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Introduction to Particle Accelerators

This lecture provides an overview of particle accelerators, with a focus on neutrino projects. Topics covered include superbeam and neutrino factory, target systems, proton driver scenarios, and the components of a neutrino factory. The challenges and driving issues of constructing a muon-based neutrino factory are discussed, along with examples of existing schemes. The lecture also highlights the history of the neutrino factory concept and the ionization cooling technique. The progress and goals of the MICE experiment, which aims to demonstrate the feasibility of muon ionization cooling, are also presented.

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Introduction to Particle Accelerators

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  1. Introduction to particle accelerators Walter Scandale CERN - AT department Roma, marzo 2006

  2. Lecture VI - neutrino projects topics • Superbeam & Neutrino Factory & Muon Collider • Target • Proton driver

  3. Scenarios for Neutrino beams • The basic blocks • – Proton driver 1 to 4 MW • – Muon accelerator • - Muon storage ring (decay ring / m - m collider) • This suggests (at least) 3 stages towards a neutrino factory: • Neutrino superbeam from pion decay with uo to 4 MW proton driver. (Stages 1a, 1b, 1c might be 1, 2, 4 MW proton driver performance.) • Add a muon capture channel + a muon accelerator • Add a storage ring to produce muon decay neutrinos nF (3a) and a m - m collision storage ring (3b) Neutrino Beams: – Superbeam neutrinos from π± -> m± + nm (anti nm) . (Pions from pA -> π±X.) – Factory neutrinos from m± -> e± anti nmne (nm anti me). (Muons from π± -> m± + nm (anti nm) ) – b-beam neutrinos from 6He -> 6Li e- anti ne, 18Ne -> 18Fe+ne

  4. Components of a Neutrino Factory • Proton Driver — primary beam on production target • Target, Capture, Decay — create π, decay into µ • Bunching, Phase Rotation — reduce ∆E of bunch • Cooling — reduce transverse emittance • Acceleration — 130 MeV ==> 20 GeV • Decay Ring — store for ~500 turns; long straigth section I > 1 x 1020 µ decays / year @ one s.s.

  5. Driving issues of a Neutrino Factory • Constructing a muon-based nF is challenging • — muons have short lifetime (2.2 µs at rest) • puts premium on rapid beam manipulations • – requires high-gradient RF for longitudinal cooling (in B field) • – requires presently untestedionization cooling technique • – requires fast acceleration system • — muons are created as a tertiary beam (p==> p ==> µ) • low production rate • – target that can handle multi-MW proton beam • large muon beam transverse phase space and large energy spread • – high acceptance acceleration system and storage ring • — neutrinos themselves are a quaternary beam • even less intensity and “a mind of their own” • — developing solutions requires substantial R&D effort • R&D should aim to specify: • – expected performance, technical feasibility/risk, cost (matters!)

  6. Examples of Neutrino Factories KEK scheme

  7. The UK scheme FFAG I (3-8GeV) FFAG II (8-20GeV) FFAG III (20-50GeV) ( Muon Decay Ring Near Detector R109 Neutrino Factory To Far Detector 1 Far Detector 1 Far Detector 2 To Far Detector 2

  8. The Super Beam

  9. A brief history of the Neutrino Factory Muon storage ring is an old idea: • Charpak et al. (g – 2) (1960), Tinlot & Green (1960), Melissinos (1960) • muon colliders suggested by Tikhonin (1968) • but no concept for achieving high luminosity until ionization cooling suggested by O’Neill (1956), Lichtenberg et al. (1956), • muon ionization cooling proposed by Skrinsky & Parkhomchuk (1981) and Neuffer(1979, 1983) • Neuffer and Palmer (1995) suggested that a high-luminosity muon collider might be feasible • Neutrino Factory and Muon Collider Collaboration started in 1995 has since grown to 47 institutions and >100 physicists • Snowmass Summer Study (1996) • study of feasibility of a 2+2 TeV Muon Collider [Fermilab 1996] • First neutrino Factory suggested by Geer (1997)

  10. The piece of cake: the ionization cooling Energy loss dE/dx Momentum recovery though RF • - RF cavities between absorbers replace E–> Net effect: • reduction in pat constant p||, i.e., transverse cooling. • - Reduce heating by Coulomb scattering: • Strong focusing (small ß along the channel) • Large radiation length Xo (low-Z absorber) • High field solenoid / lithium lens RF cavity Figure of merit:M = LR dE/ds

  11. Ionization cooling test experiment: MICE • Ionization cooling is a brilliantly simple idea! • • BUT: • never observed experimentally • delicate design and engineering problem • a crucial ingredient in the cost and performance optimization • Goals of MICE: • design, engineer and build a section of cooling channel giving the desired performance for a nF; • use a m beam and measure the cooling performance.

  12. Status of MICE I guess there’ll always be a gap between science and technology • Muon storage rings and Neutrino Factories may be the best way to study neutrino mixing and CPV • F technical feasibility has been demonstrated “on paper” • We need the experimental demonstration of muon ionization cooling feasibility & performance • MICE Proposal approved and Phase 1 funded • Scope and time-scale comparable to mid-sized HEP experiment

  13. Progress of MICE Focusing solenoid Cavity prototype Decay channel and its solenoid Final spectrometer

  14. Ionization cooling: B-flip of solenoid To get low ß and hence to produce small emittance use a big S/C solenoids & high fields! ==> expensive envelop

  15. Ionization cooling: alternative lattices Lattice design questions • Many alternative configurations • Alternating solenoid • FOFO • Super-FOFO • (+ RFOFO, • DFOFO, • Single-Flip, • Double-Flip) —both with cooling and non-cooling ==> arrive at baseline specifications • end-to-end simulations — correlations in beam and details of distributions have significant effect on transmission at interfaces (muons have “memory”) — simulation effort will tie all aspects together Alternating gradient allows low b with much less superconductor

  16. Longitudinal cooling ? • Transverse ionization cooling self-limiting due to longitudinal emittance growth, leading to particle losses • straggling plus finite E acceptance of cooling channel • need of longitudinal cooling for muon collider; could also help for F • Possible in principle by ionization above ionization minimum, but inefficient due to straggling and small slope d(dE/ds)/dE

  17. Neutrino factory based on extreme cooling “extreme cooling” via emittance exchange in helical focusing channel filled with dense low-Z gas or liquid proposed by R. Johnson, Y. Derbenev, et al. (Muons, Inc.) Ecm = 5 TeV <L> ~ 5·1034 cm-2s-1 prototype helical solenoid+rotating-dipole +quad magnet from AGS “Siberian Snake”

  18. µ production µ–: 6 – 11 GeV µ+: 9 – 19 GeV • 4-MW Proton Beam on target • 10-30 GeV p-beam appropriate for both Superbeam and Neutrino Factory. ⇒ 0.8-2.5 ×1015 pps; 0.8-2.5 ×1022 protons per year of 107 s. • Rep rate 15-50 Hz at Neutrino Factory, as low as 2 Hz for Superbeam. ⇒ Protons per pulse from 1.6 ×1013 to 1.25 ×1015. ⇒ Energy per pulse from 80 kJ to 2 MJ. • Small beam size preferred: ≈ 0.1 cm2 for Neutrino Factory, ≈ 0.2 cm2 for Superbeam. ⇒ Severe materials issues for target AND beam dump.

  19. Target / capture / decay Critical issues • Radiation Damage - Melting - Cracking (due to single-pulse “thermal shock”). • Optimum target material — solid or liquid — low, medium, or high Z • Intensity limitations — from target — from beam dump • Superbeam vs. Neutrino Factory trade-offs — horn vs. solenoid capture — can one solution serve both needs? — is a single choice of target material adequate for both? Is there hope for a 4 MW target ? • Several “smart” materials or new composites should be considered: — new graphite grades — customized carbon-carbon composites — Super-alloys (gum metal, albemet, super-invar, etc.) • While calculations based on non-irradiated material properties may show that it is possible to achieve 2 or even 4 MW, irradiation effects may completely change the outlook of a material candidate. • ONLY way is to test the material to conditions similar to those expected during its life time as target.

  20. Horns Carbon composite target with He gas cooling (BNL study): Mercury jet target (CERN SPL study): • For secondary pions • with Eπ <∼ 5 GeV (Neutrino Factory), a high-Z target is favored, • but for Eπ >∼ 10 GeV (some Superbeams), low Z is preferred.

  21. Solenoids • Palmer (1994) proposed a solenoidal capture system for a Neutrino Factory. • Collects both signs of p’s and m’s, • Solenoid coils can be at some distance from proton beam. ⇒ ≥ 4 year life against radiation damage at 4 MW. ⇒ Proton beam readily tilted with respect to magnetic axis. ⇒ Beam dump out of the way of secondary p’s and m’s. Solenoidal capture magnet (≈ 20 T) with adiabatic transition to solenoidal decay channel (≈ 1 T). • Mercury jet target and proton beam tilt downwards with respect to the horizontal magnetic axis of the capture system • The mercury collects in a pool that serves as the beam dump (F) . ⇒ Point-to-parallel focusing for ⇒ Narrowband neutrino beams (less background)

  22. Liquid / solid target Liquid target/dump using mercury, or a Pb-Bi alloy. ⇒ F ≈ 400 J/gm to vaporize Hg (from room temp), ⇒ Need flow of > 104 g/s ≈ 1 l/s in target/dump to avoid boiling in a 4-MW beam. Energy deposited in the mercury target (and dump) will cause dispersal, but at benign velocities (10-50 m/s). 1-cm-diameter Hg jet in 2e12 protons at t = 0, 0.75, 2, 7, 18 ms (BNL E-951, 2001). Solid Targets (Superbeams) alternative A solid, radiation-cooled stationary target in a 4-MW beam will equilibrate at about 2500 C. ⇒ Carbon is only candidate (in He atmosphere to suppress sublimation.) A moving band target (tantalum) could be considered in a toroidal capture system

  23. Muon production based on FFAG Osaka Univ. FFAG Magnetscaling KEK

  24. Proton driver for a Neutrino Factory Proton Driver Questions • Optimum beam energy — depends on choice of target ==> consider C, Ta, Hg • Hardware options — FFAG, linac, synchrotron ==> compare performance, cost • Beam dynamics • – beam current limitations (injection, acceleration, activation) • – bunch length limitations and schemes to provide 1-3 ns bunches, approaches for bunch compression • – repetition rate limitations (power, vacuum chamber,…) • – tolerances (field errors, alignment, RF stability,…) • Superbeam versus Neutrino Factory • Factory requirements • - required emittance and focusing • - staging

  25. Proton drivers Intensity history of multi-GeV proton accelerators. The numbers in parenthesis indicate the typical repetition rate. High proton beam power machines presently operating, under construction, or planned

  26. Existing and Proposed Proton Drivers The pulse structure is given in terms of the pulse duration tp, the number of bunches Nb making up each pulse, and the final compressed rms bunch length tb.

  27. Driver I: 4 MW, 50 Hz, 5 GeV 180 MeV, 280 MHz H- Linac Achromat for momentum and betatron collimation Two 50 Hz Rapid Cycling Synchrotrons, with two bunches of 2.5 1013 protons in each. Energy 180 MeV to 1.2 GeV Momentum ramping Two rings each, stacked vertically Two 25 Hz Rapid Cycling Synchrotrons, 4 bunches in each. Energy 1.2 GeV to 5 GeV. Bunch compression to 1 ns rms at pion target Mean radius 65m

  28. Driver II: 4 MW, 25 Hz, 15 GeV Two 12.5 Hz Rapid Cycling Synchrotrons, 6 bunches in each. Energy 3 GeV to 15 GeV. Bunch compression to 1 ns rms at pion target Mean radius 150m Achromatic arc for collimation 180 MeV, 280 MHz, H- Linac Two rings each, stacked vertically Momentum ramping Two 25 Hz Rapid Cycling Synchrotrons, with three bunches of 1.11 1013 protons in each. Energy 180 MeV to 3 GeV

  29. Challenges of the RCS • Large aperture magnets and much higher RF voltages per turn due to a low energy injection and a large and rapid swing of the magnetic field, • Field tracking between many magnet-families under slightly saturated conditions, • RF trapping with fundamental and higher harmonic cavities, • H- charge stripping foil, • Large acceptance injection/dump/extraction section, • Ceramic chambers, • Beam instabilities, • Comparison with full-energy linac+storage ring approach from view point of the radiation protection. 20 ÷ 25 kV/m cavity

  30. Other applications of Proton Drivers

  31. The b-beam concept Lorentz boost high ADVANTAGES OF BETA-BEAMS : Pure ( or ) beams. Well known neutrino fluxes. Strong collimation. b-beam Piero Zucchelli • A novel concept for a neutrino factory: the beta-beam, Phys. Let. B, 532 (2002) 166-172. CONVENTIONAL METHODS : Neutrinobeams are produced using the decay of pions and muons. A NOVEL METHOD TO PRODUCE INTENSE, COLLIMATED, PURE HIGH ENERGY ne BEAMS FROM BOOSTED RADIOACTIVE IONS.

  32. CERN: b-beam baseline scenario SPL SPS Decay Ring ISOL target & Ion source ECR Cyclotrons, linac or FFAG PS Rapid cycling synchrotron Nuclear Physics Br = 1500 Tm B = 5 T Lss = 2500 m • An annual integrated flux of n • 2.9*1018 anti-neutrinos (from 6He at g=100) • 1.1*1018 neutrinos (from 18Ne at g=100) With an Ion production in the target to the ECR source: • 6He= 2*1013 atoms per second • 18Ne= 8*1011 atoms per second

  33. CERN: b-beam baseline limitations • Isotope production • The self-imposed requirement to re-use a maximum of existing CERN infrastructure • Cycling time, aperture limitations, collimation systems etc. • The high intensity ion bunches in the accelerator chain and decay ring • Space charge • Decay losses Typical intensities of 108-109 ions for LHC injector operation (PS and SPS)

  34. Decay ring design aspects • The ions have to be concentrated in a few very short bunches • Suppression of atmospheric background via time structure. • There is an essential need for stacking in the decay ring • Not enough flux from source and injector chain. • Lifetime is an order of magnitude larger than injector cycling (120 s compared with 8 s SPS cycle). • Need to stack for at least 10 to 15 injector cycles. • Cooling is not an option for the stacking process • Electron cooling is excluded because of the high electron beam energy and, in any case, the cooling time is far too long. • Stochastic cooling is excluded by the high bunch intensities. • Stacking without cooling “conflicts” with Liouville

  35. Lecture VI - neutrino projects • reminder • Neutrino physics is very appealing • Neutrino beam devices are complex and expensive • Superbeam is the basic initial block os a modern neutrino facility, it relies on the construction of a multimegawatt proton driver • Muon accelerators are the next step and rely on a performing target system capture channel and on the very challenging ion cooling • Neutrino factories and muon muon colliders are the last step (cost is matter • Beta-beams are a clever shortcut

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