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Luminosity Issues in TESLA Linear Collider

This presentation discusses the various luminosity issues in the TESLA linear collider, including emittance tuning, bunch compressor, and machine protection. It also explores the impact of low repetition rate and high beam charge on luminosity. Additionally, the presentation addresses the effect of wakefields, beam-beam disruption, and alignment errors on luminosity. The TDR parameters, beam dynamics, and long-range wakes are also discussed.

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Luminosity Issues in TESLA Linear Collider

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  1. TESLA Linear ColliderLuminosity Related Issues Nick Walker (DESY) ITRP Meeting - RAL - 28.02.04

  2. Content • Luminosity Issues • oft quoted advantages of s.c. RF in a nutshell • Main linac dynamics • emittance tuning • Bunch Compressor • Undulator-Based Positron Source • Damping Ring • many critical issues • Beam Delivery System • head-on collision scheme • machine (collimator) protection philosophy • Luminosity Stabilisation & Feedback ITRP Meeting - RAL - 28.02.04

  3. The Luminosity Issue ITRP Meeting - RAL - 28.02.04

  4. The Luminosity Issue • Low repetition rate: 5 Hz • limited by cryogenics power • impact on ground motion stabilisation (feedback) ITRP Meeting - RAL - 28.02.04

  5. The Luminosity Issue • Compensated by • long bunch train: nb = 2800– fast intra-train orbit stabilisation (feedback) • High bunch charge: N = 2×1010 ITRP Meeting - RAL - 28.02.04

  6. The Luminosity Issue • Emittance Preservation: • low wakefields (low frequency) • relatively loose tolerances ITRP Meeting - RAL - 28.02.04

  7. Ratio of deflecting wakefield to accelerating field for 1mm offset 10-3 10-4 10-5 10-6 TESLA C-band CLIC X-band Wakefields (alignment tolerances) Transverse Wakefield Kick f3 ITRP Meeting - RAL - 28.02.04

  8. High Beam-Beam Disruption (Enhancement) • factor ~2 for luminosity • collision is unstable (kink instability) • tighter tolerance on emittance dilution banana effect The Luminosity Issue ITRP Meeting - RAL - 28.02.04

  9. 500 GeV C.M. Parameters ITRP Meeting - RAL - 28.02.04

  10. the TESLA TDR linear collider LuminosityIssues: • ML dynamics • Damping Ring • Sources (e+) • Beam Delivery & IR ITRP Meeting - RAL - 28.02.04

  11. TESLA Linac Beam Dynamics • Emittance Preservation • Alignment tolerances • Beam based alignment ITRP Meeting - RAL - 28.02.04

  12. TESLA Long Range Wakes • Random detuning • HOM absorbers 337 ns bunch spacing 36 cavity average,0.1% frequency spread All modes damped below 1105 ITRP Meeting - RAL - 28.02.04

  13. vertical offset (mm) bunch number TESLA Long Range Wakes Effect of 1sy oscillation along linac • Pattern remains the same (difference at nm level) • Result of loose tolerances (cavity offsets) • Static part (almost all) can be fixed with feed forward ITRP Meeting - RAL - 28.02.04

  14. V/pC/m z (mm) Single Bunch Wakefields Accurate calculation of single-bunch transverse wakefield 30% less transverse kick than previous TDR estimate. I. Zagorodnov, T. Weiland (2003) ITRP Meeting - RAL - 28.02.04

  15. Assumed Alignment Errors • quad offsets: 300 mm • cavity offsets: 300 mm • cavity tilts: 300 mrad • BPM offsets: 200 mm • CM offsets: 200 mm • BPM resolution: 10 mm wrt CM axis single-shot these values have been used in simulations of linac tuning ITRP Meeting - RAL - 28.02.04

  16. 45.0 with incoming jitter fitted out 40.0 no jitter 35.0 norm. vertical emittance (nm) TDR budget 30.0 25.0 20.0 0 50 100 150 200 250 300 350 Quadrupole # Dispersion Free Steering The effect of upstream beam jitter on DFS simulations for the TESLA linac. 1sy initial jitter 10 mm BPM noise uncorrected cavity tilts cause problems for TESLA average over 100 random machines ITRP Meeting - RAL - 28.02.04

  17. Ballistic Alignment systematically turn off sections of linac Use ‘ballistic beam’ to define (straight) reference line. • Less sensitive to • model errors • beam jitter average over 100 seeds ITRP Meeting - RAL - 28.02.04

  18. average over 100 seeds Ballistic Alignment systematically turn off sections of linac Use ‘ballistic beam’ to define (straight) reference line. 3% Energy Spread from Bunch Compressor • Less sensitive to • model errors • beam jitter ITRP Meeting - RAL - 28.02.04

  19. Ballistic Alignment We can tune out linear <yd> and <y’d> correlation using bumps or dispersion correction in BDS average over 100 seeds ITRP Meeting - RAL - 28.02.04

  20. dispersion corrected 100 Random Machines 94% 85% ITRP Meeting - RAL - 28.02.04

  21. Ballistic Alignment TO DO • Control of Ballistic Beam • Show that ‘fat’ ballistic beam can be safely transported through linac • Large cavity irises (Ø70mm) a benefit • Study additional potential problems • stray magnetic fields etc. • Confident we can achieve desired budget ITRP Meeting - RAL - 28.02.04

  22. critical TESLA systems Other Sub-Systems • Spin Rotation / Bunch Compression • Source • ‘dog bone’ damping ring • undulator-driven positron source • Beam Delivery System ITRP Meeting - RAL - 28.02.04

  23. TDR Bunch Compressor • Compression factor of 20 in single stagesz = 6 mm  300 mmdrms = 1.3‰  3% • RF (4 cryomodules) with Vpk ~1 GeV,f = -155° DV = -423 MV • Wiggler section (~100 m) to generate required R56 • Problems: • Cavity Tilts in Module (see later) • Large 3% DP/P • Tuning! ITRP Meeting - RAL - 28.02.04

  24. TDR Ring-to-Linac (RTL) RF wiggler SpinRotator BunchCompressor Diag-nostics (emittancemeasurement) ITRP Meeting - RAL - 28.02.04

  25. BC Cavity Tilts • Slope of tilted RF results in correlated z-y kick along long bunch (sz = 6 mm) • 300 mrad RMS tilts gives average of Dey~140% for TDR design! ITRP Meeting - RAL - 28.02.04

  26. acceleration (along bunch) Resultingcorrelation (dispersion) cavity tilt kick BC Cavity Tilts ITRP Meeting - RAL - 28.02.04

  27. BC Cavity Tilts Results of tracking simulations. Emittance estimated at exit of RF section mean: 138% mean: 2% Emittance after removing d correlation [best you can achieve] different scale! 1000 random seeds ITRP Meeting - RAL - 28.02.04

  28. RTL Emittance Dilution • Tuning ‘dispersion’ (bunch tilt) out downstream • requires tuning knobs (bumps) • emphasis on emittance measurement • final achievable emittance set by resolution (10% ?) • Re-think of design • stronger focusing in RF section (smaller b) • possible two-stage compression system • No De budget in TDR • assumed De = 0 ITRP Meeting - RAL - 28.02.04

  29. TESLA TDR Positron Source • Photons (~20 MeV g) produced by high energy electron beam in undulator placed at exit of e- linac (upstream of BDS and IR) • Thin target (0.4X0) converts the gto e+e- pairs ITRP Meeting - RAL - 28.02.04

  30. TDR e+ Source Parameters ITRP Meeting - RAL - 28.02.04

  31. TDR e+ Source Parameters see damping ring ITRP Meeting - RAL - 28.02.04

  32. Advantages • significantly reduced power deposition in thin target (~5 kW) • smaller emittance beam produced • less multiple coulomb scattering • reduced acceptance requirements for DR • no pre-DR foreseen • much cheaper / less complex than equivalent ‘conventional source’ for TESLA • if conventional source is even possible! • Naturally allows upgrade topolarised e+ source ITRP Meeting - RAL - 28.02.04

  33. Disadvantages • Requires e-linac with 150 GeV • TDR solution to use main e- linac • coupling e- to e+ production raises questions of • operability • reliability • commissioning strategy • Never been done before • although physics is well understood! • E166 experiment at SLAC can be mitigated through R&D no real show stoppers ITRP Meeting - RAL - 28.02.04

  34. TESLA Damping Ring • TESLA bunch train 2820 × 337 ns = 950 ms  285 km long • Extract every bunch separately, bunch spacing given by shortest kicker rise/fall time 20 ns × 2820  56 ms  17 km long • Save tunnel cost: DR in main linac tunnel and short return arcs  dogbone ITRP Meeting - RAL - 28.02.04

  35. Dogbone DR Concept Need ~450m of wiggler to achieve required damping time (28 msec) • B2dl= 605T2m • Permanent Magnet Wiggler withBmax = 1.6 T, l=0.4 m • Radiated Power (160 mA) over 450 m : 3 MW ITRP Meeting - RAL - 28.02.04

  36. TESLA DR Parameters ITRP Meeting - RAL - 28.02.04

  37. Wiggler ITRP Meeting - RAL - 28.02.04

  38. Space Charge Tune Shift • Unusually large circumference / energy ratio • final emittance is space-charge limited! • Quantitive effect on steady-state ey unknown, but probably >factor 2 increase. • Solutions: • Increase energy • difficult lattice in arc • more RF needed • cost optimum turns out to be at 4-5 GeV • increase transverse beam size in long straight sections through local x-y coupling • radical! • multiple ring designs (cost!) ITRP Meeting - RAL - 28.02.04

  39. Kicker Requirements 0.6 mrad ±0.05% 0.01 Tm 337 ns Ripple: 0.05% 40 ns • 2820 pulses with 3 MHz repetition rate • 5 Hz repetition rate of macro-pulse ITRP Meeting - RAL - 28.02.04

  40. RF Kicker • RF kicker system • Delahaye 93 • Koshkarev 95 • Gollin et al. 2002 • INFN-LNF 2003 • With enough harmonics very sharp pulse possible • No flexibiliy for different bunch distances ITRP Meeting - RAL - 28.02.04

  41. Stripline Kicker • Stripline Kicker (1996) • C-Yoke Kicker (2000-.....) • Kicker technology available • Main Challenge: Pulser • IGBT Transformer Switch • MOSFET Stacks ongoing R&D (XFEL needs fast kickers too!) ITRP Meeting - RAL - 28.02.04

  42. Frank Obier (DESY), Guido Blokesch (IPP) TTF Measurements averaged over 50 pulses / point ITRP Meeting - RAL - 28.02.04

  43. Dynamic Aperture • Large average injected beam power • 224 kW • Wiggler dominated dynamics leads to too small (dynamic) aperture for e+ ring: • acceptance approx. factor 2 too small • culprit: wiggler non-linearities • Needs additional study • R&D on wiggler to reduce non-linearities • introduction of octupoles into lattice • Do have factor 2 safety margin in e+ production • requires careful collimation in DR transfer line to reduce losses in ring ITRP Meeting - RAL - 28.02.04

  44. Emittance Control • BPM and H+V steerer at each quadrupole (800) • Skew windings on every sextupole (300) • Combined orbit and dispersion correction with steerer • Skew correction linear response approach ITRP Meeting - RAL - 28.02.04

  45. Emittance Control Simulated alignment errors BPM resolution critical for required level of dispersion control for all LC DR ITRP Meeting - RAL - 28.02.04

  46. Emittance Control (simulation) Simulation of vertical emittance after application of orbit tuning algorithm 100 random alignment seeds 88% of machines below achieved <14 nm (goal) space-charge coupling bumps not included (vertical correction only). goal ITRP Meeting - RAL - 28.02.04

  47. Emittance Stability • Quadrupole vibration of 350 nm (RMS) gives 10% increase in emittance • Slow drifts [based on ATL model] indicate the following corrections will be needed: • closed-orbit correction every 2 minutes • dispersion correction every 11 hours ITRP Meeting - RAL - 28.02.04

  48. Collective Effects • IBS no issue because of high energy • Coupled Bunch • HOM‘s suppresed by SC cavities • resistive wall damped with feedback • ion trapping requires P1×10-10mbarin straight sections (nb: no synch. rad.) • more studies needed for fast beam ion instability (common problem) • e-cloud seems OK because of bunch distance [input from LHC] e+/e- e+/e- e+/e- e- e- e+ ITRP Meeting - RAL - 28.02.04

  49. Stray Field Problems • Time varying stray fields at beginning of linac beam pulse from Klystron turn-on • Measured to be > 1 mT • Effect checked by simulating 5 mT m at each klystron position (every 48 m) ITRP Meeting - RAL - 28.02.04

  50. Stray Field Problems • Leads to variation of closed orbit • dispersion at extraction • Blow-up of projected emittance • Fast correction needed: • dispersion correction (difficult) • fast turn-by-turn distributed orbit feedback [accuracy 75 mm RMS] ITRP Meeting - RAL - 28.02.04

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