Scandale, Walter

Todesco, E

Zimmermann, F

CERN Document Server

HHH-2008 DISCUSSION, COMMENTS, AND QUESTIONSW. Scandale, E. Todesco, F. Zimmermann, CERN, Geneva, SwitzerlandAbstractWe attempt to summarize the discussions, comments andquestions from the CARE-HHH workshop “HHH-2008”in Chavannes-de-Bogis, 24-25 November 2008, which wasdevoted to scenarios for the LHC upgrade and FAIR, andfrom some follow-up meetings.LHC PHASE-1 UPGRADEIt was debated whether an optics with interaction-point(IP) beta functions down to β∗ = 0.25 m is feasible us-ing Nb-Ti technology without moving the quadrupoles Q4and Q5 longitudinally. The answer was most likely not[J. Miles]. At the LHC IR upgrade review the Phase-1 IRparameters were determined to be 120 mm diameter, 120T/m gradient, and β∗ equal to 0.25 m. However, no opticssolution has yet been found which matches these condi-tions exactly. A close solution with 126 T/m exists, but itproved to be unstable. A shift of the longitudinal positionof the matching quadrupoles Q4 and Q5 would provide amore robust optics. Without moving the quadrupoles, a tar-get value β∗ ≈ 0.3 m may be realistic. A final optics isneeded as soon as possible. The present scheme of cor-recting the off-momentum beta beating in the two cleaninginsertions, involving asymmetric excitation of the arc sex-tupole families, introduces a lower bound on β ∗ of about0.25 m [S. Fartoukh].The reduction of β∗ provides about 50% improvement ofluminosity; the rest has to be obtained by an intensity in-crease. To get to 2.5×1034 cm−2s−1, an intensity increaseof about 30% is needed. The induced head-on beam-beamtune shift appears to be reasonable. However, the numberof long-range collisions grows significantly and the result-ing beam intensity limit for the Phase-1 interaction region(IR) may turn out to be more severe than the correspond-ing limit for the nominal LHC. In addition, the Phase-1 IRmight lose the βx ≈ βy location that is well qualified foraccommodating a long-range beam-beam compensator inthe nominal LHC layout. The trade-off between β ∗ and in-tensity limitations from long-range beam-beam effects mayneed to be explored both for the nominal and the Phase-1optics.Schedule is a very critical issue, for the near-term plan-ning as well as for the longer-term operation. Radiationissues can affect the duration of the Phase-1 operation.According to the design specification, the new Phase-1IR quadrupoles can sustain about 3 times more integratedluminosity than the present quadrupoles before radiationdamage occurs. Since the Phase 1 is supposed to have a 2.5times larger luminosity, the actual lifetime of the Phase-1 inner triplet will be approximately the same as for thepresent one, i.e. a replacement after about 5 years of op-eration will again be needed. This matches the Phase-2upgrade schedule.LHC PHASE-2 UPGRADEWith Nb3Sn magnet technology at 1.9 K for the innertriplet, and keeping the 23 m of distance between the tripletand the IP, one can reach a β∗ ≈ 0.14 m, if the LHC sex-tupoles are only used for correcting the linear chromaticity.Another solution must then be found for the off-momentumbeta beating. If the distance to the IP is reduced to 13 m,and under otherwise identical assumptions, one can arriveup to β∗ ≈ 0.11 m. The lifetime of the Nb3Sn magnetsneeds to be determined, and should be larger than the life-time of the inner triplets for the nominal LHC and for Phase1, i.e. its radiation resistance should be at least a factor fourlarger than for the Phase-1 magnets.Five scenarios for Phase 2 have been suggested. The firsttwo rely on a minimum β∗ ≈ 0.14 m, and recover the ge-ometric reduction factor either through an early separationscheme or via crab cavities. In these cases the Phase-2 goalcan be reached with a moderate increase of beam inten-sity. The third scenario relies on a large Piwinski angle toallow a higher beam intensity without reaching the beam-beam limit. Two new alternative scenarios have been pro-posed at HHH-2008 and in a follow-up meeting. The firstof these, the fourth scenario in the list [R. Garoby], con-siders a smaller transverse emittance, a higher brightness,and a large Piwinski angle, the second one, scenario no. 5[S. Fartoukh], a blow up of the emittance after accelera-tion, achieving a similar effect as the large Piwinski angle,i.e. allowing for a much larger beam intensity. In the thirdand fifth scenario, the number of proton per bunch neededto reach a luminosity of 1035 cm−2s−1 is about 4−5×1011at either 50 ns or 25 ns spacing. The feasibility of this largeintensity in the LHC and possibly in the injector chain, es-pecially for 25-ns spacing, may be a challenge. The fifthscenario seems to imply an unacceptable heat load in theLHC itself, and might not be maintained.Concerning the feasibility of the lower transverse emit-tance, it was noticed that the electron-cloud instability andintrabeam scattering (IBS) get worse with lower emittance.Since the luminosity lifetime also decreases, the shorterIBS rise time might not be a problem. With smaller emit-tance, the sensitivity to injection errors, to kicker rippleetc. becomes more critical, however. So far these errorsare not much better than the nominal specification [G. Ar-duini]. The damage limit for collimation is already “at theedge” for the nominal LHC. The situation will deterioratewith lower emittance [R. Assmann]. In fact, it was re-HHH-2008 PROCEEDINGS163ported that the present emittance represents an approximateoptimum (smaller emittance is bad in regard to collimatordamage, whereas larger emittances are bad with respect tocleaning efficiency [R. Assmann]). Also transverse Landaudamping will be reduced for a smaller emittance [S. Far-toukh]. On the other hand, a large emittance could lowerthe TMCI threshold in the SPS as the beneficial effect ofspace charge would be reduced [G. Rumolo].Beam-screen heat load, due to synchrotron radiation, im-age currents, and electron cloud, introduces important con-straints on the bunch population. For the nominal bunchspacing of 25 ns, the maximum acceptable bunch chargecorresponds to about 2.3 or 2.4 × 1011 protons. For in-creased spacings, larger bunch charges are possible.The upgrade will profit from a higher beam intensity, en-tering a regime that cannot yet be accessed experimentallyin the LHC or the SPS. At present, already the ultimatebunch charge of 1.7 × 1011 protons is difficult to achievein the SPS; however, the PS Booster can already deliverthe ultimate intensity with 20% margin and an emittancesmaller than the one finally needed in the LHC [E. Metral].An SPS study at this intensity was planned for 2009. Abunch-intensity level of 2.4×1011 protons will be exploredonce the new Linac4 is operational, from 2012 onwards. Inaddition to the beam availability from the injectors, otherintensity limits are encountered in the LHC itself. Colli-mation is a prominent example (both cleaning efficiencyand impedance effects seem to prevent reaching the nom-inal LHC intensity). A second limit, equal to the ultimatebunch charge of 1.7 × 107 protons comes from the LHCRF system [J. Tuckmantel]. Bunch charges higher than ul-timate will require an upgrade of this RF system.The question was raised, based on two talks at this work-shop (by N. Hessey and E. Tsesmelis) whether one shouldassume that the number of events per bunch crossing mustalways stay below a value of 200 rather than 300, the limitwhich had been considered in the past. It was also askedwhether the maximum acceptable pile up depends on theextent of the luminous region. Unfortunately, a larger ex-tent of the luminous region improves the situation only fora few detector subsystems [N. Hessey]. Another questionpertained to the cost of this limitation.Given the stringent limit on the event pile up, a level-ing of the luminosity during a physics store is clearly de-sired. A leveling demonstration or test could be attemptedin the LHC, once a reasonable performance level has beenestablished. The feasibility of leveling is controversial. Ithas not been successfully applied at operating colliders, butthere was no need for leveling in these machines either. Or-bit correction during the store is possible both at RHIC andat the Tevatron without losing the beam or other negativeconsequences [W. Fischer, J.-P. Koutchouk]. This experi-ence bodes well for leveling at the LHC. Leveling also is avery natural option for crab cavities [R. Calaga].ENERGY DEPOSITIONEnergy deposition does not appear to be a fundamentalissue, as far as quenches are concerned. One can alwaysadd shielding (possibly at the expense of a larger β ∗). Theenergy deposition for Phase 1 looks acceptable. Apply-ing a simple scaling argument, the energy deposition forPhase 2 should be about equal to 4 times the one of Phase1. On the other hand, Nb3Sn has a factor 3 higher quench(or radiation) tolerance with respect to NbTi. For this rea-son, the energy deposition might not be a problem for ei-ther upgrade phase. However, critical parameters like thequadrupole gradient, crossing angle etc. can affect the losspattern in the inner triplet region, and should be taken intoaccount [F. Cerutti]. If this rule is followed, there is no ob-vious showstopper. A question concerned the adequacy ofthe triplet cooling capacity. It was pointed out that the mod-eling of heat deposited in the yoke was incomplete, due tosome missing thermodynamics in the FLUKA code. Howdoes the projected magnet lifetime for Phases 1 and 2 varywith β∗ and the crossing angle?LESSONS FROM HERA UPGRADEThe experience of the HERA upgrade highlighted theimportance of magnet alignment and stability. The ques-tion was raised if this HERA experience is taken into ac-count for the LHC upgrade Phases 1 and 2 [B. Holzer].In the workshop discussion, the problems at HERA wereattributed mostly to insufficient preparation and design,which would not be a problem for the LHC. However, itwas stressed that the effect of the CMS stray field on nearbyramping magnets could lead to effects similar to those seenat HERA and should be evaluated [B. Holzer].EARLY-SEPARATION DIPOLESA slot inside the detector at about 13–14 m from the IPmay be available for both ATLAS and CMS. It was askedwhich other inputs were missing to advance the early-separation solution. One constraint may come from the IRvacuum chamber, which must fulfill a number of require-ments: the longitudinal extent of the innermost chamberis of order +/-50 cm. The two beams will be separated atinjection. Operation with different values of β ∗ should re-main possible.LHC INJECTOR UPGRADE & FAIRWhat will be the brightness, emittance and intensityavailable from LINAC4, SPL, PS2 and the upgraded SPSfor the LHC? How are emittance and intensity correlatedin this new complex, and how do they depend on the LHCbunch spacing (25 ns vs. 50 ns)? How will the beam for theLPA scheme—flat bunches of 5 × 1011 protons spaced by50 ns—be generated?Some concern was voiced regarding the low periodic-ity of the proposed “racetrack” PS2 optics and the as-HHH-2008 PROCEEDINGS164sociated high density of structure resonances [O. Boine-Frankenheim].How flat (longitudinally) do the bunches of the “LargePiwinski Angle” scenario for Phase 2 need to be, and whichtolerances apply to the bunch-by-bunch flatness variation?A generation and maintenance scheme for the LHC mustbe defined, either in the injector complex and/or in the LHCitself.For FAIR, is a beam-pipe aperture of 2-3 σ sufficient[F. Zimmermann], and can one rely on space charge forbeam-loading compensation & pre-compression (presenta-tion by O. Boine-Frankenheim) [E. Shaposhnikova]?The presentation by F. Me´ot triggered some questions onFFAGs. Can they be used at CERN? Are they a viable (andcheaper) alternative to a linac [O. Bruning]?ADVANCED S.C. MAGNETSThe following questions arose: Will Nb3Sn, MgB2and/or Bi2212 magnets provide a path to smaller β∗ and/orto higher energy? Are fast-cyling s.c. magnets a viable al-ternative technology for the PS2? Will they allow for anSPS2 at 1 TeV?BEAM-BEAM EFFECTSCan we predict LHC beam-beam lifetime within a factorof 2 [W. Fischer, S. Peggs]? It seems that we still cannotcompute the tune shift limit in hadron colliders. Whereand how can the functionality of electron lenses for head-on beam-beam compensation be verified? Can long-rangebeam-beam compensation be made to work for Phase 1,e.g. what is the efficiency of a compensator at a place withunequal beta functions?ELECTRON CLOUDElectron-cloud build up is predicted to be more severein the PS2 than in the higher energy SPS [G. Rumolo].What is the impact of the electron cloud on the PS2 design(e.g. choice of chamber dimensions)?Which is the role and a proper model of the re-diffusedelectrons?The electron cloud could have an effect on collimationand, e.g., trigger an ion avalanche, both in the LHC andin some of the new injectors. Do the novel coatings andclearing schemes solve the electron-cloud problem at leastfor the future machines?COLLIMATIONMachine protection considerations yield a limit on thetotal energy stored in the beam. For higher intensity,the beam dumping system and the collimators require up-grades.With realistic errors the beam intensity will be limitedto 3-4% of the nominal value for the present collimationsystem. It is expected that the collimator set-up time canbe reduced from initially 15 hours to a practical value of 1minute. What is the cleaning efficiency for the “phase 2”of the collimation system with realistic errors - is the idealgain by a factor 10 with respect to the present “phase-1”collimation system maintained when errors are included,and is it sufficient if the collimator phase 1 only allowsfor 4% of the nominal intensity? The so-called n1 num-ber should never decrease below about 7, as the collimatorscannot approach the beam to less than 5 σ [R. Assmann].There is little margin against collimator destruction foran asynchronous beam dump. The present system mightnot withstand a train of ultimate bunches in case of suchasynchronous dump.Hollow electron lenses acting as low-amplitude scrap-ers are an interesting refinement option for the collimationsystem [R. Assmann]. Are rotatable collimators, cryogeniccollimators, crystals, hollow electron lenses or a combina-tion thereof the solution for higher intensity? Beam impacton the rotatable collimators would be detected by tempera-ture sensors and microphones.For the LHC Phase-2 upgrade, a new dispersion suppres-sor will be needed in addition to a new inner triplet.CRAB CAVITIES AND FP7 EUCARDThere has been an excellent progress on crab cavities.One open question concerned the procedure for the planneddown selection.The formats of the meetings and of the dissemination inFP7 EuCARD deserve attention.ACKNOWLEDGEMENTSWe thank G. Arduini, R. Assmann, O. Boine-Frankenheim, O. Bru¨ning, R. Calaga, F. Cerutti, W. Fis-cher, B. Holzer, J.-P. Koutchouk, F. Me´ot, J. Miles,S. Peggs, W. Scandale, E. Shaposhnikova, P. Spiller,J. Tu¨ckmantel, and many others for helpful comments andcontributions.HHH-2008 PROCEEDINGS165

HHH-2008 Discussion, comments, and questions

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