Koutchouk, Jean-Pierre

Zorzano-Mier, M P

CERN Document Server

SUMMARY OF SESSION 5: FROM LEP TO LHC, SOME IMPORTANTISSUESJ.-P. Koutchouk, M. P. Zorzano Mier, CERN, Geneva, SwitzerlandAbstractThe aim of this session is to introduce some importantconcepts of LHC related to performance and beam dy-namics: very high energy and luminosity (Lyn Evans),super-conducting magnets and consequences on field qual-ity (Luca Bottura), sensitivity to beam losses (Jean-BernardJeanneret), machine optics, parameters and most impor-tant beam dynamics issues (Jean-Pierre Koutchouk, DanielBrandt and Werner Herr). The overall impression may beone of complexity. In this summary, we bring together theissues and the solution(s) adopted.1 INTRODUCTIONThe LHC is a hadron machine designed for wide-band ex-ploration and discovery. For that purpose, it must run at thehighest energy possible in the LEP tunnel (7 TeV×7 TeV)and at the very high luminosity of 1034cm−2s−1.The first requirement is fulfilled by using super-conducting magnets cooled at 1.9◦ K. Due to the large en-ergy range (450 GeV to 7 TeV), the magnets are operated atlow current at injection. The dc and ac magnetization of thecables causes relatively large multipolar field componentswhich are significant for the beam dynamics.The second requirement (high luminosity) imposes avery large number of bunches in each beam with importantconsequences for the beam-beam effect.The overall impression may be that of complexity, fur-ther amplified by the requirement to foresee all hardwarein advance as the compact design makes any late modifica-tion difficult and expensive. The main issues have howeverbeen studied over years and experience of the former orexisting hadron colliders has been taken into account to an-ticipate new requirements and investigate the robustness ofthe chosen machine parameters.2 SINGLE PARTICLE ISSUES2.1 Effect of the Multipolar Content of the Mag-netic FieldsThe situation is rather contrasted. On the one hand, the to-tal relative field errors of the dipoles versus the transverseposition expressed in σ’s are not so different when com-paring LEP to LHC magnets (factor 3 at most). On theother hand, the multipolar content of the LHC dipole fieldis much richer. Significant components extend to the de-capole in the dipoles and to the 20-pole in the quadrupoles.Altogether, the beam motion is unstable at a few σ’s if nocorrective actions were made.The cures have been of three types:• improvement of the field quality (adjustment of coilposition, compensation between different sources ofimperfections, strategy in the assembly),• comprehensive set of multipolar correctors,• use of the optics (integer tunes or cell phase advance)to avoid the strongest resonances.A target field quality could be defined and it is nowreached. The dynamic aperture reaches 12σ at 10 5turns in LHC version 6. This should give at least therequired 6σ for the actual machine.2.2 Variation in Time of the MultipolesThe movie on field quality showed convincingly the largevariation in time and with B˙ of the multipoles (up to 30%).The perturbation of the beam dynamics is such as tocause a beam loss. The cures are as follows:• magnetic models based on bench measurements andphysics of the magnets, predicting the magnetic fieldgiven the current, its variation and the magnetic his-tory. The predictability is now about 80%,• reference magnets with on-line multipole measure-ment,• a slow start of the ramp,• feedback from the beam if the reproducibility is insuf-ficient.2.3 Variations with the Magnetic HistoryThe strength of the multipoles, the amplitude of the decayand snap-back change drastically with the magnetic history.This is minimized as follows:• a one-hour degauss cycle,• strict operational procedures,• logging of the magnetic history and use of the mag-netic model.2.4 Mechanical ApertureThe mechanical aperture is about 9σ without collimationand 6σ with the collimators in place. These figures in-clude rather tight tolerances on the imperfections (align-ment, closed orbit,. . . ) A special effort is being carried outto meet the tolerances and maximize the space available tothe beam. The constraint of a sufficient mechanical aper-ture is included at the optics design stage, i.e. as a matchingconstraint. The LHC aperture is indeed smaller then that ofLEP but larger than in HERA.13Chamonix X2.5 Sensitivity to Beam LossesDepending on the circumstances (energy, time constant ofthe loss, . . . ), a quench may be produced by 0.001 to lessthan 1 ppm of the LHC beam intensity. To face this chal-lenge, the following solutions have been adopted:• pilot pulses to set up the machine, with an intensitysuch that quenches will be unlikely,• two very complete collimator sets (betatron, momen-tum) with a safety margin of 100, requiring tight con-trol of the local orbit (a small fraction of σ=0.3mm attop energy).The collimators should be the first aperture restrictions.They will be equipped with loss detectors to help protectingthe machine.3 COLLECTIVE EFFECTSIn this field, LHC is not notably different from other ma-chines. The dynamics was studied already long ago. Itis not expected to limit the LHC performance. A new ef-fect (for LHC) was however identified recently: the build-up of an electron cloud. It is reported in a dedicatedsession. With the gradual design and construction of thebeam environment, the impedances which were estimatedneed now to be calculated/measured for the actual struc-tures. Presently, the longitudinal impedance is well belowthe threshold above which instabilities would occur. Thedecision was made not to build a feedback. The trans-verse impedance however is expected to cause several sin-gle bunch and coupled-bunch head-tail modes to becomeunstable. Great care is required to keep it within tolerance(Cu plating, . . . ) The transverse instabilities are dampedby a transverse feedback for the dipole mode and by twofamilies of octupoles for the higher-order modes.An interesting issue concerning the shielding propertiesof thin resistive layers was recently confirmed. An exper-iment in EPA showed that a ceramic chamber coated witha layer much thinner than a skin depth did indeed provideefficient shielding provided care is taken to avoid any lowimpedance by-pass. These shielding properties will actu-ally by used for the LHC dump kickers and possibly by theLHC experiments.4 BEAM-BEAM ISSUESThe beams crossing at an angle, two qualitatively differentphenomena should be considered: head-on and long-rangeinteractions, further complicated by the PACMAN effect.4.1 Head-on InteractionApart from the crossing angle, the parameters of the head-on collisions are those of the Spp¯S, with a total beam-beamtune shift less than 0.01. They allow reaching the nominalluminosity of 1034cm−2s−1.4.2 Long-Range InteractionsThere are 120 of these interactions, where the beam sepa-ration is mostly in the range of 7 to 13σ. They produce lin-ear effects, like closed orbit distortions and tune shifts andmore subtle non-linear effects. The latest findings seem toshow that they limit the stability of the motion at large am-plitude, while the head-on interaction does not give rise toan instability.To minimize the linear tune shift, the plane of crossingis horizontal in one point and vertical in the other. Thecrossing angle may still be increased by 20% if it wouldbecome necessary.4.3 PACMAN bunchesDue to the kicker gaps in the bunch train, some bunchesdo not experience the same number of long-range interac-tions as the nominal bunches. This gives a spread in orbits,tunes,. . .The linear tune spread is cancelled by alternating thecrossing angles. The orbit spread gives rise to a beam sep-aration of about 0.2σ, at the limit of significance. A furtherincrease of the crossing angle decreases as well the ‘PAC-MAN’ spread.4.4 Coherent Beam-Beam OscillationsAt nominal performance, the π-mode is shifted away fromthe incoherent beam-beam frequencies and becomes poten-tially unstable. This may easily be avoided by selectingdifferent betatron tunes for the two rings: one ring couldbe operated on the nominal tune (close to 3rd-order reso-nances) while the other could be tuned close to 4th-orderresonances.5 CONCLUSIONTo reach a very high performance level, the LHC beam dy-namics is involved. Solutions(s) have been devised to reachthe nominal performance in a reliable way. The margins arenot as large as in other machines and require a continuousfollow-up of the actual hardware parameters. Some flexi-bility is implemented in the machine (mainly in its optics)to face new requirements. The beam observation and con-trol systems will be important issues for an accurate controlof the machine and beam parameters.Chamonix X14

Summary of session 5: from LEP to LHC, some important issues

Abstract

Similar works

Full text

Available Versions

CERN Document Server