Recent Developments at the Brookhaven Source Development Laboratory

Transcription

1 Recent Developments at the Brookhaven Source Development Laboratory Brian Sheehy National Synchrotron Light Source Brookhaven National Laboratory Beam Physics Seminar Jefferson Laboratory October 15, 2004

2 The SDL Team G. L. Carr, E. D. Johnson, S. Krinsky, H. Loos, J. B. Murphy, J. Rose, T. Shaftan, B. Sheehy, Y. Shen, X.-J. Wang, Z. Wu, L. H. Yu National Synchrotron Light Source; Brookhaven National Laboratory Facility Overview Diagnostics/Control High Gain Harmonic Generation (HGHG) Cascading Tunability Optical Compression and and Shaping coherent FEL output SPIDER and CPA Other Sources MV/cm peak field THz source

3 Facility Overview Nisus Wiggler FEL seed at 800 nm Modulator Undulator 177 MeV Normal incidence 77 MeV Ion Pair Imaging Experiment at 88 nm NISUS pop-in monitors FEL Measurements Energy, Spectrum, Synchronization and Pulse Length Measurements at 266 nm Dispersion Magnet RF zerophasing CTR Monitor Trim Chicane Adjustable Chicane Photoinjector 30 mj Ti:Sapphire Amplifier BNL Gun IV photoinjector, S-band, 4.5 MeV 4 stage Linac up to 200 MeV upgrade to MeV near completion Magnetic Chicane Compressor R 56 = 5 cm Seed at λ s = 800 nm in 1 m undulator K=1.67, followed by dispersive section NISUS undulator, 10 m, 256 period, K = 1.1 fundamental at λ s /3 = 266 nm, output 100 µj 1 µj at third harmonic λ s /9 = 89 nm

4 over 30 Ce:YAG pop-in beam position monitors (BPM), including 17 in the radiator Automated beam matching and emittance measurements Optical longitudinal electron beam tomography CSR instability question temporal beam shaping electro-optic electron beam measurements Diagnostics/Control RMS size (um) ε n 4.78 ± 0.42 µm β 3.73 ± 0.38 m α ± 0.09 Unmatched horizontal ε 200 n 4.12 ± 0.10 µm vertical β 4.03 ± 0.15 m α 0.51 ± Automated beam matching in NISUS Distance (m) ε n 4.00 ± 0.19 µm β 3.26 ± 0.20 m α 0.09 ± 0.07 Matched ε 100 n 4.23 ± 0.15 µm β 3.16 ± 0.16 m α 0.28 ± Transverse beam parameters

5 Photocathode Drive Laser: 250 fsec resolution cross correlation with oscillator shaping: emittance, THz Power meter Signal Phase matching angle (mrad) 250 fs blue BBO crystal Diagnostics/Control 100 fs IR -4-2 Time (ps) 5ps UV 0 2 Reference Detector 266 nm HGHG light Layout of the two-photon absorption pump-probe autocorrelator Wedged beam splitter Micrometer delay stage BBO Crystal Detector two photon absorption autocorrelator for 266 nm output picosecond resolution synchroscan streak camera visible & XUV monochromators SPIDER: complete field measurement of FEL output

6 RF Zero Phase Uncompressed beam on pop14 Tail Tail Mild compression Tail Strong compression. Head Head Head Yikes! CSR Instability? Accelerate bunch at RF zero-crossing Positive RF slope Zero RF slope Negative RF slope Bending magnet generates dispersion Image at screen depends on energy spread Use linac phase to streak the bunch on screen But! This is really an energy measurement, not a current measurement

7 Longitudinal phase space tomography (H. Loos) t E Each value of the chirp manifests a different projection of the phase space. E (kev) Current (A) Intensity E-t corr current Drive laser Time (ps) Energy (kev) Spread (kev) Time (ps)

8 The energy projection can be very deceptive Huang & Shaftan NIMA 528, 345 (2004) Current and energy profiles of a chirped beam (a) without energy modulation, (b) with energy modulation.

9 Degree of modulation observed inconsistent with CSR models S. Heifets, G. Stupakov and S. Krinsky PRST-AB 5, (2002) Z. Huang & H.-J.Kim PRST-AB 5, (2002) Z. Huang T.Shaftan SLAC-PUB-9788, 329. Longitudinal Space Charge model: small modulation in photocathode drive laser small current modulations due to drive laser modulations at photocathode longitudinal space charge forces result in enhanced energy modulations in the bunch these dominate the horizontal distribution in zero-phase measurements experimental confirmation lack of coherent enhancement of the IR in coherent transition radiation modulation behavior with chicane strength, trans beam size, energy, etc phase space tomography Theory: Huang & Shaftan NIM A 528, 345 (2004) Experiment: Shaftan et al NIM A 528, 397 (2004) potential threat for short pulse short wavelength FEL s can convert to current modulation; larger energy spresd goes away for perfectly uniform laser temporal profile

10 800 nm 9 nj 100 fsec Temporal Shaping (in progress) Ti:Sapph Oscillator At SDL in collaboration with SPARC and SLAC Dazzler Amplified and compressed IR pulse 5 nj psec 25 mj psec 15 mj psec Stretcher Amplifier Compressor Intensity (arb units) 266 nm 1.8 mj ω tripler Time (psec)

11 Spectrum Analyzer GCR-150 YAG Laser GCR-170 YAG Laser ω + 2ω = 3ω Power Mon Monument Photocathode Power Atten Spatial Filter Optical Relay, 14 meters Regenerative Amplifier Variable Power Divider ω + ω = 2ω Spot Imaging 2-pass Amplifier Power Atten Autocorrelator Autocorrellator 2-pass Amplifier Compressor 1 Compressor 2 Stretcher Seeding And Diagnostics Optical Relay, ~35 meters Optical Isolator Dazzler Millennia Laser Tsunami Laser Photodiode

12 Electro-Optic e-beam meaurements Analyzer Fiber Spectrometer Laser Electrons Modulator λ/4 Plate ZnTe Trim Accelerator to NISUS Trim Dipole Polarizer Monitor Delay or chirp Seed Laser (800 nm) Retardation induced by e-bunch field E vac 3 2π n0 r Evac l ϕ = 41 ( 1+ ε ) λ Asymmetry in transmitted/reflected gives ϕ T R sin( ϕ) = T + R Chirp seed and spectrally resolve the asymmetry single shot measurement (800 fsec resolution). Jitter 150 fsec rms over 20 seconds

13 High Gain Harmonic Generation (HGHG) 3 Energy Fluctuations e - p(e) 2 1 σ= 41% SASE Self amplified Spontaneous Emission (SASE) Spontaneous emission microbunching enhanced emission Noisy Broad Bandwidth Not longitudinally coherent HGHG Seed modulates e - energy coherent microbunching emission Short wavelength : tune radiator to harmonic of seed Stable Narrow bandwidth, higher brightness Longitudinal coherence p(e) E/<E> Spectrum σ = 7% HGHG

14 Cascading HGHG to soft X-ray wavelengths (L.H Yu) 1-ST STAGE 2-ND STAGE 3-RD STAGE FINAL AMPLIFIER MODULATOR λ w = 11 cm Length = 2 m Lg = 1.6 m AMPLIFIER λ w = 6.5 cm Length = 6 m Lg = 1.3 m MODULATOR λ w = 6.5 cm Length = 2 m Lg = 1.3 m AMPLIFIER λ w = 4.2 cm Length = 8 m Lg = 1.4 m MODULATOR λ w = 4.2 cm Length = 2 m Lg = 1.4 m AMPLIFIER λ w = 2.8 cm Length = 4 m Lg = 1.75 m AMPLIFIER λ w = 2.8 cm Length = 12 m Lg = 1.75 m DISPERSION dψ/dγ = 1 DISPERSION dψ/dγ = 1 DISPERSION dψ/dγ = 0.5 e- e- LASER PULSE e- e- DELAY Fresh Spent DELAY DELAY electrons electrons Fresh Spent electrons electrons FRESH BUNCH CONCEPT 1.7 GW 500 MW 266 nm SEED LASER 400 MW 800 MW 70 MW nm nm 5 e-beam 750Amp 1mm-mrad 2.6GeV σ γ /γ= total L w =36m nm

15 A proposed 2 stage cascade for the SDL P in =1.5 MW 266nm 56 MW 133nm P out =140 MW 66.5 nm 0.8m MINI 6 m NISUS e-beam 600Amp 250 MeV 2.7 mm-mrad σ γ / γ = m VISA Pulse length ~ 0.5ps 70µJ

16 A Novel Tunability scheme for HGHG (T. Shaftan) Seed with fixed λ. Radiator E [MeV]. E [MeV] Modulator E [MeV] DS t [ps] after DS (Dispersive Section) after Modulator before FEL Dispersive Section (DS) converts energy modulation into bunching DS also compresses the energy modulation wavelength a small but measureable effect in our machine, but could be optimized to yield a tuning range of 20%

17 Optimal tunability configuration HGHG energy time Radiator XRF DS XRF Modulator Klystron Seed with fixed λ Compression or stretching in the dispersive section can be used to modify the period of the microbunching. This is ordinarily a small effect, but it could be optimized to yield ~20% tunability.

18 SDL Experiment 190 MeV 130 MeV 72 MeV 35 MeV Gun 4.5 MeV Tank 4 Tank 3 Tank 2 Tank 1 seed 266 nm NISUS DS Mod 0.8 um Chirp is provided by shifting beam off-crest in tank 4 (E max = 58 MeV) Tank 4 phase shift: from +25 to -45 Tank 4 DS is set to maximum current (200 A) energy gain Nothing else was changed! Spectrum of HGHG is measured Tank for different amounts of chirp. 4 phase

19 HGHG output spectra for various tank 4 phases: HGHG intensity, a.u Wavelength, nm

20 FEL ρ Wavelength versus chirp λ/λ 1 % Wavelength versus energy Wavelength [nm] Wavelength [nm] sin(phi4) Fit, based on R 56 (DS)=0.34 mm Fit including R 56 of DS and radiator Energy detuning, % Additional compression in the radiator

21 Spectral Phase Measurements; chirping and shaping FEL output Optical Compression and and Shaping coherent FEL output Measuring Spectral Phase SPIDER technique Application at 266nm for picosecond laser pulses Measurements HGHG Unchirped, narrow bandwidth Near transform limit Chirping and Compressing

22 frequency e - energy time time High Gain Harmonic Generation (HGHG) and Chirped Pulse Amplification (CPA) T b ω frequency time e - e - Optical compressor Match optical seed chirp to electron energy chirp Resonant frequency in modulator matches seed at each moment in the bunch Output pulse is also chirped Longitudinal coherence permits optical compression to transform limit femtosecond pulses Sensitive to spectral phase distortion Li Hua Yu et al Phys Rev E 49, 4480 (1994) frequency T<<T b time

23 Shaping HGHG Coherent control at short wavelengths For both chirping and shaping, the question is: How will phase modulation in the seed transfer to HGHG? Can distortions be used as a probe of e - beam and radiator dynamics Potential Problems / Interesting Questions synchronization jitter stability noise & harmonics optical field is bipolar, electron density is not.

24 Measuring the spectral phase: SPIDER (Spectral Interferometry for Direct Electric-Field Reconstruction) D(ω c ) 2π/τ ω c (Walmsley group, Oxford) 800 nm 400 nm 266 nm C. Iaconis and I. A. Walmsley, Opt. Lett. 23, (1998).

25 Compressor used as stretcher DOWNCONVERSION SPIDER LAYOUT. Delay Line 800 nm 266 nm 800 nm (seed) nm (HGHG) Spectrometer Michelson interferometer 400 nm Filter BBO Separate seed pulse (800 nm) and HGHG stretch seed to 60 psec make 2 HGHG pulse replicas in interferometer and separate by τ=3.5 psec Downconvert to 400 nm in BBO frequency shift is Ω=0.2 THz set spectrometer to λ c =800 nm measure 400 nm SPIDER trace in 2 nd order block seed, remove filter and measure 266 nm calibration trace in 3 rd order

26 ang

27 intensity (arb units) radians Typical Spider Trace Spidering a laboratory 266 nm source ω - ω 0 (PHz) Reconstructed phase and amplitude amplitude phase fit chirp phase-chirp stretch a 100 femtosecond 800 nm Ti:Sapph chirped-pulse-amplification system Frequency-triple in BBO to 266 nm(spoil phase matching to create an asymmetry in the time profile) Compare scanning multishot cross-correlation of the 266 nm and a short 800 nm pulse with the average reconstruction, convolved with 250 fsec resolution of the x-correlator intensity (arb units) Comparison with x-correlation spider cross correlation 900 fsec FWHM ω - ω 0 (PHz) time (psec)

28 UNCHIRPED HGHG frequency time ω frequency e - energy T b time e - e - time Stretch seed to 6 psec optimize compression / minimize e - energy chirp minimize output bandwidth

29 UNCHIRPED HGHG * 3 Spectral Phase Frequency vs time flat phase across the pulse residual seed chirp not visible frequency vs time constant Temporal Phase

30 UNCHIRPED HGHG 168 fs rms 207 fs rms number of shots shots σ ω = THz σ ω σ τ = 1.1, twice transform limit for a Gaussian pulse FWHM = fsec pulses are not Gaussian Define transform limit as the pulse when spectral phases are set to zero. pulses are times transform limit width tran ltd width number of shots rms spectral width σ (THz) σ τ = psec rms temporal width σ (psec)

31 frequency time ω CHIRPED HGHG frequency e - energy time T b e - e - time Chirp e - bunch and optical seed together optical seed: 3.8 THz/psec e - bunch: 2.7 THz/psec (resonant frequency) broader bandwidth already observed Doyuran et al PRST AB 7, (2004)

32 CHIRPED HGHG * 3

33 CHIRPED HGHG * 3

34 CHIRPED HGHG * 3

35 CHIRPED HGHG * 3

36 CHIRPED HGHG Frequency (THz) Distribution of chirps fit over a 200 fs window around peak center seed chirp * 3 Time (psec) Sources of instability optical chirp / e - chirp mismatch synchronization (150 fsec rms) compression instability rf curvature The seed chirp is clearly observed in the HGHG output over part of the pulse distortion in the pulse wings deteriorates compressibility

37 Matching Electron and Optical Chirp Optical Beam Electron Beam λ γ t Electron beam has curvature due to sinusoidal acclerating field If chirp is not matched, resonance occurs only over a short portion of the electron bunch correlation between compressibility and uncompressed pulse length? Mismatched is more sensitive to synchronization jitter.

38 correlation between compressibility and uncompressed pulse length Compression Factor Uncompressed rms pulse width (fsec)

39 Fit b for each shot and compress : rms width compressed width Compression factor compressed width/transform limit most pulses compressible in principle to ~ twice transform limit quadratic spectral phase (defines compressor) not determined only by chirp a reasonable fixed compressor compresses only 15% of pulses

40 CPA Summary Successfully demonstrated SPIDER at shortest wavelength and longest pulse lengths reported. Characterized spectral phase of High Gain Harmonic Generation near transform limited Chirped Pulse Amplification Imparted positive chirp commensurate with seed chirp poorly matched electron chirp sensitivity to other factors still unclear shown the viability of CPA and potential for more complex pulse shaping

41 Typical Laser-based THz Source UF laser GaAs THz Transient current is subpicosecond, Ea ~ 10 kv/cm Relaxation is slow: half-cycle pulse THz yields: 1 nj (250 khz) to 1 uj (1 khz), size scaling limit 1-10 uj optical rectification yields are similar w/ higher freqs. SDL LINAC-based THz Source Coherent THz dipole radiation ~ 300 fs, 700pC electron bunches (can be shorter w/ less charge) dipole chicane compressor Note: <10 Hz rep. rate photocathode electron gun Intensity Coherent THz transition radiation 0 Frequency (THz) 2 bunch length ~ λ, coherent enhancement energy of 80 uj measured in this setup ( E ~ 1 MV/cm) 2 orders of magnitude larger than other sources Scales as N e- 2 spectral content to 2 THz higher for shorter bunches This machine is not optimized for THz production

42 Coherent Transition Radiation Transition radiation occurs when an electron crosses the boundary between two different media. For a relativistic electron (β v/c 1) incident on a perfect conductor, the number of photons emitted per solid angle and wavelength range is: dn dλdω 2 α β sin = 2 π λ 2 θ cos 2 θ 2 2 ( 1 β cos θ ) Intensity is 0 on axis, peaks at θ ~ 1/γ. Polarization is radial Intensity (rel.) Far field distribution for γ = 200 Coherent radiation emission: dw N /dω = N 2 dw 1 /dω f (ω) 2 where f iωnˆ r / c ( ω ) e S( r) = dr Angle (mrad) (Nodvick & Saxon)

43 Electro-Optic measurement of THz field Paraboloid Vacuum Window Electron Beam EO Detection method: T.F. Heinz/Columbia & X.-C. Zhang/Rensselaer Accelerators: Yan, Van der Meer et al PRL 2000 Wilke et al PRL 2002 Loos et al PAC 2003 Chirped sampling: Jiang and Zhang, APL (1998) Delay Polarizer ZnTe Analyzer Ti:Sa Laser Coupling Hole, 2 mm λ/4 Lens CCD Polarization of synchronized optical field is retarded by instantaneous THz field in the ZnTe crystal by Pockels effect analyzer gives time resolved 2-dimensional distribution of electric field component

44 Image Processing for electric field recovery Use compensator waveplate to detect sign of polarization change. Reference I R (left) and Signal I S (right) obtained simultaneously. Rescale and normalize both. Calculate asymmetry A of Signal. Subtract asymmetry pattern w/o THz. Pixels A = 2I S /I R -1 Vertical (mm) Pixels Horizontal (mm)

45 Coherent THz Transition Radiation Pulses from the SDL Linac Coherent THz dipole radiation ~ 300 fs, 700pC electron bunches (can be shorter w/ less charge) dipole chicane compressor Note: <10 Hz rep. rate photocathode electron gun Coherent THz transition radiation Frequency [THz] Pyroelectric Detector Response [µj] Up to 80 µj (!) per pulse -> consistent with calculation Intensity [arb.] Intensity to 2 THz (higher with shorter bunches) Time [ms] Frequency [cm -1 ]

46 THz beam path and analyzer

47 Beam Paths in Analyzer OAP ZnTe λ/4 Pol. Ref Signal BS Camera Camera

48 CTR simulation (H. Loos) Decompose electron beam coulomb field in Gauss-Laguerre modes. Calculate complex transmission factors through experiment for THz spectral range. 30 mm Use bunch form factor to reconstruct time dependence. 20 ps

49 Cross section of E-field at focus as a function of time Temporal-spatial E-field profile of coherent transition radiation pulse at ~ f/1.5 focus Note: opposite sides are asymmetric, as shown (radial polarization) Measurement E-field along horizontal plane Calculation m (m )

50 Opportunities in Magnetism for THz pulses (E peak = 1 MV/cm B peak = 3 kilogauss) Ultra-Short Pulses and/or High Fields -- D. Arena / NSLS Current state of the art for ultra-fast dynamics experiments: exchange interaction Stoner excitations Role for THz! Time: ~100 fs (lasers) ~100 ps (synchrotron) Field: ~ gauss (stripline) spin waves (low q limit) spin-lattice relaxation manganites precessional rotation and damping spin coherence and spin diffusion magnetic viscosity Excitation / Interaction time (sec) field (gauss) Coercivity / Saturation rare-earth magnets a Fe16 N 2 dilute magnetic semicond. soft manganites transition metals & alloys permalloy

51 Soft Modes in Ferroelectrics & Perovskites (PbTiO3) G. L. Carr THz HCP THz HCP E THz HCP E E(t) E DOE Workshop on Ultrafast X-rays Use half-cycle pulse to coherently drive atoms, probe motion as a function of time (needs diffraction probe). Observe shift in diffraction spot(s)

52 Why Make Terahertz Pulses? Imaging/Remote Ranging non-ionizing medical safely use to monitor public or battlefield environments CHEMICALLY AND BIOLOGICALLY SENSITIVE explosives, weapons detection explosives phonon modes in DNA (Woolard et al Phys Rev E 65, ) remote ranging with bacterial species identification a lot of work remains in sources, detection, and characterization. 1 km weapons Woolard et al THz Differential Absorption Radar model (Bio early warning)

53 Some More Reasons Ultra-fast dynamics (< 1 ps time scale) directly measure complex dielectric response of sample strong coupling of optical/thz excitations in correlated e - systems (e.g. high- T c superconductors) Low frequency, non-linear properties of materials nanoparticles / quantum dot arrays in dielectrics Optronics: Ultrafast components in all-optical circuits faster, EMP resistant Orienting Molecules spectroscopy and chemistry from coherent rovibrational states manipulate internal molecular fields / fundamental measurements & coherent control Structural transitions Large E-field to coherently shove atoms

54 THz Summary The DUVFEL THz source pulse energy exceeds other sources by 2 orders of magnitude (DUVFEL 80 uj, Laser sources 1 uj, JLab ERL 0.5 uj) fundamental dynamics ( magnetic systems, strongly correlated e - systems, nanoparticles ) large area imaging single shot detection explore nonlinear effects THz as pump in pump-probe experiments MV/cm E-fields and kilogauss B-fields THz pulse shaping (through e-beam and optically) Accelerator sources have not been optimized for THz production

55 Global Summary SDL continues to be an important test bed for FEL science and technology development, as well as a user facility answers to important questions of beam dynamics for next generation sources HGHG is an extremely promising candidate for producing longitudinally coherent short wavelength ultrafast pulses tunability chirped pulse amplification and Shaping cascading is in the works The THz production is a truly unique source capable of opening a new regime of dynamics to study