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An Introduction to the Physics and Technology of e+e- Linear Colliders Lecture 1: Introduction and Overview Nick Walker

An Introduction to the Physics and Technology of e+e- Linear Colliders Lecture 1: Introduction and Overview Nick Walker (DESY). Nick Walker DESY. DESY Summer Student Lecture 31 st July 2002. USPAS Santa Barbara 16 th June, 2003. Course Content. Lecture:. Introduction and overview

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An Introduction to the Physics and Technology of e+e- Linear Colliders Lecture 1: Introduction and Overview Nick Walker

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  1. An Introduction to thePhysics and Technologyof e+e- Linear CollidersLecture 1: Introduction and OverviewNick Walker (DESY) Nick WalkerDESY DESY Summer Student Lecture31st July 2002 USPAS Santa Barbara 16th June, 2003

  2. Course Content Lecture: • Introduction and overview • Linac part I • Linac part II • Damping Ring & Bunch Compressor I • Damping Ring & Bunch Compressor II • Final Focus Systems • Beam-Beam Effects • Stability Issues in Linear Colliders • the SLC experience and the Current LC Designs

  3. This Lecture • Why LC and not super-LEP? • The Luminosity Problem • general scaling laws for linear colliders • A introduction to the linear collider sub-systems: • main accelerator (linac) • sources • damping rings • bunch compression • final focus during the lecture, we will introduce (revise) some important basic accelerator physics concepts that we will need in the remainder of the course.

  4. Energy Frontier e+e- Colliders LEP at CERN, CHEcm = 180 GeVPRF = 30 MW

  5. Why a Linear Collider? Synchrotron Radiation from an electron in a magnetic field: Energy loss per turn of a machine with an average bending radiusr: Energy loss must be replaced by RF system

  6. Cost Scaling $$ • Linear Costs: (tunnel, magnets etc)$linµr • RF costs:$RFµDEµE4/r • Optimum at$lin= $RF Thus optimised cost ($lin+$RF) scales asE2

  7. The Bottom Line $$$

  8. The Bottom Line $$$

  9. The Bottom Line $$$

  10. bang! solution: Linear ColliderNo Bends, but lots of RF! e+ e- 5-10 km • long linac constructed of many RF accelerating structures • typical gradients range from 25-100 MV/m Note: for LC, $totµE

  11. 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.”

  12. A Little History (1988-2003) • SLC (SLAC, 1988-98) • NLCTA (SLAC, 1997-) • TTF (DESY, 1994-) • ATF (KEK, 1997-) • FFTB (SLAC, 1992-1997) • SBTF (DESY, 1994-1998) • CLIC CTF1,2,3 (CERN, 1994-) Over 14 Years of Linear Collider R&D

  13. Past and Future generally quoted as‘proof of principle’ but we have a very long way to go!

  14. LC Status in 1994 1994 Ecm=500 GeV

  15. LC Status 2003 2003 Ecm=500 GeV

  16. The Luminosity Issue Collider luminosity (cm-2 s-1)isapproximately given by where: Nb = bunches / trainN = particles per bunchfrep = repetition frequencyA = beam cross-section at IPHD = beam-beam enhancement factor For Gaussian beam distribution:

  17. The Luminosity Issue: RF Power Introduce the centre of massenergy, Ecm: hRF is RF to beam power efficiency. Luminosity is proportional to the RF power for a given Ecm

  18. The Luminosity Issue: RF Power Some numbers: Ecm = 500 GeVN = 1010nb = 100frep = 100 Hz Need to include efficiencies: RFbeam: range 20-60%Wall plug RF: range 28-40% AC power > 100 MW just to accelerate beams and achieve luminosity Pbeams = 8 MW linac technology choice

  19. The Luminosity Issues: storage ring vs LC LEP frep = 44 kHz LC frep = few-100 Hz (power limited)  factor ~400 in L already lost! Must push very hard on beam cross-section at collision: LEP: sxsy 1306 mm2 LC: sxsy (200-500)(3-5) nm2 factor of 106 gain!Needed to obtain high luminosity of a few 1034 cm-2s-1

  20. The Luminosity Issue: intense beams at IP • choice of linac technology: • efficiency • available power • Beam-Beam effects: • beamstrahlung • disruption • Strong focusing • optical aberrations • stability issues and tolerances

  21. Ey (MV/cm) y/sy The Luminosity Issue: Beam-Beam see lecture 2 on beam-beam • strong mutual focusing of beams (pinch) gives rise to luminosity enhancement HD • As e± pass through intense field of opposing beam, they radiate hard photons [beamstrahlung] and loose energy • Interaction of beamstrahlung photons with intense field causes copious e+e- pair production [background]

  22. The Luminosity Issue: Beam-Beam see lecture 2 on beam-beam beam-beam characterised by Disruption Parameter: sz = bunch length, fbeam = focal length of beam-lens for storage rings, and In a LC, hence Enhancement factor (typically HD ~ 2): ‘hour glass’ effect

  23. The Luminosity Issue: Hour-Glass see lecture 2 on beam-beam b = “depth of focus” reasonable lower limit for b is bunch length sz

  24. The Luminosity Issue: Beamstrahlung see lecture 2 on beam-beam RMS relative energy loss we would like to make sxsysmall to maximise luminosity BUT keep (sx+sy) large to reduce dSB. Trick: use “flat beams” with Now we set sx to fix dSB, and make sy as small as possible to achieve high luminosity. For most LC designs, dSB ~ 3-10%

  25. The Luminosity Issue: Beamstrahlung Returning to our L scaling law, and ignoring HD From flat-beam beamstrahlung hence

  26. The Luminosity Issue: story so far • high RF-beam conversion efficiency hRF • high RF power PRF • small vertical beam size sy • large bunch length sz(will come back to this one) • could also allow higher beamstrahlung dBSif willing to live with the consequences For high Luminosity we need: Next question: how to make a small sy

  27. The Luminosity Issue: A final scaling law? where en,y is the normalised vertical emittance, and by is the vertical b-function at the IP. Substituting: hour glass constraint by is the same ‘depth of focus’ bfor hour-glass effect. Hence

  28. The Luminosity Issue: A final scaling law? • high RF-beam conversion efficiency hRF • high RF power PRF • small normalised vertical emittance en,y • strong focusing at IP (small by and hence smallsz) • could also allow higher beamstrahlung dBSif willing to live with the consequences Above result is for the low beamstrahlung regime where dBS ~ few % Slightly different result for high beamstrahlung regime

  29. Luminosity as a function of by

  30. The ‘Generic’ Linear Collider Each sub-system pushes the state-of-the-art in accelerator design

  31. Ez Ez c c z z The Linear Accelerator (LINAC) see lectures 3-4 on linac travelling wave structure:need phase velocity = c(disk-loaded structure) bunch sees constant field:Ez=E0cos(f ) c standing wave cavity: bunch sees field:Ez =E0sin(wt+f )sin(kz) =E0sin(kz+f )sin(kz)

  32. The Linear Accelerator (LINAC) see lectures 3-4 on linac Travelling wave structure Circular waveguide mode TM01 has vp>c No good for acceleration! Need to slow wave down using irises.

  33. power lost to structure beam loading hRF would like RS to be as high as possible The Linear Accelerator (LINAC) see lectures 3-4 on linac • Gradient given by shunt impedance: • PRFRF power /unit length • RSshunt impedance /unit length • The cavity Q defines the fill time: • vg = group velocity, ls = structure length • For TW, t is the structureattenuation constant: • RF power lost along structure (TW):

  34. The Linear Accelerator (LINAC) see lectures 3-4 on linac • Steady state gradient drops over length of structure due to beam loading unloaded av. loaded assumes constant (stead state) current

  35. The Linear Accelerator (LINAC) see lectures 3-4 on linac • Transient beam loading • current not constant but pulses! (tpulse = nb tb) • for all LC designs, long bunch trains achieve steady state quickly, and previous results very good approximation. • However, transient over first bunches needs to be compensated. V unloaded av. loaded t

  36. The Linear Accelerator (LINAC) Single bunch beam loading: the Longitudinal wakefield NLC X-band structure:

  37. The Linear Accelerator (LINAC) Single bunch beam loading Compensation using RF phase wakefield RF f = 15.5º Total

  38. The Linear Accelerator (LINAC) Single bunch beam loading: compensation RMS DE/E <Ez> fmin = 15.5º

  39. Transverse Wakes: The Emittance Killer! Bunch current also generates transverse deflecting modes when bunches are not on cavity axis Fields build up resonantly: latter bunches are kicked transversely  multi- and single-bunch beam breakup (MBBU, SBBU)

  40. Damped & Detuned Structures Slight random detuning between cells causes HOMs to decohere. Will recohere later: needs to be damped (HOM dampers)

  41. tail head Single bunch wakefields Effect of coherent betatron oscillation - head resonantly drives the tail head eom: tail eom:

  42. Wakefields (alignment tolerances) • higher frequency = stronger wakefields • higher gradients • stronger focusing (smaller b) • smaller bunch charge

  43. The LINAC is only one part • Produce the electron charge? • Produce the positron charge? • Make small emittance beams? • Focus the beams down to ~nm at the IP? Need to understand how to:

  44. e+e- Sources • produce long bunch trains of high charge bunches • with small emittances • and spin polarisation (needed for physics) Requirements: 100-1000s @ 5-100 Hzfew nC enx,y ~ 10-6,10-8 m mandatory for e-,nice for e+ Remember L scaling:

  45. e- Source • laser-driven photo injector • circ. polarised photons on GaAs cathode  long. polarised e- • laser pulse modulated to give required time structure • very high vacuum requirements for GaAs (<10-11 mbar) • beam quality is dominated by space charge(note v ~ 0.2c) factor 10 in x plane factor ~500 in y plane

  46. e- Source: pre-acceleration SHB = sub-harmonic buncher. Typical bunch length from gun is ~ns (too long for electron linac with f ~ 1-3 GHz, need tens of ps)

  47. e+ Source Photon conversion to e± pairs in target material Standard method is e- beam on ‘thick’ target (em-shower)

  48. e+ Source :undulator-based • SR radiation from undulator generates photons • no need for ‘thick’ target to generate shower • thin target reduces multiple-Coulomb scattering: hence better emittance (but still much bigger than needed) • less power deposited in target (no need for mult. systems) • Achilles heel: needs initial electron energy > 150 GeV! ~ 30 MeV 0.4X0 10-2 m5 kW

  49. Damping Rings see lecture 5 • (storage) ring in which the bunch train is stored for Tstore ~20-200 ms • emittances are reduced via the interplay of synchrotron radiation and RF acceleration initial emittance(~0.01m for e+) final emittance damping time equilibriumemittance

  50. Damping Rings: transverse damping see lecture 5 dpreplaced by RF such that Dpz = dp. since (adiabatic damping again) y’ = dy/ds = py/pz, we have a reduction in amplitude: dy’ = -dp y’ y’not changed by photon (or is it?) Must take average over all b-phases: where andhence LEP: E ~ 90 GeV, Pg~ 15000 GeV/s, tD~ 12 ms

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