Report from the Luminosity Working Group of the International Linear Collider Technical Review Committee (ILC-TRC) Chairman: Greg Loew

Transcription

1 Report from the Luminosity Working Group of the International Linear Collider Technical Review Committee (ILC-TRC) Chairman: Greg Loew The ILC-TRC was originally constituted in 1994 and produced 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

2 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

3 ILC-TRC Luminosity performance working group: charge This group will play a role similar to the former Beam Dynamics group, but will broaden its scope to analyze all those factors which affect the ultimate luminosity performance (both peak and integrated) of all four machines, including but not limited to, emittance dilution, beam jitter, tunability, and reliability. It will look at all phenomena which can reduce the luminosity at each machine sub-system, so as to predict the final emittances and luminosity reachable at the interaction point. Wherever possible, the members of this group (including a few detector representatives) should set common standards 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 should be clearly spelled out.

4 ILC-TRC Luminosity performance working group: Assessment methodology Sources subgroup (electron and positron injection systems, up to the damping 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 background suppression, 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 their work, meeting roughly monthly via audio-conference, with in-person meetings at SLAC, CERN, Paris, and at DESY.

5 ILC-TRC Luminosity performance working group: Assessment methodology The subgroups reviewed the current designs for all four machines, as outlined in the machine descriptions, which form a chapter in the ILC- TRC report. The major issues affecting the luminosity at the reference energy (500 GeV c.m.) were assessed by the subgroups, based on their experience and on calculations and simulations carried out during the review process. 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 of recommended R&D items, reflecting those concerns.

6 Sources The technology and luminosity issues for the sources are closely linked, and so the assessment in this case was performed by a joint technology/luminosity subgroup. For the electron sources, the similarity to the SLC polarized source, plus ongoing R&D done to increase the cathode charge limit, has demonstrated the basic feasibility of the system. Additional R&D is required for the laser systems. The conventional positron sources (JLC/NLC and CLIC) should be able to operate within the target fracture limit demonstrated at the SLC, albeit with some significant complications (multiple target and collection systems). The undulator-based positron source (TESLA) appears feasible from the technical point of view, but the need for the high-energy electron beam to produce the positrons introduces significant operational complexity, and makes high-luminosity collider operation at c.m. energies below 300 GeV quite difficult.

7 Damping rings Overall feasibility assessment Overall, the TESLA and NLC/JLC damping ring designs are well advanced. The required emittance performance is not far from the performance achieved by the ATF. The committee examined a number of potential problems. Many of these 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.

8 Damping rings Concerns-in priority order Electron cloud instability (TESLA and JLC/NLC positron damping rings) Fast ion instability (TESLA and JLC/NLC electron damping rings ) 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)

9 Damping rings Electron cloud instability Average electron cloud density vs. secondary Electron yield (simplified chamber geometries)

10 Low emittance transport (LET) Overall feasibility assessment The consensus of the working group is that from the point of view of luminosity performance, the feasibility of each LET design has been established. The basic, error-free designs of each LET are in a mature state. The simulation codes used to assess the performance of the LET have been checked and carefully cross-compared for the case of the error-free designs, and in general agreement between the codes is good. Simulations of main linac alignment and tuning were performed on multiple simulation codes. The results agree at the level of approximately a factor of two in the worst case. The linac simulations show performance that is consistent with achieving the luminosity goals of the different designs, although much remains to be done.

11 Low emittance transport: Overall feasibility assessment A site with ground motion comparable to models A or B would be an acceptable situation for all LET designs. All LET designs, regardless of ground motion conditions, will require some form of IP collision feedback based upon the beam-beam interaction. In the presence of significant detector noise which is coupled to the final doublet magnets, active stabilization of the final doublets is essential for JLC-X/NLC and for CLIC. Train-to-train luminosity fluctuations due to element motion may be on the order of 10% or more.

12 Low emittance transport Ground motion models for dynamic misalignments A: quiet (LEP tunnel, some Ca sites) B: intermediate (SLAC tunnel, Aurora mine (Fermilab)) C: Noisy (HERA tunnel)

13 Low emittance transport Effect of dynamic misalignments on luminosity

14 Low emittance transport Effect of dynamic misalignments on luminosity

15 Low emittance transport Concerns-in order of priority All of the LET designs rely on beam instrumentation performance that meets or exceeds the state of the art. (All projects) The simulations of static tuning are not yet complete. (All projects) A more complete evaluation of in-tunnel noise sources is required. (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

16 Low emittance transport Collimator wakefields

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

18 Machine-detector interface: Overall feasibility assessment The feasibility of the proposed designs at the baseline c.m. energy of 500~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 quite mature. The collimation and machine-protection concepts have been at least partially validated in simulation and/or in actual prototypes. Extensive background-remediation studies result in predicted levels that should be easily manageable in TESLA (thanks to the large bunch spacing); for some of the sub-detectors at warm machines, the background levels per train deserve attention.

19 Machine-detector interface Concerns-in order of priority IR and extraction-line layout. (TESLA) The head-on collision scheme has been adopted for TESLA because of some inherent advantages: simpler IR geometry, looser envelope requirements for the FD quadrupoles, no need for crab-crossing cavities. However, this choice leads to unavoidable compromises in the shared section 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 several crucial extraction-line components.. Final doublet (FD) stabilization (JLC/NLC and CLIC). Extraction-line design. (CLIC). IP stability. (TESLA)

20 Machine-detector interface: TESLA spent-beam extraction

21 Conclusion The ILC-TRC report is about to be published, with about 450 pages of text, summarizing and assessing the designs of the linear colliders (LC). The luminosity performance chapter is about 120 pages long, and contains the results of considerable work assessing the LC design feasibility, identifying concerns, and defining future R&D work. It is hoped that many of the collaborative relationships developed during the TRC study (e.g., on integrated simulations, on collimation system performance) will persist into the future, leading to the formation of integrated design teams to study the outstanding issues still before us.