FEL Concepts for Future Light Source at LBNL Alexander Zholents LBNL

Transcription

1 FEL Concepts for Future Light Source at LBNL Alexander Zholents LBNL A. Zholents, Shanghai, December

2 Scientific Drivers 1. Atomic, Molecular and Optical Physics H.Chapman, Femtosecond diffractive imaging with a soft-x-ray FEL, Nature, 839(2006) 2. Chemical Physics 3. Correlated Materials 4. Magnetization and Spin Dynamics 5. Nanoscience and Coherence Report on the workshop, Berkeley, October, A. Zholents, Shanghai, December

3 Requirements for a future light source Energy range of approximately 13.6 ev to 3000 ev (resonant edges of elements from hydrogen to argon), thereby enabling both x-ray x spectroscopies and imaging High degree of spatial and temporal coherence Very high repetition rate 100 khz or greater Both pump and probe pulses in the XUV or x-ray, x or multi-color sequences, for example to actively align molecules in three dimensions Allow precise control over the pulse length and shape Generate very stable photon beams of high brightness and peak intensityi Allow control of polarization, linear and circular, which is essential sential for dichroism spectroscopy and microscopies Exquisite synchronization with ultrafast laser sources providing g short pulses from the XUV to the far IR region A. Zholents, Shanghai, December

4 Versatile FEL facility to address broad scientific interests APEX test facility at ALS A schematic of the facility Beam distribution switch yard feeding individual FELs at 100 khz bunch rep. rate High brightness FELs with long and short x-ray x pulses, variable polariz- ation and fs synchro- nization to lasers: seeded, ESASE, echo enhanced Low-emittance emittance, 1 MHz bunch repetition rate electron gun CW superconducting linac: : 2.4 GeV,, 13 MeV/m and 1MHz bunch rep. rate Lasers serving e-gun, e FELs, diagnostics and linked with a fiber- optics time distribution network experimental stations A. Zholents, Shanghai, December

5 Bird view on linac tunnel, rf gun, spreader and FEL array Machine layout is consistent with the LBNL site: rf guns is on one side of the hill and FEL array on the other leveled side, linac tunnel goes under the hill FEL array ALS Linac RF gun A. Zholents, Shanghai, December

6 X-ray FELs are as good as the brightness of the electron beam defined by: peak current slice emittance slice energy spread and state-of of-the-art electron gun and accelerator is needed to provide and maintain high brightness electron beams B = I ε 2 σ γ Normalized slice emittance peak current Slice energy spread Users anticipate x-ray x pulses with various time formats from ~1 fs and up to ~1 ps - and this defines σ z (in most cases) State of the art injector Peak current: ~70 A Emittance: ~ 1 mm-mrad Energy spread: 3 kev B 1 ~ A/(mm-mrad) 2 LCLS At the end of the linac: Peak current: ~3 ka Emittance: ~1 mm-mrad Energy spread: ~1 MeV B 2 ~ 1500 A/(mm-mrad) 2 Hypothetical seeded FEL At the end of the linac: Peak current: ~1 ka Emittance: ~1 mm-mrad Energy spread: ~100 kev B 2 ~ 5000 A/(mm-mrad) 2 A. Zholents, Shanghai, December

7 SASE FEL LCLS gain length is not sensitive to energy spread Gain length, cm nominal, with the laser heater and after compression Energy spread, MeV LCLS gain length as a function of the uncorrelated energy spread (calculated using Ming Xie formula) A. Zholents, Shanghai, December

8 ESASE- current Enhanced SASE fs pulse λ L ~0.8 to 2.2μm Modulation Acceleration Bunching E ~ 4.5 GeV Peak current, I/I 0 E ~ 14 GeV ka Electron beam after bunching Laser pulse width One optical cycle z /λ L Laser peak power ~ 10 GW ( easy ) Short wiggler, ~ 10 periods Excellent synchronization to the laser for pumpprobe experiments A. Zholents, Shanghai, December

9 ESASE- current Enhanced SASE (2) Example for LCLS with β=12 m - not compatible with current LCLS lattice Start-to-End simulations 3-m FODO lattice period drifts+quads occupy ~ 0.5m ESASE standard LCLS courtesy W. Fawley ESASE cases saturate by 50 m with times power contrast over unmodulated part of the electron bunch - the opportunity for an absolute synchronization of a probe x-ray pulse to a pump laser pulse A. Zholents, Shanghai, December

10 ESASE- current Enhanced SASE (3) ( z) I I 0 B 1 z / 4 + λ L B 2 Δz = λl / 2B Problem: current pulse is too narrow to support slippage over many gain lengths if radiation wavelength is large (soft-x-ray) Solution: use periodic delays of the electron beam on one period to position electron microbunches ahead of the radiation field *) courtesy McNeil δ l s Electron bunch Radiation pulse *) N.R.Thompson and B.W.J.McNeil, PRL, 100, (2008) A. Zholents, Shanghai, December

11 Seeded FEL 1 for a laser like x-ray output 1) Csonka 1980; Kincaid 1980; Bonifacio 1990; L.-H. Yu 1990 laser light bunching chicane e nm 48 nm modulator radiator time delay chicane e - light modulator bunching chicane radiator light 48 nm 12 nm e - evolution of e-beam phase space Radiator resonant at λ L /n, π π π π phase ρ z Fresh electron technique Non fresh electron technique light signal light signal used electrons energy tail tail head fresh electrons head A. Zholents, Shanghai, A. Zholents, December San-Diego, 2008 April,

12 Seeded FEL (2) Bunching efficiency at a given harmonic n depends on the ratio of the energy modulation amplitude ΔE to the energy spread σ E 2ΔE 2σ E b n J n ( nkr 56 ΔE ) e E 1 σ E ( n ) 2 ΔE 2 e inkr 56 ΔE cos( ωt) E k = 2π / λ ω = kc Only one optical cycle is shown modulation wavelength small σ E allows to go to higher n for a given ΔE or to use smaller ΔE and smaller light power for a given n ΔE ~ small σ E is very important for a non-fresh bunch technique P A. Zholents, Shanghai, December

13 Preparation of bright electron beams APEX - Advanced Photoinjector EXperiment Electron gun is a critical technology for a high repetition-rate rate facility Detail physics design of cavity Power density on walls, multipactoring, ion back-bombardment Beam dynamics including a design of the entire injector Detail engineering design of cavity Heat dissipation and cooling Tuning Vacuum 20 MV/m, 0.7 MeV Fabrication and materials Supports Costs Vacuum Torr, NEG pumps in plenum Cathode mounted on coaxial support Beam transport and diagnostics CW 187 MHz accelerating cavity APEX program is testing of high repetition rate (MHz) photocathode RF gun A. Zholents, Shanghai, December

14 Preparation of bright e-beams: longitudinal space charge, CSR and microbunching instability *) Initial density modulation induces energy modulation through longitudinal space charge forces, converted to more density modulation by a compressor Current λ 1% gain=10 10% t courtesy Z. Huang Energy λ Space charge compression saturation due to overmodulation stops the growth Microbunching is inevitable, turned on by a shot noise *) Saldin, Schnedimiller, Yurkov, NIM A, 483, 516 (2002) A. Zholents, Shanghai, December t

15 Microbunching instability - induces irregularities and fragmentation in the phase space After the first bunch compressor Before the second bunch compressor After the second bunch compressor Vlasov solver, M. Venturini A. Zholents, Shanghai, December

16 Suppression of microbunching instability with the Laser Heater 1,2 Laser heater increases energy spread inside the electron bunch and, thus, increases mixing of the microbunching structures courtesy P. Emma 1) E.L. Saldin, E.A. Schneidmiller, and M.V. Yurkov, DESY Report No. TESLA-FEL , ) Z. Huang, et. al, Phys. Rev. ST Acc. and Beams, V.7, , (2004). A. Zholents, Shanghai, December

17 Microbunching instability (2) Minimizing microbunching instability This example is taken from accelerator studies for FERMI project *) *) M.Cornacchia et.al., LBNL-62765, (2007); M. Venturini, A. Zholents, NIM A, 593, 53(2008). A. Zholents, Shanghai, December

18 Impact of MBI on seeded FEL MBI induces fragments of energy variation along the electron bunch as seen in illustration 1 σ ΔE( t) ( n ) b E n J n ( nkr e 2 56 ) Δ e E ΔE( t E E 2 ) inkr56 Distance to the FEL resonace varies along the bunch ΔE(t) causing modulation of bunching efficiency Bunching efficiency analytical simulation with GINGER A. Zholents, Shanghai, December

19 Impact of MBI on harmonic cascade FEL (2) Additionally, modulation in the bunching phase can be large and can cause a significant frequency chirp 1,2 analytical simulation with GINGER 1) S. G. Biedron, S.V. Milton, and H.P. Freund, NIM A 475 (2001)401 2) T.Shaftan et al., Phys. Rev. E, 71, (2005) A. Zholents, Shanghai, December

20 An example of the impact of energy modulation on the x- ray signal bandwidth One important goal for seeded FELs is generation of FT limited pulses with a narrow bandwidth Simulation for FERMI project Quadratic energy chirp Δ d dz 2 w ~ 2 ΔE E Δw FWHM 40meV Elegant Flat Δw FWHM 10meV Longitudinal phase space Power spectrum (courtesy G. Penn) A. Zholents, Shanghai, December

21 Timing and synchronization Peak current distrubution at the end of the accelerator z, mm Peak current Sample errors: Charge: 1% RF phase: 0.1 degree RF amplitude: 0.01% 1000 random seeds seed laser pulse Useful part A. Zholents, Shanghai, December

22 RF signal distribution via controlled fiber links Russell Wilcox, Gang Huang, Larry Doolittle, John Byrd, Alex Ratti, John Staples, LBNL RF Hi RF Lo Transmitter Receiver Remote client 1 RF out nm CW fiber laser Rb freq. locker AM N temp. control, 0.01C ref. arm RF signal fiber Acoustic optical modulator +50MHz temp. control, 0.01C freq. shifter 100MHz 100MHz beat RF in LO in RF out RF in LLRF controller (FPGAbased) Interferometer beat signal at 100 MHz Remote client 2 Transmit RF signal as a modulation of optical carrier Use digital controller to measure the line length Remote clients lock to original RF signal and each other Remote client 3 Remote client N A. Zholents, Shanghai, December

23 Test experiment in SLAC tunnel Fsec Long Measured fiber correction drift in (in pspsec) in long fiber QuickTime?and a decompressor are needed to see this picture. Ch A-B Differential Drift vs Time Relative drift (in fsec) Jan Days Short fiber correction 15 fsec Two fibers continuously stabilized to ~15 fsec peak-to to-peak relative drift over 24 days while SLAC linac was running. Corrections applied to tunnel fiber correlate with daily and long term average tunnel temperature variation. Strong correlation found to humidity A. Zholents, Shanghai, December

24 Echo effect for harmonic generation of x-rays *) ΔE/σ E z/λ Mitigates the problem with high harmonic b n J n ( nkr 56 ΔE ) e E *) G. Stupakov, talk at SLAC-PUB (2008). 1 σ E ( n ) 2 ΔE 2 e inkr 56 ΔE cos( ωt) E A. Zholents, Shanghai, December

25 Echo effect for harmonic generation of x-rays (2) Peak current modulation Bunching efficiency at various harmonics A. Zholents, Shanghai, December

26 Echo effect for harmonic generation of x-rays (3) GENESIS simulation of the FEL producing 10 nm radiation beginning with 240 nm energy modulation A. Zholents, Shanghai, December

27 Acknowledgement I used some material in this talk provided by: John Corlett,, John Byrd, Paul Emma, William Fawley, Zhirong Huang, Brian McNeil, Gregory Penn, Fernando Sannibale,, John Staples, Gennady Stupakov, Marco Venturini,, Russell Wells, Russell Wilcox, Dao Xiang and I greatly benefited from discussions with them and lots of other o people. Thank you for your attention A. Zholents, Shanghai, December

28 Scientific Challenges Challenges in chemical physics Attosecond probe and control of electron dynamics in atoms and molecules -- X-ray pump and probe Understanding excited state chemistry that is not determined by simple adiabatic potential surfaces Femtosecond probe and control of non-born Born-Oppenheimer chemical dynamics Conical intersections dominate photochemistry involved in energy applications (e.g., photosynthesis, inorganic and organic photochemistry) For large (and many small) molecules there potential energy surfaces are multiple conical intersections (hω<10 ev) that determine photochemistry hν FEL will address the fundamental time scale for vibrational or reactive atomic motion vibrational period (T( vib = fs) ) with intensities and repetition rates sufficient to characterize non-born Born-Oppenheimer dynamics A. Zholents, Shanghai, December

29 Scientific Challenges Challenges in magnetism Magnetic and Spin Dynamics and Correlated Materials Understanding materials that are not described by single electron band structure models, i.e., complex correlated materials Direct observation of the structure and dynamics of the valence electrons Observe, understand and control magnetism and magnetic response on the femtosecond temporal and nanometer spatial scale -- simultaneously Extend observations of material fluctuations to ultrafast time scale A. Zholents, Shanghai, December

30 Performance Goals THREE PRINCIPAL MODES OF OPERATION Sub-femtosecond Short-pulse High-resolution Wavelength range (nm) ~40-1 ~ 90-1 ~ 90-1 Photon energy (ev) Pulse duration (fs) Repetition rate(khz) Peak power (GW) Photons/pulse (@1 nm) 1.5x10 8 (in 100 as) 5x10 11 (in 100 fs) 2x10 12 (in 500 fs) Power in third harmonic few% of fundamental few% of fundamental few% of fundamental Polarization Variable, linear/circular Variable, linear/circular Variable, linear/circular A. Zholents, Shanghai, December

31 Assess stability of x-ray x parameters and design feedback systems in the accelerator Shot-to-shot variations in X-ray output UV seed Harmonic FEL cascade HHG seed A. Zholents, Shanghai, December

32 Design FELs with a realistic assessment of capability for producing various x-ray x outputs X-ray peak and average flux and brightness Pulse repetition rate Wavelength Bandwidth Pulse duration Polarization etc. 100 kw λ=30 nm 1 GeV beam 500 A 1.2 micron emittance 75 kev energy spread Modulator λ=30 nm, L=1.8 m Modulator λ=30 nm, L=1.8 m Radiator λ=3.8 nm, L=12 m A. Zholents, Shanghai, December

33 Impact of wake fields, rf waveform and other nonlinearities Energy variation along the electron bunch causes frequency chirp in the output signal ΔE/E C ρ head ω head z az 2 tail ω tail z z z Quadratic energy chirp with superimposed energy modulation in the modulator Compression factor: C C 1 = 1+ hr 2aR z ; h ( ΔE / E) dz More compression at the tail than at the head produces electron bunch with modulated density ω head < ω tail Note: what is important is the speed of energy variation, thus, same modulation on a shorter time scale causes more bandwidth broadening than on a longer time scale. = d A. Zholents, Shanghai, December

34 Requirements to the e- beam beyond brightness (2) Energy variation along the electron bunch causes frequency chirp in the output signal * ΔE/E C ρ head ω head z az 2 tail ω tail z z z Quadratic energy chirp with superimposed energy modulation in the modulator Compression factor: 1 C = 1+ hr C 2aR z ; h ( ΔE / E) dz More compression at the tail than at the head produces electron bunch with modulated density ω head < ω tail = d *) S. G. Biedron, S.V. Milton, and H.P. Freund, NIM A 475 (2001)401. T.Shaftan et al., Phys. Rev. E, 71, (2005) A. Zholents, Shanghai, December

35 Beam switch yard (spreader) Pulser Distance between orbits at the beginning of a septum All dimensions are in mm 150 F D m FID GmbH: 15 kv, 10 ns, 100 khz Jitter: 0.1%, 20ps F D kicker septum kicker septum kicker F D F D F D F D A schematic of a beam take-off section: only two branch lines are shown Kicker: 2 m long stripline,, 20 kv/cm. Electro-magnet for a quasi continuous beam switching can be added a top of the stripline A. Zholents, Shanghai, December

36 Minimizing microbunching instability Non isochronous spreader R56=0.9 mm Δ d dz 2 w ~ 2 ΔE E theory solid line 1) dots Vlasov solver 2) At the end of the spreader and 10m downstream Isochronous spreader 1) S. Heifets, G. Stupakov, S. Krinsky, PRST-AB, 5, (2002) 2) M. Venturini et.al., PRST-AB, 10, (2007) A. Zholents, Shanghai, December