Towards the International Linear Collider (ILC)

1. Stony Brook seminar P. Grannis Oct. 5, 2004 Towards the International Linear Collider (ILC) • There has been much progress in the past few years • In formulating the physics case • In specifying the accelerator technology • In establishing an organizational model • In initiating contacts with government funding agencies The Linear Collider world is a maze of impenetrable acronyms. If progress is measured in silly names, the LC is in good shape!

2. A Little History A Possible Apparatus for Electron-Clashing Experiments (*). M. Tigner Laboratory of Nuclear Studies. Cornell University - Ithaca, N.Y. M. Tigner, Nuovo Cimento 37 (1965) 1228 “While the storage ring concept for providing clashing-beam experiments (1) is very elegant in concept it seems worth-while at the present juncture to investigate other methods which, while less elegant and superficially more complex may prove more tractable.” 2/60

3. 2001: The Snowmass Workshop participants produced the statement recommending construction of a Linear Collider to overlap LHC running. 2001:HEPAP, ECFA, ACFA all issued reports endorsing the LC as the next major world project, to be international from the start 2002: The Consultative Group on High-Energy Physics of the OECD Global Science Forum executive summary stated as the first of its Principal Conclusions: “The Consultative Group concurs with the world-wide consensus of the scientific community that a high-energy electron-positron collider is the next facility on the Road Map. “There should be a significant period of concurrent running of the LHC and the LC, requiring the LC to start operating before 2015. Given the long lead times for decision-making and for construction, consultations among interested countries should begin at a suitably-chosen time in the near future.” 3/60

4. Understanding Matter, Energy, Space and Time: The Case for the Linear Collider A summary of the scientific case for the e+ e- Linear Collider, representing a broad consensus of the particle physics community. April 2003: “Consensus document” signed now by ~2700 physicists worldwide.: http://sbhepnt.physics.sunysb.edu/~grannis/ilcsc/lc_consensus.pdf ) (To join this list, go to http://blueox.uoregon.edu/~lc/wwstudy/ ) 4/60

5. Fall 2002: ICFA created the International Linear Collider Steering Committee (ILCSC) to guide the process for building a Linear Collider. Asia, Europe and North America each formed their own regional Steering Groups (Jonathan Dorfan chairs the North America steering group). International Linear Collider Steering Committee Maury Tigner, chair Physics and Detectors Subcommittee (AKA WWS) Jim Brau, David Miller, Hitoshi Yamamoto, co-chairs (est. 1998 by ICFA as free standing group) Accelerator Subcommittee Greg Loew, chair Parameters Subcommittee Rolf Heuer, chair (finished) Technology Recommendation Panel Barry Barish, chair (finished) Global Design Initiative organization Satoshi Ozaki, chair (finished) GDI central team site evaluation Ralph Eichler, chair GDI central team director search committee Paul Grannis, chair Comunications and Outreach Neil Calder et al 5/60

6. Technical Review Committee In Feb. 2001, ICFA charged a Technology Review Committee, chaired by Greg Loew of SLAC to review the critical R&D readiness issues. The TRC report in 2003 gave a series of R&D issues for L-band (superconducting rf TESLA), X-band (NLC and GLC), C-band and CLIC. The most important were the R1’s: those issues needing resolution for design feasibility. TRC R1 Issues: TESLA: Feasibility for 500 GeV operation had been demonstrated, but 800 GeV with gradient of 35 MV/m requires a full cryomodule (9 or 12 cavities) and shown to have acceptable quench and breakdown rates with acceptable dark currents. X-band: Demonstrate low group velocity accelerating structures with acceptable gradient, breakdown and trip rates, tuning manifolds and input couplers. Demonstrate the modulator, klystron, SLED-II pulse compressors at the full power required. • R1 issues pretty much satisfied by mid-2004 6/60

7. Main elements of physics program (1) The Higgs will almost surely be discovered by LHC (Tevatron), but it will remain to determine its properties – is it the SM Higgs, or more complicated. ILC (International Linear Collider) will measure mass, width, spin and parity; measure its couplings to all particles; measure self-couplings (determine Higgs potential). Susy couplings Accuracy of BR determination in 500 fb-1 approx. errors Susy Higgs couplings to fermions, WW (ZZ) differ from SM as Susy parameters change. Precision BR measurements→ new physics required; if Susy, BRs indicate MA SM value (decoupling limit) 7/60

8. Main elements of physics program (2) If SUSY exists, measure the masses, branching ratios, quantum numbers of the accessible particles. Precision is much greater at ILC than LHC. ILC can measure the gauginos, sleptons, sneutrinos – and the LSP. e.g. can measure neutralino, chargino mass and mixing matrices, including the CP phases. Is the origin of B-Bbar asymmetry in the universe in the Supersymmetric sector? Measuring masses, couplings/mixing matrices of sparticles allows us to extrapolate to high energy, probing the unification scale, and sorting out the origin of supersymmetry breaking. Quark/lepton mass evolution Gauge mass unification To get these measurements, have to see the relevant sparticles! Almost surely this will require more than 500 GeV collisions. 8/60

9. Linear collider Main elements of physics program (3) If not SUSY, search for new phenomena connected to strong coupling, large extra dimensions etc. Significance for 1.2 TeV technirho at LHC and LC at 500, 1000, 1500 GeV. (factor 6 gain in significance going from 500 → 1000 GeV.) significance Measure the number of extra dimensions Present 68% S,T limits Possible measurements at ILC Likely want to return to the Z pole to measure precision Z (and W) properties to further illuminate the new physics. 68% S,T limits at Giga Z at LC; location of precision ellipse gives model info. 9/60

10. Parameters for the Linear Collider Based on the physics goals in the consensus document, a group drew up parameters for the Linear Collider Parameters for the Linear Collider September 30, 2003 • Baseline machine: • Ecm continuously adjustable from 200 – 500 GeV • Luminosity and reliability to allow ∫Ldt = 500 fb-1 in 4 years following the initial year of commissioning • Ability to scan at any energy between 200 and 500 GeV; downtime to set up not to exceed 10% of actual data-taking time • Energy stability and precision below 0.1%; machine interface must allow energy, differential luminosity spectrum with that precision • Electron polarization of at least 80% • 2 intersection regions for experiments; one with crossing angle to enable gg collisions • Allow calibration at the Z, but with lower luminosity and emittance 10/60

11. Parameters for the Linear Collider (2) • Should be capable of Energy Upgrade: • Energy upgrade to approximately 1 TeV • Luminosity and reliability to allow 1 ab-1 in about 3-4 years • Capability for running at any energy up to maximum energy (assume L scales as √s ) • Beam energy and stability as for baseline machine • Should preserve Options beyond the Baseline: • Ability to double∫Ldtat 500 GeV to 1 ab-1 in two additional years • Ability to collide e-e-up to full energy • Positron polarization to, or above, 50% from 90 GeV to max. energy • Operation at Z pole with L ~ few x 1033 cm-2s-1 , with positron polarization • Operation at WW threshold with few x 1033 cm-2s-1 and dE/E ~ few 10-5 (not demonstrated) • Ability to collide photons of arbitrary polarization states at up to 80% of maximum energy, and 30-50% of e+e- luminosity 11/60

12. Linear Collider technologies, specific machine realizations, and the Technology Recommendation (ITRP) process (good introduction athttp://www.desy.de/~njwalker/uspas/ • rf bands: • L-band (TESLA) 1.3 GHz l = 3.7 cm • S-band (SLAC linac) 2.856 GHz 1.7 cm • C-band (JLC-C) 5.7 GHz 0.95 cm • X-band (NLC/GLC) 11.4 GHz 0.42 cm • (CLIC) 25-30 GHz 0.2 cm • Accelerating structure size is dictated by wavelength of the rf accelerating wave. Wakefields related to structure size; thus so also is the difficulty in controlling emittance growth and final luminosity. • Bunch spacing, train length related to rf frequency • Damping ring design depends on bunch length, hence frequency So the frequency dictates many of the design issues for LC 12/60

13. Effects of frequency on design Power lost in structure walls: dP/dz = - E02/Rs where Rs is the shunt impedence, E0 = peak E field; gradient dV/dz = E0/2 Rs ~ f1/2 for normal conducting surface: Rs ~ f-1 for superconducting; thus prefer high frequency for good efficiency for warm structures and low frequency for superconducting cavities. Rs also controls size of beam loading (loss of rf power as beam bunches along the train are accelerated), so limits the bunch train length. Q = 2p (stored energy)/(energy lost per cycle) = w ws/ dP/dz (ws=stored energy/unit length) Q ~ f-1/2 for normal conducting; Q ~ f-2 for superconducting. The Q controls the filling time – time to get the energy into the accelerating structure. Superconducting Q = 1010 ; Copper structures Q = 104 ; large Q → high effic. The inference is that for high Q, high gradient and low loss, superconducting wants relatively low frequency; normal conducting wants high frequency. 13/60

14. Any linear collider requires:EnergyLuminosity • Electron source • Positron production • Pre-injector accelerators • Damping rings • Bunch compressor • rf power source/delivery • Low level rf for rf control • Main linacs • Beam diagnostics: BPMs, movers • Final focus system at IP • Machine protection system 14/60

15. LC proposals and Technology Recommendation (ITRP) in 2004: C-band: proposed by a Japanese group (largely Univ. Tokyo) to minimize risks of X-band. The maximum conceived energy for C-band is 500 GeV, but an X-band follower could boost to 1 TeV. The difficulty in achieving klystrons, modulators, structures are alleviated. C-band being used for SPRING=8 (xFEL). View this as a ‘conservative’ fall back, but given readiness of X-band design, not considered in detail. CLIC: replace the klystron based power source with a low energy, high current drive beam. Beam gymnastics give pulse train compression. Special cavities couple the power out of the drive beam and deliver to the high energy accelerated beam. Collision energy of ~3 TeV thought feasible in ~30km site. CERN is building CLIC Test Facility 3 to come online ~2008. Many issues remain for proof of design, handling high field gradients, power transfer efficiency, cost, wakefield control, emittance preservation … Not considered as possible choice for baseline LC at this time. 15/60

16. X-band: NLC (SLAC) & GLC (KEK); joint R&D and essentially identical designs Operating gradient = 52 MV/m (65 MV/m unloaded); max. energy 1 TeV; can extend to ~1.3 TeV at reduced luminosity Site length 32 km (27.6km linacs); 2 tunnels (klystrons and linac are separate ) Main linac acceleration – about 106 Cu structures fed by 11.4 GHz rf; damping slots to remove higher order modes; each structure in a set of 60 tuned slightly differently to decohere wakefields. Bunch Dt = 1.4ns; #bunches/train = 192; train length = 267ns; rep. rate 120Hz Crossing angle 1st IP = 20 mrad; Site power (500 GeV) = 200 MW; effic. = 2.5% 16/60

17. Tesla: DESY led collaboration 35 MV/m maximum gradient, limited by quench limit; Design EMAX = 0.8 TeV for TESLA site Site length (0.8 TeV) = 33 km; linac length = 30km Single tunnel (klystrons on surface) 1.3 GHz superconducting 9-cell cavities in cryomodules containing 12 cavities. Bunch Dt = 337 ns; #bunches/train = 2820; train length = 950 ms Collisions can be exactly head on as trailing bunches are far away. Site power = 140 MW; effic. = 23% US study gave paper design of 1 TeV variant, with some different design choices. 17/60

18. Getting the energy: rf acceleration of particles 18/60

19. rf power generation and acceleration: • Modulator converts ac power to dc pulse for klystron • Klystron tube accelerates electrons through a set of bunching cavities; generate microwave pulse for accelerating electrons in output cavity (rf power amplifier) • For X-band, need to shorten the pulse train; use dual delay lines and rf switching in SLED-II to chop and fold rf pulse on itself • rf pulse injected into set of rf structures on the beam line (travelling or standing wave) for accelerating particles rf pulse compression needed for high power, short bunch X-band, not for SC L-band 19/60

20. Klystrons X-band PPM klystrons – small but need ~4000 with 75 MW pulse power; 1.6 ms pulse length; permanent magnet focussing to reduce power TESLA multibeam klystron Need 572; 10 MW pulse power; 1.4ms pulse length Klystron is larger, but simpler than X-band 20/60

21. rf pulse compression for X-band(not needed for L-band) Stack reflected power to give 4X klystron power in ¼ time interval. Systems are long, most expensive subsystem of main linac. 21/60

22. Ez c z The Linear Accelerator (LINAC) Travelling wave structure; need phase velocity = c Circular waveguide mode TM01 has vp> c No good for acceleration! Need to slow wave down (phase velocity) using irises. Bunch sees constant field Ez=E0cosf SC cavity Room temp Cu structure 22/60

23. X-band accelerating structures About 106 Cu structures, with different aperture to detune wakefields; 60 structures in a section mounted on a girder. Slots couple out higher order modes. Room temperature. Input power coupler is simple. Iris size = 0.45 cm rf breakdown observed in earlier structures has been cured (lower group velocity, better design, modified coupler) 23/60

24. TESLA rf cavities 9-cell cavities (ultra-pure grade Nb, electropolished, baked). Electropolishing gives much smoother surface than Buffered Chemical Polishing. 12 cavities in a single cryostat “cryomodule”, operated at high vacuum and 2K. Iris size 3.5 cm BCP EP Making cost effective, high gradient superconducting cavities, has been the primary challenge for the cold machine. 24/60

25. TESLA cryomodules 3 cryomodules (36 cavities) served by one klystron rf power input couplers are quite complex: transition from room temp. and atmosphere to 2K vacuum. Extreme care needed to protect Nb surfaces. 25/60

26. TESLA rf cavities Cavities reaching 35 MV/m (suitable for 800 GeV in the TESLA site footprint) and Q > 5x109 have been built. Not operated in cryomodule. Possible cavity redesign suggested by KEK could increase gradient to closer to the quench limit, set by critical B field at surface. Recent results from AC70 • First cavity processed in DESY EP facility 26/60

27. TESLA rf cavities Surface defects cause field emission; some electrons are captured by rf and accelerated. Not a problem for the beam emittance, but (a) extra load on cryo-system and (b) radiation damage to nearby electronics. Need to limit to 50 nA/cavity to keep within cryo limit; electropolishing helps by factor 10, but projection to 35 MV/m (I ~ eGRADIENT ) gives some cause for worry. But annealing of emitters will occur. Red: BCP Blue: EP 27/60

28. Achieving the luminosity Emittance control – like herding cats – is the nearly impossible trick required for high luminosity 28/60

29. Low emittance beams are prepared in the damping rings Synchrotron radiation in arcs, wiggler sections, with energy restored by rf damps the transverse emittances. Very tight control of orbits, vacuum, and instabilities are required. X-band damping ring has been built at KEK, and has shown the desired low emittance. 29/60

30. TESLA damping rings The long bunch train in L-band means the damping ring to hold the full train is 100’s of km long. Fold the bunches into 20 ns spacing to get 17 km length for damping ring, and put the rings in the long linac tunnels, with ‘dogbone’ sections to turn the beams around. Convert flat beams in wiggler arcs into round beams in straight sections to avoid excessive tune shifts. Need very fast kickers to eject a single bunch without disturbing the neighbors. 30/60

31. Electron cloud buildup in positron DR destroys the low emittance; need secondary emission coefficient at wall of less than 1.3. (similar effect from ion buildup in electron DR) Problem is somewhat worse for X-band; also affects the bunch compressor section following the DRs, and perhaps the final focus. Needs further R&D 31/60

32. tail head Transverse Wakefields – the emittance killer Wakefields: off axis beam sets up electric field due to induced charges in cavity walls; these cause deflections of tail of same bunch and on subsequent bunches. Within a single bunch, betatron oscillation in head of bunch creates a wakefield that resonantly drives the oscillation of the tail. Can be cured by BNS energy spread from head to tail; lower energy tail focussed harder by quadrupoles; L-band wakefields 1000x less than X-band, but energy spread needed for X-band is “only” 10x that for SC, but still a factor 8 less than SLC. tail head eqn: head tail eqn: 32/60

33. Transverse Wakes Multibunch wakefield Bunch current generates transverse deflecting modes when bunches are not on cavity axis Fields build up resonantly: latter bunches are kicked transversely Long range wakefield (on subsequent bunches) cured for X-band by detuning the structures so that effects cancel at following bunches. Ultimately the wakefield currents die out. For TESLA with 337 ns bunch interval, this effect is naturally smaller. 33/60

34. Beam alignment Beam alignment: Must align the cavities to a few m . Quadrupoles to ~100 nm; final focus elements to 10 – 100 nm, and final lens to ~nm! Natural thermal, vibration, ground motion takes the beam out of alignment so must re-establish the ‘gold orbit’. First survey the linac elements to 100 m level (harder in SC since don’t ‘see’ cavities inside cryomodules, and have to allow for thermal contraction). Beam Position Monitors in all magnets. X-band structure gives beam position to 10 m. Hard to measure beam within the superconducting cryomodule, so rely on measurements at the ends. Steer the beam to the gold orbit: detune quads and measure steering (beam based alignment). Tuning the orbit can be done semiautomatically by tracking position drifts in BPMs and fast feedback at IP, so that invasive tuning can be kept to relatively long intervals. Aligning final focus is biggest task. 34/60

35. Beam alignment Beam alignment systems will be a big deal! Need reliable, precise BPMs, redundancy, and well-automated procedure. Alignment of SC machine is less critical due to large apertures and smaller wakefield. 35/60

36. US study of L-band collider US group made design of 1 TeV SC collider with several differences: Length to allow 1 TeV Space for helical undulator to give positron polarization (test underway at SLAC) Allow positron production at fixed energy (150 GeV) to allow stable operation for any energy collisions 2 tunnels Non-zero crossing angle (desirable to allow measurement of energy, luminosity profile, polarization before and after collision) They estimated that cost of this cold machine is (1.25  .10)X that for warm at same energy Also performed risk and availability studies 36/60

37. Superconducting rf at lower gradient has been used in several HEP applications; it is becoming the rf system of choice for accelerators. 37/60

38. And superconducting rf is the basis for many new applications in a broader spectrum of accelerator-based physical and biological science 38/60

39. X-band has many applications to non-scientific uses. These stem from the smaller size (higher gradient) of X-band compared the more normal S-band: military radar on airplanes communications industry medical accelerators food sterilization semiconductor lithography 39/60

40. ITRP -------------------- Linear Collider Technology Recommendation Barry Barish FALC Meeting CERN 17-Sept-04 Funding Agencies – Linear Collider Executive summary is public: http://www.ligo.caltech.edu/~donna/ITRP_Home.htm 40/60

41. ITRP in Korea 41/60

42. Barish Sept. 17, 2004 Evaluating the Criteria Matrix • We analyzed the technology choice through studying a matrix having six general categories with specific items under each: • the scope and parameters specified by the ILCSC; • technical issues; • cost issues; • schedule issues; • physics operation issues; • and more general considerations that reflect the impact of the LC on science, technology and society • We evaluated each of these categories with the help of answers to our “questions to the proponents”, internal assignments and reviews, plus our own discussions 42/60

43. Barish Sept. 17, 2004 Evaluation: Scope and Parameters • The Panel’s general conclusion was that each technology would be capable, in time, of achieving the goals set forth in the Parameters Document. • The Panel felt that the energy goals could be met by either technology. • The higher accelerating gradient of the warm technology would allow for a shorter main linac. • The luminosity goals were deemed to be aggressive, with technical and schedule risk in each case. • On balance, the Panel judged the cold technology to be better able to provide stable beam conditions, and therefore more likely to achieve the necessary luminosity in a timely manner. 43/60

44. Barish Sept. 17, 2004 Evaluation: Technical Issues • In general, the Panel found the LC R&D to be far advanced. The global R&D effort uncovered a variety of issues that were mitigated through updated designs. • For the warm technology, major subsystems were built to study actual performance. • The KEK damping ring was constructed to demonstrate the generation and damping of a high-intensity bunch train at the required emittance, together with its extraction with sufficient stability. • The Final Focus Test Beam at SLAC was constructed to demonstrate demagnification of a beam accelerated in the linac. • As a result, the subsystem designs are more advanced for the warm technology. 44/60

45. Barish Sept. 17, 2004 Evaluation: Technical Issues • In general, the cold technology carries higher risk in the subsystems other than the linacs, while the warm technology has higher risk in the main linacs and their individual components. • The accelerating structures have risks that were deemed to be comparable in the two technologies. • The warm X-band structures require demonstration of their ability to run safely at high gradients for long periods of time. • The cold superconducting cryomodules need to show that they can manage field emission at high gradients. • For the cold, industrialization of the main linac components and rf systems is now well advanced. 45/60

46. Barish Sept. 17, 2004 Evaluation: Technical Issues • Many cold technology components will be tested over the coming few years in a reasonably large-scale prototype through construction of the superconducting XFEL at DESY. • A superconducting linac has a high intrinsic efficiency for beam acceleration, which leads to lower power consumption. • The lower accelerating gradient in the superconducting cavities implies that the length of the main linac in a cold machine is greater than it would be in a warm machine of the same energy. • Future R&D must stress ways to extend the energy reach to 1 TeV, and even somewhat beyond. 46/60

47. Barish Sept. 17, 2004 Evaluation: Technical Issues • In a superconducting rf structure, the rf pulse length, the length of the bunch train, and interbunch time interval are all large. This offers many advantages. • The disadvantages are mainly related to the complex and very long damping rings, and the large heat load on the production target for a conventional positron source, which might require a novel source design. • Storage rings are among the best-understood accelerator subsystems today, and much of this knowledge can be transferred to the linear collider damping rings. • Beam dynamics issues such as instabilities, ion effects, and intrabeam scattering have been well studied in those machines. 47/60

48. Barish Sept. 17, 2004 Evaluation: Technical Issues • Achieving design luminosity will be a critical measure of the collider’s success. A number of arguments indicate it will be easier with the cold technology. • The cold technology permits greater tolerance to beam misalignments and other wakefield-related effects. • Natural advantage in emittance preservation because the wakefields are orders of magnitude smaller • The long bunch spacing eliminates multi-bunch effects and eases the application of feedback systems. • This feedback will facilitate the alignment of the nanometer beams at the collision point. • For these reasons, we deem the cold machine to be more robust, even considering the inaccessibility of accelerating components within the cryogenic system. 48/60

49. Barish Sept. 17, 2004 Evaluation: Cost Issues • The Panel spent considerable effort gathering and analyzing all information that is available regarding the total costs and the relative costs of the two options. • At the present conceptual and pre-industrialized stage of the linear collider project, uncertainties in estimating the total costs are necessarily large. • Although it might be thought that relative costing could be done with more certainty, there are additional complications in determining even the relative costs of the warm and cold technologies because of differences in design choices and differences in costing methods used in different regions. 49/60

50. Barish Sept. 17, 2004 Evaluation: Physics Operations Issues • Several factors favor the cold machine: • The long separation between bunches in a cold machine allows full integration of detector signals after each bunch crossing. In a warm machine, the pileup of energy from multiple bunch crossings is a potential problem, particularly in forward directions. • The energy spread is somewhat smaller for the cold machine, which leads to better precision for measuring particle masses. • If desired, in a cold machine the beams can be collided head-on in one of the interaction regions. Zero crossing angle might simplify shielding from background. • But a nonzero crossing angle permits the measurement of beam properties before and after the collision, giving added constraints on the determination of energy and polarization at the crossing point. 50/60