Ultrashort radiation pulses from storage rings

Transcription

1 Ultrashort radiation pulses from storage rings Lawrence Berkeley National Laboratory page 1

2 Overview Introduction to synchrotron radiation Coherence of Synchrotron Radiation? Challenges for generating CSR Tailored terahertz pulses via laser-beam modulation X-ray pulse compression via transverse beam chirping Summary page 2

3 Science Opportunities: Michael C. Martin, Wayne R. McKinney, Dimitri Basov, Daniel Chemla, Ben Feinberg, Robert Kaindl, Jim Krupnick, Laszlo Mihaly, Joe Orenstein, Al Sievers, Jason Singley, Neville Smith Accelerator Physics:, Fernando Sannibale, David Robin, Hiroshi Nishimura, Weishi Wan, Christoph Steier, Warren Byrne, Tom Scarvie, Agusta Loftsdottir Laser Slicing: Robert Schoenlein, Sacha Zholents, Max Zolotorev, Zhao Hao Engineering: Ross Schlueter, Jin-Young Jung, Dawn Munson, Ken Baptiste, Walter Barry, R.J Benjegerdes, Alan Biocca, Daniela Cambie, Mike Chin, John Corlett, Stefano De Santis, Rick Donahue, Mike Fahmie, Slawomir Kwiatkowski, Derun Li, Steve Marks, David Plate, J.A. Paterson, Greg Stover, Will Thur, J.P. Zbasnik Collaborations: Marco Venturini - LBNL, Etienne Forest - KEK, Gennady Stupakov - SLAC, Jim Murphy - NSLS-BNL - Larry Carr, NSLS-BNL, Wim Leemans - LBNL, Bout Marcelis - LBNL/Eindhoven, Bob Warnock - SLAC, Rui Li - JLab, Gode Wustefeld - BESSY, Peter Kuske - BESSY,... page 3 Acknowledgements Work supported by LBNL LDRD funding since FY01.

4 page 4 Synchrotron radiation: White noise Radiated electric field of an electron = = N k t k t e t E 1 ) ( ) ( = = N k t e i k e E 1 ) ˆ( ) ( ˆ ω ω ω Total power is a sum over the relative phases of individual electrons = = = = = N m k t t i N m t i N k t i m k m k e e e e e e P 1, ) ( * ) ˆ( ) ( ) ˆ ˆ( ) ( ω ω ω ω ω ω ω Average power is + = ) ˆ( ) ˆ( ) ( ω ω ω f N N e P coherent radiation incoherent radiation frequency spectrum of longitudinal bunch distribution

5 Coherence of Synchrotron Radiation long bunch (λ>σ z ) E( t) = N k= 1 e( t t k ) Total electric summed over N electrons distributed at time t k. Bunch spectral distribution short bunch (λ<σ z ) 2 2 P( ω) = eˆ( ω) N + N fˆ( ω) Incoherent 2 Coherent Log Flux coherent incoherent page 5 Log Frequency

6 Vacuum Chamber acts as a High Pass Filter effective source size beam size Challenges for generating CSR long bunch spectrum shielded impedance h short bunch spectrum vacuum chamber When the effective size of the SR source is equal to the height of the vacuum chamber, SR is suppressed. Most rings can not make short enough bunches to generate stable CSR! free space impedance Frequency Nodvick, Saxon, Phys. Rev. 96, 1, p. 180 (1954) Shielding by the vacuum chamber limits the SR emission to wavelengths above the waveguide cutoff condition πσ < λ <2h h ρ 1/2 page 6

7 Dreams Come True... Recently, two different facilities experimentally demonstrated the feasibility of such sources: The Jefferson Lab Energy Recovery Linac The BESSY-II Storage Ring Intensity (Arb. Units) Integrated THz Intensity (Arb. Units) Measured Intensity N 2 Fit Beam Current (µa) 50 µa 80 µa 105 µa 110 µa 170 µa 230 µa 0 page Wavenumbers (cm -1 ) L. Carr et al., Nature 420, 153 (2002) M. Abo-Bakr et al., Phys. Rev. Lett. 88, (2002) M. Abo-Bakr et al., Phys. Rev. Lett. 90, (2003)

8 Our Dream An Optimized Ring Based CSR Source Up to 9 orders of magnitude higher power than in existing sources! Revolutionary impact on the THz based science page 8

9 Radiation Force e - opening angle~ψ r ~ λ 2πρ 1/3 ρ In free space E φ = Z 0c 2e 1 4π 3 4 ρ 2 1/3 s 4/3 for s>0 Total voltage on a bunch V(s) =2πρ s ds'e φ (s s')i(s') E φ Front wake accelerates bunch front nominal bunch distribution Back (de)focussing gradient page 9

10 Bunch distortion from radiation Radiation force additionally focusses the front of the bunch. Limit reached when back of bunch is defocussed. 10Q 0 Q 0 Steep leading bunch edge extends coherent emission spectrum to shorter wavelengths. Coherence occurs at sharp front edge of the bunch-shorter bunches. Log Amplitude front back Bane, Krinsky, & Murphy, Microbunches Workshop, AIP Conf. Proc. 367, 191 (1996). Murphy, Krinsky, & Gluckstern, Part. Accel. 57, 9 (1997). page 10 Log Frequency

11 Simulation of the BESSY Results The CSR Gain CSR Gain = P TOT TOT /P INC INC = 1 + N g(ω) g The measured quantity: f S The input quantities: α C, σ 0 Other effects: Uneven filling of the buckets Indeterminacy on h Systematic experimental errors page 11

12 Simulation of the BESSY Results Distribution Asymmetry Tail Head Leading edges much sharper than trailing ones, in agreement with BESSY II observations page 12

13 CSR Instabilities CSR can drive a microbunching instability in the electron bunch, resulting in a periodic bursts of terahertz synchrotron radiation, resulting in a noisy source. Bolometer signal (V) 10 ma 10.5mA 28.8mA mA ms Time (msec) Time (msec) page 13 Simulated instability showing bunch shape Bursts of far-ir CSR observed on a bolometer. Threshold depends on beam energy, bunch length, energy spread, and wavelength.

14 Microbunching Model Small perturbations to the bunch density can be amplified by the interaction with the radiation. Instability occurs if growth rate is faster than decoherence from bunch energy spread. δ/σ δ Nonlinear effects cause the instability to saturate. Radiation damping damps the increased energy spread and bunch length, resulting in a sawtooth instability. page 14 z/σ S. Heifets and G. Stupakov, PRST-AB 5, (2002). M. Venturini and R. Warnock, PRL 89, (2002).

15 ALS microbunching results Bursting (ma) Burst threshold (ma) ALS studies show first confirmation of CSR driven sub-microwave instability mm threshold 3.2 mm threshold Model predictions Energy (GeV) Instability thresholds understood: -agreement w/observations at other sources -coherent power enhanced w/lower S/N -possibility of raising threshold using nonlinear momentum compaction Standard FTIR techniques require clean sources. A CSR source must be below the instability threshold. J. Byrd, et. al. PRL 89, , (2002). page 15

16 Bessy-II Microbunching Bursting threshold Agrees well with predicted microbunching thresholds page 16 G. Wuestefeld, Napa CSR Workshop, Oct. 2002

17 Filling the Terahertz (THz) Gap The most scientifically rich, yet underutilized region of the EM spectrum Tom Crowe Hz 1 THz = 4.1 mev = 33 cm -1 = 300 µm page 17 THz Science: collective excitations, protein motions & dynamics, superconductor gaps, magnetic resonances, terabit wireless, medical imaging, security screening, detecting explosives & bio agents Much brighter terahertz beams are required for scientific and technological applications... Large average and peak powers could be used to manipulate and alter materials, chemical reactions and biological processes. -Mark Sherwin, Nature News & Views 520, 131 (2002).

18 T-rays are hot! led to unprecedented creativity in source development. "The size of the community is increasing with a clear growth potential to support a large THz user s network including user facilities." The opportunities are limitless page 18

19 CIRCE: Coherent InfraRed CEnter ALS LINAC ALS Booster CIRCE ALS page 19

20 CIRCE Main Characteristics CIRCE Parameters (NC RF): E = 600 MeV f RF = 1.5 GHz V RF = 0.6 MV U 0 = 8.62 kv I total = 8-90 ma I bunch = µa L = 66 m # buckets = 330 σ τ0 = 1-3 ps σ δ = α= ρ = m 2h = 4 cm Σ = Periodicity = 6 DBA lattice 50 nm emittance (diffraction limited in far-infrared) Variable momentum compaction with 3rd order correction Magnets pre-aligned on girders Shielding fits directly over magnets (i.e. no tunnel access) CIRCE Parameters (SC RF): Same as the normal conductive case but: V RF = 1.5 MV I total = ma I bunch = µa α = page 20

21 E=600 MeV f rf =1.5 GHz V rf =0.6 MV L=66 m h=330 ρ = m rms pulse length [ps] CIRCE Performance Table Total power [w] Pulse peak power [kw] Energy per pulse [nj] Horizontal Acceptance = 300 mrad Power integrated between 1 and 100 cm -1 Total current [ma] Current per bunch [µa] Particles per bunch Momentum Compaction Mode Mode Mode With Superconductive RF: Vrf ~ 1.5 MV - Power & Energy increase a factor Currents increase a factor Momentum compaction increases a factor 2.5 page 21

22 MIT/Bates as a T-ray source Longitudinal equilibrium distributions (Haissinski equation solutions) and relative spectra for three different modes. Stable solutions, no IR bursts present (SR driven instability). The different modes trade between bandwidth and power. The hump-like shape on the 2.4 ps distribution indicates significant shielding from the vacuum chamber. F. Sannibale - September 2004 page 22

23 Novel techniques for using CSR Self-synchronized Electro-optic sampling provides functionality of benchtop setup w/1.5 GHz rep-rate use inherent synchronization of optical and THz beams optical source can be dipole (very weak) or undulator self-mixing techniques also possible. optical beam l W IR beam electron bunch wiggler or bend page 23

24 Pulse stacking Because input pulses are coherent, it is possible to resonate the signals to gain high pulse power levels. Input CSR pulses Peak power limited by cavity Q and phase stability of pulses page 24 T. Smith, et al., NIMA 393 (1997)

25 Development of CIRCE 1994, First idea of a storage ring based CSR source (1994) Murphy & Krinsky, NIM A 346, 571 (1994). Spring 2000, CIRCE Idea Conceived Presented at the FIR Workshop at the ALS Users Meeting, October 2001, CIRCE LBL Internal Review (March 14, 2001) Positive report. High priority to the CSR Mode of operation 2002, First demonstration of stable CSR at BESSY Abo-Bakr et al., PRL 88, (2002) 2002, Microbunching instability predicted, simulated and experimentally verified. Heifets & Stupakov, PRSTAB 5, (2002). Venturini & Warnock, PRL 89, , (2002). J. Byrd, et. al. PRL 89, , (2002). 2003, BESAC 20 year BES Facilities Roadmap (February 22, 2003) Importance of the THz science. Need of organizing a nationwide workshop 2003, First science with CSR successful Singley, et al., PR B 69 (9), (2004). 2003, Model for CSR production in storage rings Sannibale, et al., PAC 2003, to appear in PRL in , DOE-NSF-NIH THz Science Workshop (February 2004, report issued Sept 04) page 25

26 Laser Tailoring of Beams Laser slicing is a new technique for generating ~ fsec xray pulses in a storage ring. In operation at ALS since 2002, and being constructed on Bessy-II and SLS. page 26 R.W. Schoenlein, et al., Science, Mar 24, (2000) A. Zholents, M. Zolotorev, Phys. Rev. Lett. 76, 912, (1996).

27 Holy Bunches 1/24 ring after slicing 3/4 ring after slicing Holes spread due to time of flight disperson (i.e. momentum compaction) page 27 Calculated distributions for ALS with nominal and twice nominal momentum compaction.

28 Slicing CSR signals 1 msec laser rep rate Raw bolometer signal shows a signal synchronous with the laser repetition rate. The signal spectra extend up to 2 THz and depend on the initial laser pulse and proximity to the slice. Fine structure in spectra is due to measurement details. long slice short slice page 28

29 Micromodulation at CIRCE: Predicted Performance Longitudinal Distribution Modulation at Radiator Laser Modulation: 6 energy spread sigmas Laser pulse length: 50 fs FWHM Distance modulator-radiator: radiator: 2.5 m [ps] M. Zolotorev Current per bunch: 10 ma Horizontal Acceptance 100 mrad (single mode) Energy per pulse: 8.5 µj Max reprate: : khz page 29

30 X-ray pulse compression via vertical beam chirp δy ( z = σ ) z eu E beam β β rf undul 2πσ z λ rf SCRF deflecting cavity (TM110 mode) Electron trajectory Collimating slits SCRF deflecting cavity X-ray compression in asymmetric-cut crystals Undulator Radiation from tail electrons >> σ + σ 2 y' Radiation from head electrons Collimating mirror 2 x ray l Input x-ray pulse >> diffraction limited size and natural beamsize page 30

31 Examples for ALS and APS Avoids natural limits on short electron bunches high rep rate beam dynamics under study at APS and ALS page 31 Trajectory for an electron with z=σ z and 200 µrad kick high frequency SC cavity needed for sufficient deflection of high energy beams two cavities needed to allow local effect brightness reduced by compression factor

32 Summary Opportunities still abound for ultrafast pulses from storage rings CSR emission in storage is a powerful source of T-rays observed in several rings; new proposals for T-ray operation mode. possibilities of new range of techniques with high power: pulse stacking, two-color pump/probe laser tailoring allows coherent control of ultrafast T-ray pulses Vertical chirp pulse compression a viable technique for high rep rate sub-picosecond x-ray pulses page 32