Laser Based Beam Diagnostics

Transcription

1 Laser Based Beam Diagnostics G A Blair BIW08, Lake Tahoe 6 th May 008 Introduction Emittance measurement Laser optics; Gaussian beams Laser-wire principle Interferometric techniques PETRA and ATF laserwire projects Signal etraction Use in H- machines Summary

2 Lasers in Beam Diagnostics Transverse emittance (LW) Polarimetry Energy Spectrometry (Compton end-point) Longitudinal emittance (LW + EO) H - measurements (LW, stripping) Isotope selection (ecitation) Others? See talk by J.Van Tilborg

3 General Scheme Light transport laser α Final Focus Laser Diagnostics Vacuum window Detector α α α = π for LW Particle beam α 0 for polarimeter 3

4 Luminosity L = f n1n 4πσ σ y Big L small ε ε = πσ β Need 4 scanners per BDS arm 4

5 Linac ILC 5

6 Laser wire : Measurement precision Phys. Rev. ST Accel. Beams 10, (007) I. Agapov, G. B., M. Woodley The Goal: Beam Matri Reconstruction NOTE: Rapid improvement with better σ y resolution Reconstructed emittance of one ILC train using 5% error on σ y Assumes a 4d diagnostics section With 50% random mismatch of initial optical functions The true emittance is µm µrad 6

7 Skew Correction y u φ 1 σ φ = optimal tan σ y at ILC σ y σ u σ ILC LW Locations E b = 50 GeV Error on coupling term: σ (µm) 39.9 σ y (µm).83 φ opt ( ) 86 σ u (µm) 3.99 δ y δσ u = σ 4 σ y σ u δσ + σ δσ y + σ y

8 8 Mawell Wave Equation 0 ),,, ( 1 = t z y E t c ) ( ), ( ),,, ( ct ik e r u t z y E = z y r + = Mawell: Write: where: = r u r r r u u ik 1 (assume aial symmetry)

9 9 Paraial Wave Equation = r u r r r u u ik 1 Drop this term; valid provided divergence of any ray is less than about 1 radian. = r u r r r u ik 1

10 10 Gaussian Solution ( ) ( ) ( ) = 0 ep, r I r u σ πσ ( ) ( ) ( ) ( ) r i e r I r u, 0 4 ep 1, φ σ σ π = Attempt to find a solution of the form: so: Needed to conserve energy ( ) + = 0 1 R σ σ λ σ π 0 = 4 R ( ) ( ) R r r R 1 tan, λ π φ = ( ) R R + = Rayleigh range

11 Classical approimation 99% of energy within D D σ( f ) D = πσ ( f ) θ 0 =0 f θ0 = tan 1 σ ( f ) f

12 r / R wave fronts D σ( f ) θ 0 R() r σ 0 σ ( f ) f σ 0 R = λ 4πσ 0 f σ 0 = 0.5λ = 0. 5λf D #

13 General Spherical Gaussian Waves Comple radius: 1 q = 1 + im ( s) R( s) ( ) λ 4πσ s ( s + ) ( s + ) = A C B D ( s) ( ) s ' optics A B C D ' q ( s + s) = Aq Cq ( s) + B ( s) + D 13

14 Practical Considerations Obtaining theoretical values of D for a given optical layout is hard (alignment, stability, ). In the following, assume a practical factor; to be conservative, k p ~ is chosen in the following: σ = kpσ 0 σ λf l l # Beware of confusing conventions: e w σ laser spot size 14

15 Higher Order Modes N_Comptons per bunch TM Delta_y (microns) Their presence increases the effective emittance of the laser σ σ pure TM 00 = λ M 4π λ 4π (M >1) property of a realistic laser 15

16 Incoming beam is now outside aperture M >1 MD D θ 0 σ 0 M =1 Mθ 0 Mσ 0 f

17 M >1 Rematched upstream optics MD D θ 0 Mθ 0 M σ0 Mσ 0 M >1 f

18 Important Lesson In any practical environment, including light transport, alignment, finite aberrations etc., it would be unwise to trust the formulae. Measurement of the light distribution near the focus is essential. Absolute measurement with knife-edge scans (see below). Relative frequent measurements of an imagepoint with a CCD camera. 18

19 Spotsize measurement Achromat Linos Halo f150/.16 H Window Blade... 5mm 6mm 7mm mm 15 mm Blade moved with stepping motor Intensity measured as a function of position Photodiode 19

20 Spotsize results for test laser Single scan Sigma vs z Fit to function: σ ( z) = σ 0 1+ M λ( z z π (σ ) 0 0 ) 0

21 Laserwire 1

22 Measuring the Beam Profile Traditional method is to sweep a solid wire across the beam. Measure background vs relative position of wire and beam. Micron-scale precision required for LC Solid wires would not stand the intense beams of the LC Solid wires could ablate, harming SC surfaces nearby. So: replace wire with a laser beam. Count Comptons downstream.

23 R σ l Summary of Laser Conventions laser beam σ ey σ e I l (, y, z) For TM 00 laser mode: = I 0 πσ l f R 1 y + z ep ( ) σ f ( ) l R σ l electron bunch f R ( ) 1 = + R σ l = M λ R = f # 4π M λ f# Note: here, f # includes an M-factor

24 Scan of an ILC Train of Bunches σ L =cτ L α train σ e σ e σ scan α J σ e N train bunches σ e (1 + s train ) Not to scale!

25 Machine Contributions to the Errors σ e = [ σ scan ( ) α σ ( ηδ ) J e E 1 ] Bunch Jitter δσ e σ e σ BPM 5 10 α J nm Dispersion BPM resolution of 0 nm may be required Assuming η can be measured to 0.1%, then η must be kept < ~ 1mm δσ σ e.3 δη [ η / mm] η e 5

26 Compton Statistics N Detected = 11ξ 1 πσ m ep 1 σ y m Approimate should use full overlap integral (as done below ) Where : ξ = η Pl MW e-bunch occupancy Ne 10 λ 53 nm Compton sec factor ( ω) f 0. det µ 10 m Laser peak power Laser wavelength Detector efficiency (assume Cherenkov system) 6

27 Laserwires at ILC/CLIC 7

28 TM 00 Mode Overlap Integrals N_comptons per bunch σ σ ey e = 1µ m, = 10µ m N_comtpons per bunch σ σ ey e = 1µ m, = 100µ m Delta_y (microns) Delta_y (microns) Rayleigh Effects obvious Main Errors: Statistical error from fit ~ ξ -1/ Normalisation error (instantaneous value of ξ) assume ~1% for now. Fluctuations of laser M assume M known to ~1% Laser pointing jitter ψ δσ e σ e ψ µ rad δψ /10% ψ δσ e σ e # M λf σ e δm M 8

29 Laser Requirements Wavelength Mode Quality Peak Power Average power Pulse length Synchronisation Pointing stability 53 nm MW 0.6 W ps 0.3 ps 10 µ rad ILC-spec laser is being developed at JAI@Oford based on fiber amplification. L. Corner et al 9

30 E_stat Statistical Error From 19-point scan TM 00 mode Optimal f-num for λ= 53nm Then improve M determination f- lens about to be installed at ATF f-number Relative Errors E_las Error resulting from 5% M change f_num ATF LW; aiming initially at f ; eventually f 1? 30

31 Towards a 1 µm LW preliminary Resultant errors/10-3 Goals/assumptions Wavelength 66 nm Mode Quality 1.3 Peak Power 0 MW FF f-number 1.5 Pointing stability 10 µrad M resolution 1% Normalisation (ξ) % Beam Jitter 0.5σ BPM Resolution 0 nm Energy spec. res 10-4 E ξ.5 E point E jitter. 5.0 E stat 4.5 E M Total Error Final fit, including dispersion Could be used for η measurement E η 31

32 Alternative Scan Mode R&D currently investigating ultra-fast scanning (~100 khz) using Electro-optic techniques Alternative: Keep laser beam fied and use natural beam jitter plus accurate BPM measurements bunch-by-bunch. Needs the assumption that bunches are pure-gaussian For one train, a statistical resolution of order 0.3% may be possible Single-bunch fit errors for σ 1 µ m, σ = 10µ m ey = e Error on Sigma_ey (%) Error on Sigma_ey (%) Relative Bunch Jitter (alpha_j) Beam jitter fied at 0.5σ BPM Resolution (um) BPM resolution fied at 100 nm 3

33 TM 01 Mode used for scans N_Comptons per bunch Delta_y (microns) TM01 gives some advantage for larger spot-sizes Relative Statistical Error E_stat TM 01 E stat TM 00 Relative M error (E_M) TM 00 TM 01 E M sigma_ey (microns) sigma_ey (microns)

34 Laser propagation Interferometric methods θ y N γ σ ey Small σ ey Large d d y 34

35 Shintake Monitor N γ ( ) [ ( )] k y cosθ ep σ 1+ cos y k y y k y = π θ sin λ FFTB measured σ y =37 nm Balakin et al PRL 74, 479 (1995) 35

36 LW Eperimental Programme: Started at CTF PETRAII ATF SNS FETS 36

37 Laser-wire People BESSY: T. Kamps CERN: I. Agapov DESY : E. Elsen, V. Gharibyan, H. C. Lewin, F. Poirier, S. Schreiber, K. Wittenburg, K. Balewski JAI@Oford: B. Foster, N. Delerue, L. Corner, D. Howell, L. Nevay, M. Newman, R. Senanayake, R. Walczak JAI@RHUL: A. Aryshev, G. Blair, S. Boogert, G. Boorman, A. Bosco, L. Deacon, P. Karataev, S. Malton, M. Price, KEK:, H. Hayano, K. Kubo, N. Terunuma, J. Urakawa SLAC: A. Brachmann, J. Frisch, M. Woodley FNAL: M. Ross 37

38 Took part in CTF laser-wire: Problems with backgrounds (CTF energy 45 MeV) But we learnt a lot! 38

39 PETRA LW Routine scans of two-dimensions were achieved PETRAII programme now finished; preparing for PETRAIII Fast scanning system with 130kHz laser at RHUL planned Collaborating with DESY on fast DAQ Look forward to installation in new location for PETRAIII this year 39

40 Optics built and tested at RHUL 40

41 PETRA interaction chamber viewport BPM 41

42 Vertical bread-board design; D scanning: 4

43 43

44 Laser Transverse Profile 44

45 Q-switched, unseeded Laser Longitudinal Profile Eample of a single shot measurement of the profile 500 ps window, resolution 5 ps 60 ps 66 ps 45

46 Laser Temporal Profile Unseeded Seeded 46

47 Laser Pointing Jitter Translates to longitudinal so not relevant Translates into a 1.6 µm jitter at the LW IP for Both horizontal and vertical scans 47

48 PETRAII Results: Vertical Convoluted profile (takes<50s) Fit to Rayleigh formula 48

49 PETRAII Results: Horizontal Convoluted profile Fit to Rayleigh formula NIM A accepted 49

50 LW in the ATF Etraction Line pulsed laser-wire location 50

51 ATF Parameters Beam energy : Beam intensity single bunch operation : multi bunch operation : Beam reputation : X emittance (etrapolated to 0 intensity) : Y emittance (etrapolated to 0 intensity) : Typical beam size : 1.8 GeV electrons/bunch electrons/bunch 0 bunch Hz rad.m (at 1.8GeV) rad.m (at 1.8GeV) 70µm 7µm (rms horizontal rms vertical) 51

52 ATF Laser-Wire Aiming at micron-scale vertical scans 5

53 ATF/ATF Laser-wire At ATF, we will aim to measure micron-scale electron spotsizes with green (53 nm) light. Two locations identified for first stage (more stages later) 1) 0.75m upstream of QD18X magnet ) 1m downstream of QF19X magnet Nominal ATF optics LW-IP (1) LW-IP () σ = 38.9 µm σ = µm σ y = 7.74 µm σ y = 7.94 µm ATF LW-test optics P. Karataev LW-IP (1) LW-IP () σ = 0.43 µm σ = 0 µm σ y = 0.9 µm σ y = 1.14 µm Ideal testing ground for ILC BDS Laser-wire system 53

54 Prelminary ATF LW Results Detector Signal (a.u.) y (micrometres) Laser M not yet optimised Laser astigmatism not yet corrected Rayleigh Range effects well described by fit 54

55 Lens Design + Tests M. Newman, D. Howell et al. Low f-number Radiation hard Spectral width? Designs for f-1 optics are currently being studied, including: Aspheric doublet Vacuum window 55 N. Delerue et al.

56 56

57 Signal Etraction PETRA Downstream of Laser-wire Compton Spectrum At peak, ~1000 photons For 7 GeV Petra; Mean E = 0.5 GeV per photon 57

58 PETRA Detector Requirements for detector material short decay time (avoid pile up) short radiation length small Moliere radius Cuboid detector crystals made of PbWO4 33 matri of mm crystals Energy resolution better than 5% 58

59 PETRA Detector Response DESY test beam Calorimeter BPM Online system 59

60 ATF Detector 60

61 Geant4 Simulation of Beamlines

62 BDSIM BDSIM is a geant4 based programme for detailed simulation of BDS. Fast machine-style tracking combined with full shower simulation and physics processes. Original BDSIM: CLIC-Note-509 (00) Backgrounds from Halo loss along BDS, especially in collimation region. Laserwire process included (among many others) 6

63 Compton Spectrum at CLIC Energy Spectrum at 1.5 TeV: electrons photons Both electrons and photons need to be tracked downstream. 63

64 Laserwire Simulation (CLIC) 64

65 65

66 Laser-wires in the ILC LINAC? To etract signal: - Quadrupole fields with scattered electrons. - Earth s curvature for photons? Photon shower emerges ~650 m downstream laser-wire in warm section? 66

67 Linac Module Steel supports Aluminium heat shields Titanium pipe Liquid Helium Length 1.5m 8 sets of 9 cavities Steel vessel shell Niobium cavities Invar rod Titanium pipe around cavity (not shown) 67 L. Deacon

68 H - Neutralisation H The process has threshold energy ~0.75 ev + γ H 0 + e so it can be driven by a Nd:YAG laser operating at 1060 nm. A focussed laser beam can thus be used to Measure emittance of H- beam Enable proton production by laser-induced stripping. All the previous technical issues apply 68

69 Schematic Operation Front End Test Stand (RAL) electrons + neutrals SNS (detect electrons) 69

70 SNS laser-wire system Laser e - detector dipole to etract e - 70

71 Fiber Laser R&D Ecellent pointing stability Reliability Spectral width? (important for final focus optics) A mini-workshop on this topic would be timely Eperimental arrangement seed 1030 nm, 800 fs <1 J µ spectrometer/ power meter seed 1053 nm, 5 ps.5 nj PM double clad fibre L. Corner dichroic mirror aspheric lens aspheric lens dichroic mirror pump 976 nm 0-0W 71

72 Summary Very active + international programme in laser-based diagnostics: Hardware Optics design Advanced lasers Emittance etraction techniques Proton machines Data taking + analysis Simulation Important effects: Laser pointing M monitoring Low-f optics Fast scanning High precision BPMs Signal etraction is not trivial Full simulations in progress. 7