Linac-based light sources. Ivan Bazarov Cornell University

Transcription

1 Linac-based light sources Ivan Bazarov Cornell University

2 Talking points Synchrotron radiation sources of today Motivation for a linac based source Physics of high-brightness electron injectors Energy Recovery Linac (ERL) Phase 1 Summary May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 2

3 Free e as medium Relativistic free electrons the only medium for tunable light production in widest spectral range Hard x-ray range is the subject of this talk May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 3

4 Free e as medium hard x-ray Relativistic free electrons the only medium for tunable light production in widest spectral range Hard x-ray range is the subject of this talk May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 3

5 X-ray brightness Average brightness: measure of transversely coherent flux Peak brightness: proportional to the number of photons per coherence volume the photon degeneracy F c = B avg 3 λ λ 1 c = B peak May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/ λ 2 2 λ c

6 SR/XFEL/ERL SR XFEL ERL efficient avg. brightness workhorse technology peak brightness short pulse new user-base avg. brightness existing user-base XFEL as a subset short pulse May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 5

7 Exp1: diff. imaging of proteins R. Neutze, et al., Nature, 406, 752 (2000) Fienup s algorithm Briefly: calculations were done for T4 lysozyme (diameter 32 Å, N C ~ 1000); flux X-rays/Å 2 with ~ 2000 primary ionization events; elastically scattered ~ 200 photons. If pulse is sufficiently short (<10 fs), lysozyme nanocrystal will scatter to <2Å resolution. Key feature: coherent x-rays May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 6

8 Radiation damage: biomaterials Shen, Bazarov, Thibault, J. Sync. Rad., Vol. 11 (2004) 432 May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 7

9 Exp2: fs chem. reaction movie Broad class of pump-probe experiments providing structural (core e s) conformational changes in the initial stages (mol. vibrational timescale 10 s fs) of photo-induced reactions Mb Time-resolved Laue Crystallography (Phil Anfinrud, NIH) 1 mm Mb*CO May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 8

10 Storage ring Equilibrium Quantum Excitation vs. Radiative Damping E ph ρ = p eb d σ dt 2 E & 2 ~ N phe ph Emittance (hor.), Energy Spread, Bunch Length Tighter focusing (higher tune) stronger 6-poles for chromaticity correction smaller dynamic aperture & lifetime May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 9

11 Why linac? Beam never in equilibrium 6D phase space (x,p x,y,p y,e,t) defined by the electron source Longitudinal phase space (E,t) gymnastics : exchange momentum spread for shorter bunches (3 orders of magnitude) Colder intense beams (transverse coherence) < 20 fs pulses achievable May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 10

12 Emittance basics What exactly is emittance? (2D projected) E.g. rms emittance of laser Gaussian beam ε = λ/4π (diffraction limit) Adiabatic damping of geometric emittance May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 11

13 Diffraction limited e-beam In properly tuned undulator x-ray phase space is convolution of e-beam with diff. limit x ε elec electron phase space in undulator x x x-rays phase space x ε x-rays ~ ε elec +ε ph Goal: for 1 Angstrom ε x ~ λ/4π = 8 pm geometric, or ε nx = 0.08 µm if energy is 5GeV E.g. best storage ring performance as of today: ε x / ε y = 3000 / 15 pm May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 12

14 Energy recovery concept hν production Ib, dec V c, V g I b,acc µ-wave source e- source RF structure same-cell May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 13

15 Energy recovery concept 2 hν production Ib, dec V c, V g I b,acc 2 µ-wave source e- source RF structure same-cell May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 14

16 Energy recovery concept 3 hν production Ib, dec V c, V g I b,acc 3 µ-wave source e- source RF structure same-cell extends linac operation to high average currents reduces beam dump energy May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 15

17 ( ε λ / 4π )( ε + λ / 4π ) x E.g. ESRF I avg = 200 ma, ε x,y = 4 nm / 0.02 nm I + y ERL: e-source limited ERL I avg = 100 ma, ε x,y = 0.1 nm (ε nx,y = 1 µm if 5GeV) is superior + bonus short bunches Caveat: state-of-the-art in high brightness injectors I avg 10 ma; ε n 10 µm (ε n ~ 2 µm for pulsed) Overall missing 3 orders of magnitude in beam brightness May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 16

18 ERL Phase1 Energy Max Avg. Current Charge / bunch Emittance (norm.) 100 MeV 100 ma pc 2 µm@77 pc Injection Energy 5 15 MeV E Q Bunch Length ps Eds. Gruner, Tigner; Bazarov, Belomestnykh, Bilderback, Finkelstein, Fontes, Krafft, Merminga, Padamsee, Shen, Rogers, Sinclair, Talman, ERL prototype proposal to the NSF, 2001 May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 17

19 ERL Phase11a Energy Max Avg. Current Charge / bunch Emittance (norm.) 100 MeV 100 ma pc 2 µm@77 pc Injection Energy 5 15 MeV E Q Bunch Length ps Eds. Gruner, Tigner; Bazarov, Belomestnykh, Bilderback, Finkelstein, Fontes, Krafft, Merminga, Padamsee, Shen, Rogers, Sinclair, Talman, ERL prototype proposal to the NSF, 2001 May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 17

20 Beam breakup: challenge y TM 110 y I th = e( R / Q) λ Q 2 p λ k r λ c R 12 sinω t λ r z x Bazarov, Krafft, Merminga, PAC (2001) 3347 E B below I th near I th above I th 1.5E E E+02 momentum [ev/c] 1.0E E E E E E E E E+00 momentum [ev/c] 1.5E E E E E E E E E E E+01 momentum [ev/c] 1.5E E E E E E E- 0.00E E E E E E E E E E E E+02 position [m] position [m] position [m] Highest current recirculated in SRF linac was 4.5 ma in year 2001 May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 18

21 Beam breakup: figured out BBU code & theory developed for ERLs Bazarov, Hoffstaetter, EPAC (2004) 2194 Hoffstaetter, Bazarov, Phys. Rev. ST: AB 7, (2004) Benchmarked with good accuracy in JLAB FEL Douglas, Jordan, Merminga, Pozdeyev, Tennant, Wang, Smith, Simrock, Bazarov, Hoffstaetter, Phys. Rev. ST: AB 9, (2006) Various suppression techniques successfully tested trouble cryomodule JLAB FEL May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 19

22 Phase 1a 0.6 MW dump merger quad telescope booster buncher gun diagnostics chicane laser entrance HV DC gun based photo-injector up to 100 ma average current, 5-15 MeV beam energy norm. rms emittance 1 µm at 77 pc/bunch rms bunch length 0.6 mm, energy spread 0.1% Bazarov, Sinclair, PAC2003, IEEE (2003) 2062 May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 20

23 Photo-gun Cathode Cs:GaAs Laser rep. rate 1.3 GHz Wavelength 520 nm Duration (rms) 10 ps Vacuum <10 12 Torr goal: 750 kv hν cathode field laser spot space charge limit q = 2 4πε 0E cath σ x May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 21

24 Physics of e- photoguns Two main limiting mechanisms: Phase space scrambling due to nonlinear space charge 3D Gaussian initial distribution Optimal initial distribution ε n,x = 1.7 µm ε n,x = 0.13 µm Photocathode thermal emittance ε = σ n, th x, y kt mc 2 transverse temperature of photoemitted electrons May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 22

25 Photocathode requirements (1) photon excites electron to a higher-energy state; (2) electron-phonon scattering (~0.05 ev/collision); (3) escape with kinetic energy in excess to E vac Ideal photocathode: E th kt 25 mev response time 1 ps high QE 10% kt for GaAs General trend λ Q.E. kt τ May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 23

26 Superlattice photocathode? A B C Q.E. small large large kt small large small equipped to accurately (~mev) measure transverse temp. of e at different wavelengths photoemission temporal response resolution (~ps) Gun development lab in Wilson May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 24

27 May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 25 Space charge basics ] [ R c cb E m e R K R th n c s c s r f + = + γ ε β γ θ && R R E m e p c s r ) ( ω γ = Beam envelope equation: m n e p γ ε ω =

28 Beam temperature Diffraction limited beam at 1Å ε x = 8pm at 5GeV ε nx = 0.08 µm 10 MV/m gradient σ laser = 0.3 mm Transverse temp. needed kt = 25 mev focusing potential beam self-potential kt beam F 2 ~ std σ x R 1 250ε 0 eq σ z tail I head Equilibrium kt beam ~ 100 ev!! May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 26

29 S.c. compensation concept Serafini PRE 55, 7565 focusing s.c. emittance Needle beam: linearize equilibrium flow condition for slice oscillation frequency current independent May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 27

30 S.c. compensation concept Serafini PRE 55, 7565 focusing s.c. emittance Needle beam: linearize equilibrium flow condition for slice oscillation frequency current independent May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 27

31 Design approaches Cut the number of decision variables to some reasonable number (2-4) perhaps by using a simplified theoretical model to guide you in this choice Large regions of parameter space remain unexplored Optimize the injector varying the remaining variables with the help of a space-charge code to meet a fixed set of beam parameters (e.g. emittance at a certain bunch charge and a certain length) One ends up with a single-point design without capitalizing on beneficial trade-offs that are present in the system Primary challenge in exploring the full parameter space is computational speed May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 28

32 Doing it faster work harder work smarter get help processor speed algorithms parallel processing Solution: use parallel MOGA MultiObjective Genetic Algorithm throw in all your design variables map out whole Pareto front, i.e. obtain multiple designs all of which are optimal use realistic injector mode Master Genetic operators: selection, cross-over, etc. Slaves Objectives evaluation May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 29

33 Multi-objective optimization Bazarov, Sinclair, Phys. Rev. ST:AB 8, (2005) Bazarov, Padamsee, PAC (2005) 2188 Vilfredo Pareto, May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 30

34 Evolving into optimal injector design Parallel Multiobjective Evolutionary Algorithm May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 31

35 Injector decision variables Fields: DC Gun Voltage ( kv) 2 Solenoids Buncher SRF Cavities Gradient (5-13 MV/m) SRF Cavities Phase Bunch & Photocathode: E thermal Charge Positions: 2 Solenoids Buncher Cryomodule Laser Distribution: Spot size Pulse duration (10-30 ps rms) {tail, dip, ellipticity} 2 Total: dimensional parameter space to explore May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 32

36 Optimization results Takes ~10 5 simulations E th = 35 mev (aka 780 nm) optimization problem: minimize emittance minimize bunch length maximize bunch charge May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 33

37 Closer look: 80 pc May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 34

38 Going full steam ahead Gun development lab: 750 kv gun with diagnostics line May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 35

39 Going full steam ahead May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 36

40 Going full steam ahead May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 37

41 Going full steam ahead May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 38

42 Going full steam ahead May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 39

43 Summary Linac-based source to put Cornell on the forefronts of synchrotron radiation science for many years to come (both spontaneous and stimulated SR) Key to feasibility of ERL is the very high current, high brightness electron source (in the works) Plenty of opportunities for interdisciplinary exchange (photocathodes, surface science, laser technology, beam diagnostics & instrumentation) May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 40

44 X-ray light source dev. team May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 41

45 Thank you! May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 42

46 Backup slides May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 43

47 Light sources worldwide About 70 light sources worldwide based on storage ring technology (VUV to hard X-rays), new ones are being built / designed >20 (small) FELs operational (far IR to VUV) 3 XFELs in construction / committed to, plus half a dozen in earlier stages of planning (soft to hard X-rays) 3 labs seriously consider building ERL as a hard X-ray light source May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 44

48 Linac based approach V b V c I g I b V g µ-wave tube e- source RF structure hν production May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 45

49 Linac based approach V b V c I g I b V g 2 2 µ-wave tube 2 e- source RF structure hν production 2 May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 46

50 Contributors to emittance Thermal (cathode) RF-induced Space charge Low thermal energy photocathodes Min laser spot size Max gun voltage Rapid acceleration adiabatic focusing and bunching Transverse laser shaping Temporal pulse shaping (fast emission photocathodes) short bunch length tight focus reduced field gradient helps here, neutral elsewhere helps here, may harm elsewhere May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 47

51 Pulse duration rms 21.5 ± 1.4 ps Spot size rms ± mm Charge 80 ± 5.8 pc Solenoid1 Bmax ± kg Solenoid2 Bmax ± kg Cavity1 phase ± 1.7 deg Cavity2 phase ± 2.0 deg Cavity3-5 phase ± 2.0 deg Buncher Emax 1.73 ± 0.04 MV/m Cavity1 Emax 15.4 ± 0.3 MV/m Cavity2 Emax 26.0 ± 0.5 MV/m Cavity3-5 Emax 27.0 ± 0.5 MV/m Q1_grad ± T/m Q2_grad ± T/m Q3_grad ± T/m Q4_grad ± T/m Optima steepness Virtual injector allows absolute control of parameters, real system with a dozen of sensitive parameters will not 100 random seeds (outliers removed) ave(ε x ) = 1.04 µm std(ε x ) = 0.52 µm ave(ε y ) = 0.95 µm std(ε y ) = 0.62 µm May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 48

52 Tolerances for optimum 10% increase in emittance (p-t-p) BunPhase 3.5 Cav1Phase 3.0 Ecav1 3.8% Lphase 2.4 B1 0.37% B2 0.85% Qbunch 3.7% Trms 8.0% Vgun 0.39% XYrms 2.4% May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 49

53 Three frontiers in source dev. Avg. brightness Short pulses (< ps) & peak brightness Compactness Compact X-ray analog of Livingston plot only 2! Hard x-rays Brightness May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 50

54 Three frontiers in source dev. Avg. brightness Short pulses (< ps) & peak brightness Compactness Compact X-ray analog of Livingston plot only 2! Hard x-rays Brightness May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 50

55 State-of-the-art: BOEING gun Photocathode Performance: Photosensitive Material: K 2 CsSb Multialkali Quantum Efficiency: 5% to 12% Peak Current: 45 to 132 amperes Cathode Lifetime: 1 to 10 hours Angle of Incidence: near normal incidence Gun Parameters: Cathode Gradient: 26 MV/meter Cavity Type: Water-cooled copper Number of cells: 4 RF Frequency: 433 x10 6 Hertz Final Energy: 5 MeV(4-cells) RF Power: 600 x10 3 Watts Duty Factor: 25%, 30 Hertz and 8.3 ms Laser Parameters: Micropulse Length: 53 ps, FWHM Micropulse Frequency: 27 x10 6 Hertz Macropulse Length: 10 ms Macropulse frequency: 30 Hertz Wavelength: 527 nm Cathode Spot Size: 3-5 mm FWHM Temporal and Transverse Distribution: gaussian, gaussian Micropulse Energy: 0.47 microjoule Energy Stability: 1% to 5% Pulse-to-pulse separation: 37 ns Micropulse Frequency: 27 x10 6 Hertz Gun Performance: Emittance (microns, RMS): 5 to 10 for 1 to 7 ncoulomb Charge: 1 to 7 ncoulomb Energy: 5 MeV Energy Spread: 100 to 150 kev 433 MHz RF Gun 32 ma avg. current λ RF /4 λ RF /2 May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 51

56 State-of-the-art: JLAB FEL inj. 500 kv (350) DC gun Cs:GaAs photocatode max current 9.1 ma, routine 5 ma best simulated normalized emittance 5 µm, measured 2 larger at 60 pc 9.1 ma max avg. current 10 kw IR FEL May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 52

57 ILC linac optimal front 10 bounded decision variables, 10 constraints May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 53

58 ILC linac optimization maximize Luminosity minimize Total Cost May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 54

59 Basics of sync. rad. production x ~ 1 γ z dp dω dp dω off-axis on axis ω ~ ω 1 N ω ω ω 1 ω 1 λ λ / γ p = p x e Θ 2 sin Θ z dp dω ω = ω 1 N back to lab frame λ p 1 2 λ n = (1 + 2 K + γ 2γ n λ ~ λ after a pin-hole 2 2 θ 2 1 n nn p (for fixed θ only!) ) in e frame hω May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 55

60 Radiation field from a single k th electron in a bunch: E = E exp( iωt k Coherent enhancement Flux in the central cone I N& ω I gn ( K) ph = παn gn(k) πα n ωn e e n Inefficient! only 10 8 e-beam power converted 0 k ) Radiation field from the whole bunch bunching factor (b.f.) N 1 e b. f. = exp( iωtk ) N e k = 1 λ Radiation Intensity: I = I b. f. Ne single electron 2 1) long bunch : b. f. ~ 1/ => = I incoherent (conventional) sync.rad N e I 0N e 2 2) short bunch or µ-bunching: b. f. 1 => I I N coherent (FELs) sync.rad ~ 0 e May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 56

61 XFEL Fully transverse coherent Prerequisites for e -bunch: diffraction-limited emittance peak current 3-5 ka energy spread 10 4 r r = ev E Intense relativistic electron bunch becomes effective gain medium (e.g. use seed / amplifier setup) May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 57 dε dt e e

62 May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 58 Light brightness Geometric optics Wave optics (Wigner distribution function) Gaussian laser beam σ x σ x = λ/4π Or from uncertainty principle (light emittance): π λ ε 4 / 2 1/ ') ( ) ( ', ' 2, / ) ( ) ( = = = k x std x std kx px p p std x std x x x h h π λ ε 4, = diff x ϕ ϕ r r r r ) ;, ( xd d F d z x B = y ik x x e z y x E z y x E y d T c d z x B r r r r r r r h r r + = ϕ ω ω λ ε ω ω ϕ ) 2; / ( ) 2; / ( 2 ) ;, (, *, 2 2 0

63 Photocathode requirements (1) photon excites electron to a higher-energy state; (2) electron-phonon scattering (~0.05 ev lost per collision); (3) escape with kinetic energy in excess to E vac Dunham et al., PAC1995, 1030 Ideal photocathode: E th kt 25 mev response time 1 ps high QE 10% robust hν close to bandgap cold electrons, but long response (~10ps), low Q.E. currently studying GaAs, GaAsP other possibilities GaN, Al-doped GaAs dream: superlattice cathode May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 59

64 Photocathode dev. opportunities Transverse temp. of e at different wavelengths kt = 97 ± 3 mev Temporal response with ps resolution Gun development lab in Wilson electron shape along the injector corresponding to small emittance May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 60

65 Cathode lifetime May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 61

66 Cathode Field E th cathode pulsed! E cath = 120 MV/m τ laser = 2.7 ps rms σ laser = 0.5 mm rms τ laser z = 0.08 mm E cath = 43 MV/m τ laser = 5.8 ps rms σ laser = 0.85 mm rms τ laser z = 0.12 mm E cath = 8 MV/m τ laser = 13 ps rms σ laser = 2 mm rms τ laser z = 0.12 mm 2 18 MV/m 2 6 MV/m 2 1 MV/m E cath / E s.charge = E cath / E s.charge = E cath / E s.charge s a m e s i m u l a t e d e m i t t a n c e May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 62

67 Emittance compensation Axial cross section of bunch charge at cathode Very sensitive to optics & bunch distribution details; Need computer modeling Transverse phase space plots: (a) at cathode; (b) after drift; (c) after lens; (d) more drift May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 63

68 Challenges Achieve gun voltage of 500 kv Demonstrate photocathode longevity Cleanly couple 0.5 MW RF power into the beam without affecting its transverse emit. Control non-linear beam dynamics: over a dozen of sensitive parameters that need to be set just right to achieve the highest brightness Instrumentation and tune-up strategy Drive laser profile programming (both temporal and spatial) spatial temporal May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 64

69 A bigger picture Mission Maintain vibrant/diverse acc. physics program Provide world-class x-rays to CHESS users Future light source and new accel. technology development May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 65

70 3-staged approach to ERL III I II SPL 1150 m I upgrade to CESR to put CHESS on the synch. rad. science frontier II 5 GeV SRF linac with 10 s fs pulse capability & XFEL friendly III diffraction-limited ERL once injector performance is established May 1, 2007 I.V. Bazarov, LASSP seminar, 04/26/07 66