e + e - Linear Collider

Transcription

1 e + e - Linear Collider Disclaimer This talk was lifted from an earlier version of a lecture by N.Walker Eckhard Elsen DESY DESY Summer Student Lecture 3 rd August

2 Disclaimer II Talk is largely based on TESLA design which meanwhile evolved into the design of the International Linear Collider (ILC) some reference will be made to the specifics of the ILC (The TESLA numbers are only indicative of the foreseen ILC performance). 2

3 The TESLA Linear Collider Technical Design Report (TDR) published in March GeV LC based on SCRF technology Chosen as the basis for the International Linear Collider 3

4 Energy Frontier + ee Colliders ILC LEP at CERN, CH Ecm = 180 GeV PRF = 30 MW 4

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 radius ρ: Energy loss must be replaced by RF system 5

6 Cost Scaling $$ Linear Costs: (tunnel, magnets etc) $ lin ρ RF costs: $ RF ΔE E 4 /ρ Optimum at $ lin = $ RF Thus optimised cost ($ lin +$ RF ) scales as E 2 6

7 The Bottom Line $$$ 7

8 The Bottom Line $$$ 8

9 The Bottom Line $$$ 9

10 Solution: Linear Collider No Bends, but lots of RF! bang! e + e - 15 km For a E cm = 1 TeV machine: Effective gradient G = 500 GV / 15 km = 34 MV/m Note: for LC, $ tot E 10

11 The TESLA linear collider Now a separate project 11

12 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 clashingbeam 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

13 A Little History ( ) SLC (SLAC, ) NLCTA (SLAC, 1991-) TTF (DESY, ) ATF (KEK, 1991-) FFTB (SLAC, ) SBTF (DESY, ) CLIC CTF1,2,3 (CERN, 1994-) FLASH (DESY, 2006-) Over 18 Years of Linear Collider R&D 13

14 The Luminosity Issue particles per repetition bunch rate No. bunches in bunch train Beam-beam enhancement factor (pinch effect) LEP f rep = 40 khz beam cross-section at Interaction Point (IP) LC LEP: f rep = µm 2 Hz! LC: nm 2 14

15 The Luminosity Issue Requirements for Next Generation LC: L cm 2 s 1 E cm = TeV 15

16 The Luminosity Issue P beam L = 1 4πE cm (E cm f rep n b N) N σ x 1 σ y H D Requirements for Next Generation LC: L cm 2 s 1 E cm = TeV 16

17 The Luminosity Issue Efficiency L = 1 4π ηp RF E cm N σ x 1 σ y H D Beamstrahlung (energy loss δe/e) Requirements for Next Generation LC: L cm 2 s 1 E cm = TeV 17

18 Beamstrahlung e + RMS Energy Loss for a flat beam (σ x» σ y ) 18

19 Beamstrahlung e γγ e + γ hard γs radiated by intense electric field of approaching beam = Beamstrahlung RMS Energy Loss for a flat beam (σ x» σ y ) δ BS E cm σ z ( N σ x ) 2 19

20 Luminosity re-visited IP focusing & Emittance L ηp RF E 3/2 cm ( δ BS σ z ) 1/2 1 σ y H D Requirements for Next Generation LC: L cm 2 s 1 E cm = TeV 20

21 Emittance and Strong Focusing σ y * ε y β y * = 5 nm = m = 0.4 mm 21

22 Limit on β* σ z y β* = depth of focus reasonable lower limit for β* is bunch length σ z Thus set β* = σ z s IP (s = 0) 22

23 Final Luminosity Scaling Law High Beam Power Superconducting RF Small IP vertical Technology beam size High current (n b N) High efficiency (P RF P beam ) L ηp L ηp RF ( δ RF ( δ BS ) 1/2 BS E E ɛ ) 1/2 cm y ɛ y Small emittance ε y Strong focusing (small β* y ) 23

24 TESLA superconducting 9-cell Niobium cavity operated at 2 K 24

25 The Superconducting Advantage Low RF losses in resonator walls (Q compared to Cu 10 4 ) high efficiency η AC beam long beam pulses (many bunches) low RF peak power large bunch spacing allowing feedback correction within bunch train. 25

26 The Superconducting Advantage Low-frequency accelerating structures 1.3 GHz (for Cu 6-30 GHz) very small wakefields relaxed alignment tolerances high beam stability 26

27 Wakefields Just Ohm s Law: The TESLA concept: Put a large current through a low impedance 27

28 Wakefields (alignment tolerances) Misplaced cavity Banana 28

29 Wakefields (alignment tolerances) Transverse Wakefield Kick f Ratio of deflecting wakefield to accelerating field for 1mm offset TESLA C-band X-band CLIC 29

30 The TESLA Test Facility (TTF) Cavity strings are prepared and assembled in ultra-clean room environment at TTF 30

31 The TESLA Test Facility (TTF) TTF Test Linac constructed from completed Cryomodules now user facility: FLASH 31

32 The TESLA Linac Damping Ring Cryomodule B M A E Cavities 32

33 The TESLA Linac ~1m 1 9-cell 1.3GHz Niobium Cavity 33

34 The TESLA Linac ~16m 12 9-cell 1.3GHz Niobium Cavity 1 Cryomodule 34

35 The TESLA Linac K 36 9-cell 1.3GHz Niobium Cavity 3 Cryomodule 1 10MW Multi-Beam Klystron 35

36 The TESLA Linac Per Linac (Ecm = 500 GeV): 10,296 Cavities 858 Cryomodules 286 Klystrons Gradient: 23.4 MV/m (inc. 2% overhead) LENGTH 14.4km (fill factor: 74%) 36

37 Cryoplants Each linac divided into 6 Cryo-units (~140 cryomodules) 7 refrigeration (liquid He) plants housed in 7 surface halls (~5km) 37

38 Cryohalls Refrigerators LINAC tunnel 38

39 10MW Multibeam Klystron Design power and pulse length (1.5ms) at 65% efficiency reached at TTF The Thomson TH1801 multibeam klystron 39

40 10MW Multibeam Klystron Design power and pulse length (1.5ms) at 65% efficiency reached at TTF The Thomson TH1801 cathode 40

41 The LINAC is only one part of a linear collider! The SC linac can: Efficiently accelerate a high charge to high energies (high RF beam power transfer efficiency) Preserve the required small bunch volumes (small emittance) because of low wakefields Has relatively relaxed tolerances 41

42 The LINAC is only one part of a linear collider! BUT how do we: Produce the electron charge? Produce the positron charge? Make small emittance beams? Focus the beam down to 5nm at the IP? 42

43 The LINAC is only one part of a linear collider! 43

44 The LINAC is only one part of a linear collider! 44

45 The LINAC is only one part of a linear collider! 45

46 The LINAC is only one part of a linear collider! 46

47 The LINAC is only one part of a linear collider! 47

48 The LINAC is only one part of a linear collider! 48

49 The LINAC is only one part of a linear collider! 49

50 Machine Overview (TESLA) 50

51 Electron Sources Laser-driven photo-injectors 51

52 Positron Source Replacing planar undulator with HELICAL undulator gives possibility of POLARISED POSITRONS 52

53 Small Emittances Require normalised emittances of γε x = 10-5 m γε y = m Thermionic guns (ε~10-5 ) and state-of-theart RF guns (ε~10-6 ) not good enough Emittance Damping Ring required 53

54 Damping Rings ring in which the bunch train is stored for T ~200 ms ε x,y is damped down due to synchrotron radiation effects: initial emittance (~0.01m for e + ) final emittance equilibrium emittance damping time ~ 28 ms 54

55 How β-damping works y not changed by photon δp replaced by RF such that Δp z = δp. since y = dy/ds = p y /p z, we have a reduction in amplitude: δy = δp y Must take average over all β-phases: where and hence LEP: E ~ 135 GeV, P γ ~ 70 GeV/s, τ D ~ 4 s 55

56 How β-damping works y not changed by photon δp replaced by RF such that Δp z = δp. since y = dy/ds = p y /p z, we have a reduction in amplitude: δy = δp y Must take average over all β-phases: where and hence LEP: E cm ~ 135 GeV, P γ ~ GeV/s, τ D ~ 28 ms 56

57 Damping Rings for LC Typically E 3-5 GeV B bend = 0.2 T ρ m <P γ > = 240 GeV/s [330 kv/turn] hence τ D 28 ms Note: ε eq E 2 /ρ 57

58 TESLA Damping Rings Long pulse: 950ms c = 285km!! Compress bunch train into 18km ring Minimum circumference set by speed of ejection/injection kicker (~20ns) Unique dog-bone design: 90% of circumference in linac tunnel. 58

59 Limits on ε eq Horizontal emittance defined by lattice (presence of dispersion in x-plane leads to so-called anti-damping): theoretical vertical emittance limited by space charge intra-beam scattering (IBS) In practice, ε y limited by magnet alignment errors [cross plane coupling] typical vertical alignment tolerance: Δy 30 µm requires beam-based alignment techniques! 59

60 Damping Rings Dogbone Straight Sections Dogbone Arc Tunnel 60

61 Cryohall and dogbone tunnel 61

62 longitudinal phase space Bunch Compression bunch length from ring ~ few mm required at IP µm dispersive section 62

63 Bunch Compression Damping ring Bunch length After compression (300 µm) 6 mm 63

64 Bunch Compression 2.8% ΔE/E Damping ring (~ppm) After compression 5 GeV: 2.8% 250 GeV:

65 Final Focus System for small β* Optical telescope required to strongly demagnify the beam (M x 1/100, M y 1/500) Strong focusing leads to unacceptable chromatic aberrations [non-linear optics] Require 2nd-order optical correction uses non-linear elements (sextupoles) 65

66 Beam Delivery System Positron Production System 66

67 Beam Delivery System Collimation System 67

68 Beam Delivery System Final Focus System 68

69 Beam Delivery System σ y = 3 µm Demagnification: 636 σ y = 5 nm 69

70 Stability Tiny (emittance) beams Tight component tolerances Field quality Alignment Vibration and Ground Motion issues Active stabilisation Feedback systems Linear Collider will be Fly By Wire 70

71 Stability: some numbers Cavity alignment (RMS): 500 µm Linac magnets: 100 nm BDS magnets: nm Final lens : ~ nm!!! Parallel-to- Point focusing 71

72 IP Fast (orbit) Feedback Long bunch train: 2820 bunches t b = 337 ns Beam-beam kick 72

73 IP Fast (orbit) Feedback Systems successfully tested at TTF 73

74 Banana Effect : Beam-Beam Simulation Instability driven by vertical beam profile distortion Strong for high disruption Distortion caused by transverse wakefields and quad offset only a few percent emittance growth Tuning can remove static part Nominal TESLA Beam Parameters + y-z correlation (equivalent to few % projected emittance growth) Beam centroids head on 74

75 Long Term Stability hour 1 day 1 minute 10 days No Feedback IP Feedback IP Feedback & slow orbit feedback 75

76 Damping ring to IP Simulations bunch compressor linac BDS Gaussian bunch from DR Ideal machine Change of bunch compressor phase by ±2.5 deg (powerful knob at the SLC) This is just an example what we intend to study 76

77 Timeline 1995 SLAC produced X-Band design report (ZDR) 2001 TESLA TDR Published US Snowmass HEP Workshop: World-Wide Consensus 2003 KEK (Japan) X-Band GLC TDR Published 2004 International Technology Recommendation Panel Decision (X-Band or TESLA?) SCRF 1 st ILC Workshop, KEK Japan, November Formation of Global Design Initiative and Regional Design Teams, Baseline defined 2 nd ILC Workshop, Snowmass Colorado, USA, August Reference Design Report International TDR with full costs Site selection, start of construction Begin e + e - physics 77

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