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Study of Low- a p Lattice Options for a Super-PEP HER

Study of Low- a p Lattice Options for a Super-PEP HER. U. Wienands SLAC PEP-II. Context for the Super-PEP HER. Would like to maintain r dipole =165 m ∆E/turn ≈ 2.2 MV at 8.5 GeV, 1/ r 2 every arc-cell filled with a dipole in PEP-II Will likely want a smaller emittance than for PEP-II

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Study of Low- a p Lattice Options for a Super-PEP HER

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  1. Study of Low-ap Lattice Options for a Super-PEP HER U. Wienands SLAC PEP-II U. Wienands, SLAC-PEP-II Super-B Hawaii Apr-05

  2. Context for the Super-PEP HER • Would like to maintain rdipole=165 m • ∆E/turn ≈ 2.2 MV at 8.5 GeV, 1/r2 • every arc-cell filled with a dipole in PEP-II • Will likely want a smaller emittance than for PEP-II • PEP-II HER has 48 nmr nominal • For Super-B, will likely want to reduce this to 40 or even 28 nmr. • Will want a low momentum-compaction factor • PEP-II: 0.00241–>0.00167 • Super-B: 0.0006…0.0001 U. Wienands, SLAC-PEP-II Super-B Hawaii Apr-05

  3. How to Achieve low ap • ap ≈ h => high tune nx • practical limit, acceptance limit • also reduces the beam emittance • Modulate bending or focusing, forcing h down in dipoles • Teng & Ohnuma: ≤360° phase advance/period • Potential to reduce ap < 0 • Potentially increases beam emittance (≈ B2) U. Wienands, SLAC-PEP-II Super-B Hawaii Apr-05

  4. Candidate Superperiods • 90° cells: • LEB-type Period: DOFO with BB00BB bending • high dispersion in empty cells, low/negative in bending cells • dipole packing factor is low • KAON-Factory type cell: FODO, full cells, modulate quads • high packing factor • large dispersion swings for low/negative ap • In either of these, emittance is traded for momentum compaction. U. Wienands, SLAC-PEP-II Super-B Hawaii Apr-05

  5. HER Sextant, 90° cell ap = 0.0006 ex = 50 nmr 4 periods+nsup=16 cells rdipole = 165 m U. Wienands, SLAC-PEP-II Super-B Hawaii Apr-05

  6. What about negative ap? ap = -0.0002/period, but +0.00028/sext. ex = 68 nmr 4 periods+nsup=16 cells rdipole = 165 m U. Wienands, SLAC-PEP-II Super-B Hawaii Apr-05

  7. Can’t get negative ap easily, and ex already too large • It turns out the h suppressors raise ap considerably • ap = (ap(periods)*lperiods+ap(hsup)*lsup)/larc • ap(hsup) is large, therefore have to make these short! • Shorten the dipoles in the high-h cells to lower ex • this lowers the bending radius… U. Wienands, SLAC-PEP-II Super-B Hawaii Apr-05

  8. U. Wienands, SLAC-PEP-II Super-B Hawaii Apr-05

  9. Not promising! • lowest emittance achieved: 56 nmr (empty high-h cells) • sr. energy loss up by more than a factor of 2 U. Wienands, SLAC-PEP-II Super-B Hawaii Apr-05

  10. Half-length Dipoles in High-h cells ap = +0.00025/sext. ex = 61 nmr 4 periods+nsup=16 cells rdipole = 139 m U. Wienands, SLAC-PEP-II Super-B Hawaii Apr-05

  11. Where to go from here? • For our case, shortening the h suppressors only marginally sucessful • overall arc length has to stay due to tunnel geometry • => dipoles get longer, or stronger => emittance  • Need to look into more exotic alternatives • start with stronger focusing • 108°/cell, 135°/cell, … • lower natural emittance, • less modulation needed to lower ap to desired value U. Wienands, SLAC-PEP-II Super-B Hawaii Apr-05

  12. Try 135°/cell • one period = two cells: 270° • should be able to lower ap by quad. modulation • should be able to find 180° locations for sextupoles… U. Wienands, SLAC-PEP-II Super-B Hawaii Apr-05

  13. 135°/cell Period, ap=0 ap = 0/cell ex = 20 nmr 7 periods+nsup=16 cells rdipole = 165 m U. Wienands, SLAC-PEP-II Super-B Hawaii Apr-05

  14. 135° Lattice (cont’d) • Closer inspection of this lattice reveals that • X chromaticity compensation should be easy • Y chromaticity compensation ≈ impossible • high ßy locations have almost no dispersion •  • Can we modify design for vertical chroma. correction • idea: raise ßy at high-dispersion location U. Wienands, SLAC-PEP-II Super-B Hawaii Apr-05

  15. 135° Lattice with Doublet U. Wienands, SLAC-PEP-II Super-B Hawaii Apr-05

  16. Raise ßy/ßx at end to about 3:1 • may be sufficient in principle • But: • xy now up to 7 units in ∂n/(∂p/p), • likely too high for required acceptance U. Wienands, SLAC-PEP-II Super-B Hawaii Apr-05

  17. Conclusion (for now) • As expected, a low-ap lattice for a Super-PEP HER is not easy to find. • For 90°/cell, ap ≈ 0.0006 seems to be about as low as it will go, in the PEP-II context (ex, tunnel, s.r.) • very preliminary tracking suggests chromaticity correction is feasible. • For comparison, the 90° HER lattice for PEP-II will have ap ≈ 0.00167. • For 135°/cell, lower ap is feasible to 1st order, but chromaticity correction will be a major challenge. U. Wienands, SLAC-PEP-II Super-B Hawaii Apr-05

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