The CARE-HHH-APD workshop LHC-LUMI-05 on Scenarios for the LHC Luminosity Upgrade was held in Arcidosso, Italy, from August 31st to September 3rd, 2005. The workshop was organized in four plenary morning sessions supported by afternoon sessions of two parallel working groups on LHC IR Upgrade and High Energy Injectors. In this report we review the presentations and discussions in Session 2, devoted to High-Intensity Effects, emphasizing the suggestions for future studies and pointing out the open issues

Ruggiero, F

Rumolo, Giovanni

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SUMMARY OF SESSION 2: HIGH INTENSITY EFFECTS  F. Ruggiero and G. Rumolo, CERN, Geneva, SwitzerlandAbstractThe CARE-HHH-APD workshop LHC-LUMI-05 on“Scenarios for the LHC Luminosity Upgrade” was held inArcidosso, Italy, from August 31st to September 3rd, 2005.The workshop was organized in four plenary morning ses-sions supported by afternoon sessions of two parallel work-ing groups on LHC IR Upgrade and High Energy Injectors.In this report we review the presentations and discussionsin Session 2, devoted to High-Intensity Effects, emphasiz-ing the suggestions for future studies and pointing out theopen issues.INTRODUCTIONThere were four presentations on High-Intensity Ef-fects in the LHC and its injectors, addressing theneed/advantages of upgraded high energy injectors basedon fast super-conducting synchrotrons and including a pos-sible new booster in the ISR or SPS tunnel and a 1 TeVsuper-SPS with new transfer lines to the LHC: Frank Zimmermann–Progress of beam-beam com-pensation schemes [1], Elena Shaposhnikova–High brilliance and closerbunches from the LHC injectors [2], Joachim Tuckmantel–New RF systems for the Super-SPS and Super-ISR [3], Nuria Catalan–Beam collimation and control in thehigh energy injectors [4].The presentation by Elena Shaposhnikova included a dis-cussion of the RF upgrades and a tentative estimate of therelated cost for different LHC bunch spacings. This wasfurther discussed in the working group on High Energy In-jectors and later at CERN: the conclusions are reported ina separate section.PROGRESS OF BEAM-BEAMCOMPENSATION SCHEMESDifferent approaches have been proposed to boost theLHC performance: Increase crossing angle and reduce bunch length tostay at the beam-beam limit. This requires a higher-frequency bunch shortening RF system and reducedlongitudinal emittance. Reduce crossing angle and apply ‘wire’ compensa-tion.Work supported by the European Community-Research Infrastruc-ture Activity under the FP6 ”Structuring the European Research Area”programme (CARE, contract number RII3-CT-2003-506395) Crab cavities have the potential to go to large crossingangles without luminosity loss. Collide long intense bunches with large crossing an-gle, without exceeding the beam-beam limit.The two promising schemes of wire compensation and crabcrossing were discussed by Frank Zimmermann.Long-range wire compensationFrank Zimmermann presented the merits of the wirecompensation, underlining that long-range compensationwas demonstrated in the SPS using two wires (beam life-time recovery), simulations predict 1-2  gain in dynamicaperture for nominal LHC, and wire compensation allowskeeping the same–or smaller–crossing angle for higherbeam current with no additional geometric luminosity loss.Challenges and plans include further SPS experiments (3rdwire in 2007), a demonstration of the wire compensationeffectiveness with real colliding beams (at RHIC), and thestudy of options for a pulsed wire.Simulations of long-range compensation with two wiresindicate that the beam lifetime is recovered over a widetune range but not for all tunes. The measured SPS lifetimeat 26 GeV can be fitted by the expressionmsec 	 ,where  is the beam-wire separation and  the transverser.m.s. beam size. A naive extrapolation to the LHC beam-beam separation of about  would yield a beam lifetimeof only 6 minutes! This estimate is pessimistic, since theSPS beam lifetime was very short as a consequence of sev-eral noise sources. Tevatron observations with an electronlens show a cubic dependence on the ratio  . FurtherSPS tests at different energy are needed to validate the en-ergy scaling.Beam lifetimes predicted by simulation codes are muchlarger than those observed, even though the sensitivity tomachine parameters seems correct. Further understandingand beam tests are needed, e.g. at RHIC.For extreme PACMAN bunches there is over-compensation which causes the footprint to flip overor to increase instead of shrinking. To avoid degradedlifetime for PACMAN bunches, the wire should be pulsedtrain by train. It is rather challenging to make a pulsed wirefor beam-beam compensation: the required average pulserate is 439 kHz and the turn-by-turn amplitude stabilityis ﬁﬀ .Experiments at RHIC (by Wolfram Fischer et al.) with asingle long-range encounter show that the beam-beam ef-fect is visible starting from a separation of about , con-sistent with Tevatron and Daphne observations, but con-trary to LHC simulations and possibly to earlier observa-tions at the SPS collider.LHC-LUMI-05 PROCEEDINGS135Crab cavitiesFrank Zimmermann presented the merits of crab cavi-ties, underlining that a practical demonstration will be pos-sible at KEKB in early 2006, that crab cavities can avoidthe geometric luminosity loss for large crossing angles (nolong-range beam-beam effect) and have the potential ofboosting the beam-beam tune shift by a factor 2-3, as pre-dicted for KEKB. Challenges and proposed plans includethe design and prototyping of a Super-LHC crab cavity(Cornell is interested) and the demonstration that noise-induced emittance growth is acceptable for hadron colliders(possible installation and experiment at RHIC).For crossing angles above 1 mrad, the required Crabcavity RF voltage is significantly lower then for a bunchshortening RF system. The Crab cavity impedance can becontrolled by coaxial couplers or waveguide dampers andby a proper elliptical cavity shape, to shift the frequencyof unwanted modes: the instability rise time for a singleCrab cavity corresponds approximately to the rise time in-duced by 10 normal RF cavities with the same total voltage.Moreover, the phase or timing tolerances for Super-LHCcrab cavities is about an order of magnitude lower than forKEKB and even tighter than for the ILC.HIGH BRILLIANCE AND CLOSERBUNCHES FROM THE LHC INJECTORSElena Shaposhnikova presented an analysis of the beamperformance with the existing LHC injectors and with pos-sible new high energy injectors. The main conclusion isthat the SPS is the bottle-neck to reach and exceed ultimateLHC intensity. The target impedance for a Super-SPS isabout ﬃﬂ, i.e., about an order of magnitude less than thepresent SPS.Existing LHC injectorsThe main limitation to reach higher brilliance comesfrom the SPS, where the nominal LHC emittance is notyet reached in the vertical plane owing to electron cloudeffects. The limitations associated to closer bunches are anew RF system needed either in the PS (10 or 15 ns) or SPSand LHC (12.5 ns). In the SPS more problems are antici-pated with electron cloud (vertical emittance blow-up) andcoupled bunch instabilities.Intensity dependent capture losses in the SPS were re-duced in 2004, but their exact cause and therefore scalingis not clear. Coupled bunch instabilities in the SPS canbe cured by controlled emittance blow-up. This requiresa 200 MHz capture system in the LHC. Beam loading inthe 200 MHz and 800 MHz RF systems is a limit at ulti-mate intensity for known performance. A fast transverseinstability is anticipated for more MKE kickers or higherbunch intensities. Below this TMCI-like threshold therecan still be emittance blow-up. Curing the TMCI instabil-ity by a large chromaticity at high RF voltage may increasethe beam losses.Possible improvements and machine studies include afurther SPS impedance reduction, in particular for theMKE kickers, capture loss and beam lifetime SPS stud-ies possibly with shorter bunches from the PS, buncheswith ultimate intensity injected into the SPS using PS batchcompression, and electron cloud ‘scrubbing runs’ at higherintensities.Future LHC injectorsThe present SPS could be used as a booster ring injectingat 150 GeV into a new Super-SPS (to be named Hyper-PSor HPS) located in the same tunnel and reaching 1 TeV. Re-ducing the top energy in the SPS to 150 GeV allows theramp length to be reduced to 2 s, but does not improvethe coupled-bunch longitudinal beam stability on the flattop (i.e., a controlled blow-up of the longitudinal emittancemay still be necessary) and makes more difficult bunch-to-bucket transfer into the 400 MHz RF system of the nextring.With the present SPS and pre-injectors, the minimumramp length of the HPS can be 6 s. Using a 400 MHzsuper-conducting RF system requires an extra capture RFsystem and twice more voltage than with the 200 MHzsystem. Using a 200 MHz normal conducting RF systemseems to be optimal, but requires a tight impedance budgetof  !#"$ﬃﬂprobably achievable for a new machine.RF UPGRADES FOR DIFFERENT LHCBUNCH SPACINGSElectron cloud effects may exclude the possibility of re-ducing the LHC bunch spacing below its nominal value.Any bunch spacing shorter than 25 ns requires new elec-tronics for the LHC BPM system, upgraded feedback sys-tems, RF couplers, collimation, cryogenics, beam dump,etc. A tentative cost estimate for the difference between thetwo options of 12.5 ns and 10-15 ns bunch spacing is shownin Table 1 and amounts to about 30 MSfr. This does not in-clude accelerator equipment upgrades common to the twooptions and should be compared to the corresponding costdifference estimate from the LHC experiments.There are two possibilities for a reduced bunch spacingof 12.5 ns: A new low-frequency RF system in the SPS and acapture RF system at the same frequency (i.e., 160 or240 MHz) in the LHC. This is the option reported inTable 1. A new high-frequency main RF system (320 or400 MHz) and a capture RF system (160 MHz) in theSPS. No capture RF system in the LHC.The total cost would be about the same, but it could bedistributed differently between SPS and LHC.LHC-LUMI-05 PROCEEDINGS136Table 1: Additional RF equipment needed in the LHC and its present injectors and tentative cost estimate for the differencebetween the two options of 12.5 ns and 10-15 ns bunch spacing.12.5 ns bunch spacingPS: double RF voltage at 80 MHz %'& MSfrSPS: more RF power and new RF cavities at 160 or 240 MHz %)(MSfrLHC: new capture cavities at 160 or 240 MHz ( &*+( MV) %,&MSfrTOTAL (12.5 ns) % 62 MSfr10 or 15 ns bunch spacingPS: new RF system at 60 MHz %MSfrSPS: double RF power at 200 MHz %-&. MSfrLHC: more RF power at 400 MHz %/0 MSfrTOTAL (10 or 15 ns) % 35 MSfrNEW RF SYSTEMS FOR THE SUPER-SPSAND SUPER-ISRJoachim Tuckmantel presented a comprehensive reviewof the RF system requirements necessary to fulfil the de-sired specifications for the new high-energy LHC injectorsand the consequent technical implications. In particular hediscussed advantages and disadvantages of travelling wavevs standing wave cavities, comparing normal conductingand super-conducting options.He remarked that just a few cavities, either copper orsuper-conducting, can easily supply the desired voltage.The gradients have to be lowered voluntarily, since thepower coupler cannot transmit the corresponding RF powerto accelerate high beam currents and compensate reactivebeam loading. Therefore power coupler capabilities have tobe increased considerably by a vigorous R&D effort. Forsuper-conducting cavity couplers, RF losses into liquid Hemay lead to ‘de-conditioning’.The existing RF power sources for a 200 MHz system arevery space consuming and this creates a problem to housethem close to cavities underground to minimize RF feed-back delays. There is also a need to study compact RFpower transmitters at 200 MHz.In conclusion: RF power generators should be located in the tunnelto reduce feedback delays, High power transmitters at or below 200 MHz aredrastically different from higher frequency technol-ogy (klystrons). Also expertise in super-conductingRF is very different from that in travelling or stand-ing wave cavities. There are good reasons to adoptdifferent technologies for different synchrotrons, e.g.super-conducting cavities have very limited tunabil-ity and are not suited for lower energy injectors, how-ever there is also a strong incentive to adopt similartechnologies to reduce development and maintainancecosts (P+M). This may guide the design of the futureLHC injector complex.BEAM COLLIMATION AND CONTROLIN THE HIGH ENERGY INJECTORSNuria Catalan presented some ideas on possible colli-mation systems for a double ring in the SPS tunnel (a lowenergy ring with conventional magnet technology to accel-erate protons from 26 to 150 GeV, SPS, and a second highenergy ring using superconducting magnets to reach 1 TeV,SSPS). The level of tolerable losses is given by three mainlimitations: Heat load (global, superconducting). Not more than3 W/m uniformly distributed over the ring circumfer-ence Quench levels (local, superconducting). Maximumbeam power deposited in one spot before the magnetsquench must be in the range 10-50 W/m. Activation and maintenance (local, general). Local-ized loss power must not exceed 1-10 W/m.Most of the losses occur at injection, transition crossingand extraction. Identification of the loss mechanisms is akey ingredient for an efficient collimator system design be-cause the collimation efficiency depends on the number ofparticles at the large betatron amplitudes (halo population)and on the diffusion rate of the particles toward this halo.The worst case scenario consists in considering slow con-tinuous losses.The type of collimation system depends on the energy ofthe ring. The amount of power associated to the beamclearly indicates that a collimation system is needed for thehigh energy ring, whereas for the low energy ring it wouldonly be necessary to avoid excessive activation or at extrac-tion. At 25 GeV, the energy loss per impact is sufficient tokick the halo particles out of their buckets for almostany size and material collimators. Besides, particlesget significant angular kicks and have a weak nuclearabsorption. Collimation can be very efficient withinone to few turns. At 1 TeV the energy loss and the angular kick per im-pact are very small for practical size jaws, even con-sidering the heaviest elements. Therefore collimationLHC-LUMI-05 PROCEEDINGS137would definitely be a multi-turn process. The nuclearabsorption can be very close to 100% even for smallcollimator sizes, but it must be kept below a certainlimit due to heating and out-scattering (i.e., particlesthat leave the jaw before the full traversal, their num-ber decreases with increasing energy). The use of pri-mary short collimators in light material could mini-mize nuclear absorption while maximizing the angu-lar scattering. At 150 GeV the momentum loss and the angular kickper impact are significant but cannot kick the particlesout in one passage. Multi-turn cleaning would thenrequire a high momentum acceptance of the machinein order not to randomly lose the particles before theyintercept the jaws.At all energies, particles outscattered from the jaws have tobe intercepted by secondary collimators (absorbers) placeddownstream. To have a good collimation efficiency, at thelow energies the secondary collimators must be put at theright phase advance to absorb the halo particles within oneor few turns, whereas at the intermediate and high energyranges the halo particles may have to circulate many turnsin the machine before they hit the absorber, which poses arequirement on the machine acceptance.Finally, one could think of a collimator system for the dou-ble ring that is made of: Momentum cleaning at low energy in the SPS. It re-quires a heavy material scraper and local collimatorsplaced at the right phase advance. Protection absorbers in the transfer line. Both momentum and betatron cleaning at injectionand extraction energy in the SSPS. If possible onecould use the same secondary collimators but differentprimaries for different energies (this would require aspecial optics design).REFERENCES[1] F. Zimmermann, these proceedings.[2] E. Shaposhnikova, these proceedings.[3] J. Tuckmantel, these proceedings.[4] N. Catalan, these proceedings.LHC-LUMI-05 PROCEEDINGS138

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