correct accelerated orbit distortion.

  26. Correction experiment 1 (April 16) • Try correction by shifting quadrupole doublets on either side of injection septum inwards by 0.4mm • Momentum (equivalent) set to 18.0 MeV/c

  27. Compare correcction with simulation (April 16) • Zgoubi model (quads set to match NAFF tune), shift quads inward by 0.4mm • Compare with difference of COD measurements with/without shift

  28. Correction experiment 2 (April 17) • Try correction by shifting quadrupole doublets on either side of injection septum outward by 0.2mm • Momentum (equivalent) set to 12.0 MeV/c

  29. Compare correction with simulation (April 17) • Zgoubi model (quads set to match NAFF tune), shift quads outward by 0.2mm • Compare with difference of COD measurements with/without shift

  30. Conclusions • We have identified the septum stray field as being the major source of horizontal COD. • Injection septum stray field of 0.5 mTm produces a COD in broad agreement with measurements. • Agreement is better in higher momentum cases • Result of orbit correction attempts inconclusive – further optimsation of local quad shifts needed. • Change in COD after shifting quadrupole pairs agrees with simulation.

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