Basic principle Parameter list

1. Large emittance scenario for the Phase II Upgrade of the LHC S. Fartoukh with precious help from E. Laface Phase II Brainstorming meetings 24/03/2009 Basic principle Parameter list Preliminary results, optics, X-scheme, aperture, b-b footprint. Conclusions

2. S. Fartoukh, 24/03/2009 Basic idea It is a dream to gain something substantial with the optics, i.e. reducing b*, w/o reducing L*! At cst L*=23 m, the limit is given by a full chromatic correction of the inner triplet by the arc sextupole @550A (possibly 600A) and is given by (e.g. see LIUW#15, May 2008) For a gradient of ~ 120 T/m, this limit corresponds to a minimum b* of 30 cm (~25cm assuming 600A can be reached in the MS) 1. Leaving no aperture margin for NbTi (120 mm IDmax @ 120 T/m)  Phase I triplet 2. Leaving a huge aperture margin for Nb3Sn (~160 mm ID max @ 120 T/m). Of course this margin could be used for pushing the gradient above 120 T/m in order to reduce further b*, at cst J, up to reaching the Nb3Sn technology limit in terms of aperture for a given gradient. In this case the min. possible is b*~23-24 cm for Nb3Sn (~100mm IDmax @ 230 T/m) Far too expensive compared the lumi gain: only ~1025% at cst beam parameters w/o or with crab-cavity or D0!! A much better scenario is to fully count on the expected performance of the new CERN injector complex, ~5 1011 p/bunch @ nominal LHC emittance (????), and to blow up the beam emittance during the ramp (by a factor of ~3 to stick to the beam-beam limit of DQ~0.01), as partially demonstrated last year in the SPS! In this case (see later), a Nb3Sn IT (120 T/m-160 mm) with exactly the same layout as the one foreseen for PhaseI, still warrant n1~7 @ b*= 30 cm!! Then, if too pessimistic in our evaluation of the b.b. limit (e.g. see Tevatron working with a b.-b. tune shift of 0.02-0.03), nothing will prevent us from reducing the emittance blow-up during the ramp and then even further reduce b* after some upgrade of the arc sextupoles.

3. S. Fartoukh, 24/03/2009 Short parameter wish list

4. S. Fartoukh., 24/03/2009 Layout & Optics (same as Phase I) Nb3Sn Inner Triplet 120 T/m-160mm b*=30 cm  See presentation at next LIUWG (26/03/2009)

5. S. Fartoukh, 24/03/2009 X-scheme •  10 s b.-b. separation • 730 mrad full xing-angle • ~20 mm peak orbit excursion in the IT • Will need to ~double the MCBC/Y @ Q4/5/6

6. S. Fartoukh, 24/03/2009 (after correction of the spurious dispersion by H/V orbit bumps in the arcs). Aperture (with nom. matching section, inc. TAN) Q1 (ID-160mm) with 9 mm shielding D2/Q4 Q7 Q6 Phase I-D1 (ID-CB=140 mm ) Q5 New TAS (ID-65mm) TAN to be changed !! Q2-Q3 (ID-160mm) •  TAN to be changed, possibly D2/Q4/Q5, or only b.s. rotation (see LIUWG#2) • n1 just 7 in the IT (assuming 21 mm between coil ID and b.s. ID, perhaps a bit optimistic?) • In any case, why sticking to n1=7 in collision when the beam sizes will be ~180% bigger?? • i.e. some compromise to be found for impedance keeping in mind that the MO landau damping efficiency scales directly with the beam emittance, i.e. 320% improvement.

7. S. Fartoukh, 24/03/2009 Courtesy of E. Laface Tune footprint (1/2)  MO’sassumed to beused both for Phase I and Phase II to compensate for residual Q’’ in collision.  Perhaps no longer needed for Phase II, if the LHC IR tunability can improve by adding new quad’s (the logic behind this beyond purpose, see next LIUWG presentation for more details) • This explain the more or less big H/V dissymmetry observed for the PhaseI/II tune footprints. (OD’s stronger in the optics because 4 times less efficient than OF’s w.r.t. Q’’, but of course as efficient as OF’s w.r.t. amplitude detuning)  Said differently, for large emittance beam, we might well have an knob to “shape” the tune footprint

8. S. Fartoukh, 24/03/2009 Tune footprint (2/2) Courtesy of E. Laface • As expected, when MO’s switched off in the case of Phase II

9. S. Fartoukh, 24/03/2009 • Nb3Sn new triplet 160 mm-120 T/m .. Y a qu’a! • 5.5 1011 p/bunch with 25 ns bunch spacing w/o impedance problems in SPS and LHC at injection … Y a qu’a! • Controlled beam emittance blow up during the ramp: Why not (we have 20 min. to do it)? • Pile-up events not discussed, but given, as usual, as soon as the peak lumi and the bunch spacing is defined…. Perhaps bigger s* combined with smaller exp. pipe could help to push a bit further the limit?? … As usual, the final parameter list will be a compromise of several little gains, but the beam emittance is certainly the most efficient knob (at large intensity), at least by far much more efficient than b*! Conclusions