Report from the Luminosity Working Group of theInternational Linear Collider Technical Review

Committee (ILC-TRC)Chairman: Greg Loew

• The ILC-TRC was originally constituted in 1994 andproduced a report in 1995 comparing the status of LC

designs at that time• Charged by ICFA in 2001 to again review the four LC

designs: TESLA, JLC-C, JLC-X/NLC, CLIC• The design assessments were carried out by three working

groups:–energy/technology performance

–luminosity performance–reliability and operability

ILC-TRC Luminosity performance working group:membership

• R. Assmann, CERN• W. Decking, DESY• G. Dugan, Cornell (Chair)• J. Gareyte, CERN• W. Kozanecki, CEA/Saclay• K. Kubo, KEK• N. Phinney, SLAC;

• J. Rogers, Cornell• A. Seryi, SLAC• R. Settles, MPI• D. Schulte, CERN• P. Tenenbaum, SLAC• N. Walker, DESY• A. Wolski, LBNL

ILC-TRC Luminosity performance working group:charge

This group will play a role similar to the former Beam Dynamics group, butwill broaden its scope to analyze all those factors which affect the ultimateluminosity performance (both peak and integrated) of all four machines,including but not limited to, emittance dilution, beam jitter, tunability, andreliability. It will look at all phenomena which can reduce the luminosity ateach machine sub-system, so as to predict the final emittances and luminosityreachable at the interaction point. Wherever possible, the members of thisgroup (including a few detector representatives) should set commonstandards and use common computer codes to predict emittances, jitters, etc.Calculations should take into account mechanical and electrical tolerances,ground motions at various sites, etc. The standards and assumptions shouldbe clearly spelled out.

ILC-TRC Luminosity performance working group:Assessment methodology

• Sources subgroup (electron and positron injection systems, up to thedamping rings): W. Decking, leader

• Damping rings subgroup: J. Rogers, leader• Low emittance transport subgroup (from damping ring to IP: bunch

compressors, main linac, beam delivery systems, beam-beam collisions):D. Schulte, P. Tenenbaum, co-leaders

• Machine-detector interface subgroup (collimation and backgroundsuppression, spent beam extraction, detector/machine interaction at IP,machine-related and beam-beam detector backgrounds): W. Kozanecki,leader

Starting in Dec. 2001, through Dec. 2002, the subgroups carried out theirwork, meeting roughly monthly via audio-conference, with in-personmeetings at SLAC, CERN, Paris, and at DESY.

ILC-TRC Luminosity performance working group:Assessment methodology

• The subgroups reviewed the current designs for all four machines, asoutlined in the machine descriptions, which form a chapter in the ILC-TRC report.

• The major issues affecting the luminosity at the reference energy (500GeV c.m.) were assessed by the subgroups, based on their experienceand on calculations and simulations carried out during the reviewprocess.

• Luminosity issues at the upgrade energy were also briefly considered.• An overall assessment of the feasibility of the system was provided,

together with a prioritized list of concerns, and a ranked list ofrecommended R&D items, reflecting those concerns.

Sources• The technology and luminosity issues for the sources are closely linked,

and so the assessment in this case was performed by a jointtechnology/luminosity subgroup.

• For the electron sources, the similarity to the SLC polarized source,plus ongoing R&D done to increase the cathode charge limit, hasdemonstrated the basic feasibility of the system. Additional R&D isrequired for the laser systems.

• The conventional positron sources (JLC/NLC and CLIC) should beable to operate within the target fracture limit demonstrated at the SLC,albeit with some significant complications (multiple target andcollection systems).

• The undulator-based positron source (TESLA) appears feasible fromthe technical point of view, but the need for the high-energy electronbeam to produce the positrons introduces significant operationalcomplexity, and makes high-luminosity collider operation at c.m.energies below 300 GeV quite difficult.

Damping ringsOverall feasibility assessment

• Overall, the TESLA and NLC/JLC damping ring designs are welladvanced. The required emittance performance is not far from theperformance achieved by the ATF.

• The committee examined a number of potential problems. Many ofthese were found to have satisfactory solutions or negligible impact:– Correction of the emittance due to ground motion– Coupled bunch instabilities due to wake fields– Space charge– Intrabeam scattering– Beam jitter due to ground motion and vibration, and beam loading– Circumference variations– Touschek lifetime– Polarization preservation.

Damping ringsConcerns-in priority order

• Electron cloud instability (TESLA and JLC/NLC positrondamping rings)

• Fast ion instability (TESLA and JLC/NLC electron dampingrings )

• Particle loss (TESLA positron damping ring)• Broadband impedance budget (TESLA DR and JLC/NLC

MDR)• Equilibrium emittance (TESLA and JLC-NLC rings)• Average pressure and ion-clearing gap (TESLA electron

damping ring)

Damping ringsElectron cloud instability

Average electron cloud density vs. secondaryElectron yield (simplified chamber geometries)

Low emittance transport (LET)Overall feasibility assessment

• The consensus of the working group is that from the point of view ofluminosity performance, the feasibility of each LET design has beenestablished.

• The basic, error-free designs of each LET are in a mature state. Thesimulation codes used to assess the performance of the LET have beenchecked and carefully cross-compared for the case of the error-freedesigns, and in general agreement between the codes is good.

• Simulations of main linac alignment and tuning were performed onmultiple simulation codes. The results agree at the level ofapproximately a factor of two in the worst case. The linac simulationsshow performance that is consistent with achieving the luminosity goalsof the different designs, although much remains to be done.

Low emittance transport:Overall feasibility assessment

• A site with ground motion comparable to models A or B wouldbe an acceptable situation for all LET designs.

• All LET designs, regardless of ground motion conditions, willrequire some form of IP collision feedback based upon thebeam-beam interaction.

• In the presence of significant detector noise which is coupledto the final doublet magnets, active stabilization of the finaldoublets is essential for JLC-X/NLC and for CLIC.

• Train-to-train luminosity fluctuations due to element motionmay be on the order of 10% or more.

Low emittance transportGround motion models for dynamic misalignments

A: “quiet” (LEP tunnel, some Ca sites)B: “intermediate” (SLAC tunnel, Aurora mine (Fermilab))

C: “Noisy” (HERA tunnel)

Low emittance transportEffect of dynamic misalignments on luminosity

Low emittance transportEffect of dynamic misalignments on luminosity

Low emittance transportConcerns-in order of priority

• All of the LET designs rely on beam instrumentationperformance that meets or exceeds the state of the art. (Allprojects)• The simulations of static tuning are not yet complete. (Allprojects)• A more complete evaluation of in-tunnel noise sources isrequired. (All projects)• Collimator wakefields (All projects)• Final doublet stability (JLC-X/NLC, CLIC)• Beam-beam instability (TESLA).• Parasitic collisions (JLC-X).• Long-range wakefields (All projects)• CLIC bunch compressor design

Low emittance transportCollimator wakefields

Low emittance transport: Luminosity vs. correlated emittance growth (TESLA).

Machine-detector interface:Overall feasibility assessment

• The feasibility of the proposed designs at the baseline c.m. energy of500~GeV is on solid ground.– The beam-beam simulations that drive the IR geometry and the

background suppression are well understood.– The layout of the TESLA and NLC IRs, as well as the conceptual (and in

some cases, the engineering) design of their crucial components are quitemature.

– The collimation and machine-protection concepts have been at leastpartially validated in simulation and/or in actual prototypes.

– Extensive background-remediation studies result in predicted levels thatshould be easily manageable in TESLA (thanks to the large bunchspacing); for some of the sub-detectors at warm machines, thebackground levels per train deserve attention.

Machine-detector interfaceConcerns-in order of priority

• IR and extraction-line layout. (TESLA)• The head-on collision scheme has been adopted for TESLA because of someinherent advantages: simpler IR geometry, looser envelope requirements forthe FD quadrupoles, no need for crab-crossing cavities.• However, this choice leads to unavoidable compromises in the sharedsection of the beam line, resulting in- marginal stay-clear for the outgoing charged and photon beams;- potentially degraded SR masking;-overall extraction-line radiation levels significantly larger than in JLC-X/NLC,-a serious engineering challenge for the design and protection of severalcrucial extraction-line components..

• Final doublet (FD) stabilization (JLC/NLC and CLIC).• Extraction-line design. (CLIC).• IP stability. (TESLA)

Machine-detector interface:TESLA spent-beam extraction

Conclusion• The ILC-TRC report is about to be published, with about 450

pages of text, summarizing and assessing the designs of thelinear colliders (LC).

• The luminosity performance chapter is about 120 pages long,and contains the results of considerable work assessing the LCdesign feasibility, identifying concerns, and defining futureR&D work.

• It is hoped that many of the collaborative relationshipsdeveloped during the TRC study (e.g., on integratedsimulations, on collimation system performance) will persistinto the future, leading to the formation of integrated designteams to study the outstanding issues still before us.