High Energy Gain Helical Inverse Free Electron Laser Accelerator at Brookhaven National Laboratory

Transcription

1 High Energy Gain Helical Inverse Free Electron Laser Accelerator at Brookhaven National Laboratory J. Duris 1, L. Ho 1, R. Li 1, P. Musumeci 1, Y. Sakai 1, E. Threlkeld 1, O. Williams 1, M. Babzien 2, M. Fedurin 2, K. Kusche 2, I. Pogorelsky 2, M. Polyanskiy 2, V. Yakimenko 3 1 UCLA Department of Physics and Astronomy, Los Angeles, CA Accelerator Test Facility, Brookhaven National Laboratory, Upton, NY, SLAC National Accelerator Laboratory, Menlo Park, CA, HBEB Workshop on High Brightness Beams San Juan, Puerto Rico March 26th 2013

2 Outline Brief IFEL introduction IFEL experiments Rubicon IFEL project o o o Helical undulator Experimental setup Electron energy spectra 1 GeV IFEL concept IFEL driven mode-locked soft x-ray FEL

3 IFEL interaction Undulator magnetic field couples high power radiation with relativistic electrons Undulator parameter Normalized laser vector potential Energy exchanged between laser and electrons maximized when resonant condition is satisfied Courant, Pellegrini, and Zakowicz, Phys Rev A, 32, 2813 (1985)

4 IFEL characteristics Inverse Free Electron Laser accelerators suitable for mid to high energy range compact accelerators Laser acceleration => high gradients Vacuum acceleration => preserves output beam quality Energy stability => output energy defined by undulator Microbunching => manipulate longitudinal phase space at optical scale Interest lost as synchrotron losses limit energy to few GeV (so no IFEL based ILC) Recent renewed interest in compact GeV accelerator for light sources

5 IFEL experiments STELLA2 at Brookhaven - Gap tapered undulator - 30 GW CO2 laser - 80% of electrons accelerated W. Kimura et al. PRL, 92, (2004) UCLA Neptune IFEL - Strongly tapered period and amplitude planar undulator GW CO2 laser - 15 MeV -> 35 MeV in ~25 cm - Accelerating gradient ~70 MeV/m P. Musumeci et al. PRL, 94, (2005)

6 Radiabeam-UCLA-BNL IFEL CollaboratiON RUBICON Unites the two major groups active in IFEL Past experience: UCLA Neptune, BNL STELLA 2 Builds off UCLA Neptune experiment: strong tapering + helical geometry for higher gradient Collaboration paves the way for future applications Higher gradient IFEL Inverse Compton scattering Soft x-ray FEL

7 Experimental design Parameter Input e-beam energy Final beam energy Final beam energy spread Average accelerating gradient Laser wavelength Laser power Laser focal spot size (w) Laser Rayleigh range Undulator length Undulator period Magnetic field amplitude Value 50 Mev 117 MeV 2% rms 124 MV/m 10.3 μm 500 GW 980 μm 25 cm 54 cm 4 6 cm kg Parameters for the RUBICON IFEL experiment

8 Helical undulator Electrons always moving in helix so always transferring energy. Helical yields at least factor of 2 higher gradient. Especially important for higher energy (high K) IFEL's.

9 Helical undulator design First strongly tapered high field helical undulator 2 orthogonal Halbach undulators with varying period and field strength NdFeB magnets B r = 1.22T Entrance/exit periods keep particle oscillation about axis Pipe of 14 mm diameter maintains high vacuum and low laser loses Estimated particle trajectories Laser waist

10 Beamline layout

11 Timing S0/Sref Coarse alignment with stripline coincidence Germanium used for few ps timing Maximize interaction for fine timing σ=7.2 ps Δt laser NaCl Dipole Ge wafer S0 e-beam

12 Polarization 0, 4.6 J 30, 4.4 J Quarter wave plate polarizes CO2 elliptically before amplification One handedness matches undulator 60, 5.52 J 90, 6.11 J > 5 J > 4 J < 4 J 180, 4.5 J All shots have delay 1854 and 800 pc charge circular polarization linear polarization circular (opposite handedness) circular polarization *Preliminary data

13 Laser-ebeam cross correlation Cross correlation measurement of laser and 1 ps long e-beam using IFEL acceleration as a benchmark Gradient scales proportional to the square root of the laser power so scale momenta sigma = 4.5 ps Estimated rms pulse width < 4.5 ps Delay (ps)

14 IFEL acceleration 100% energy gain *Preliminary

15 Compare spectra Looks like temporal effects at play here low power tails? 300 GW 7 GW Deficit at 52 MeV likely from phosphor damage

16 Where to go from here Doubled electron energy, now increase efficiency o o o Retune undulator for higher efficiency capture Measure transverse emittance Better characterize laser Move to Ti:Sa laser o o o o More power => higher gradient Shorter wavelength => shorter undulator period >10 TW commercially available LLNL IFEL: world's first 800 nm driven IFEL Neptune undulator + 4 TW Ti:Sa 50 -> 200 MeV

17 GeV class IFEL Strongly tapered helical undulator 20 TW Ti:Sa (800 nm) GeV IFEL Input energy at focus Emittance Laser spot size Rayleigh range 100 MeV 100 μm 0.25 mm mrad 240 μm 20 cm

18 Prebunch for higher current Increase fraction captured by prebunching input beam uniform beam injected prebunched beam injected

19 Harmonic microbunching Harmonic microbunching further enhances capture and reduces energy spread of accelerated beam by increasing bunching of prebunched beam. Linearize ponderomotive force by coupling electrons to harmonics of the drive laser monochromatic prebunched input harmonic prebunched input

20 High current 1GeV IFEL Harmonic prebuncher 1 ka input B = 800 nm 40 cm 18 nm rms GeV IFEL accelerates beam 0.18% rms 100 MeV 20 TW Ti:Sa 1 m 954 MeV 98% capture 13.5 ka peak current

21 Soft x-ray FEL 5 nm SASE FEL saturates in 10 m with constant current beam But IFEL beam is microbunched Requires 50 times longer to saturate with a constant undulator => ~500 m effective gain length! Some dielectric accelerators have similar bunch trains

22 Mode locked FEL slippage in one undulator Mode locked FEL's produce short pulses with controllable bandwidth * Microbunched beam acts as a periodic lasing medium similar to a ring resonator Can enhance slippage by using chicanes so that pulses always see gain medium Slippage provided by chicanes between gain sections introduces mode coupling Periodic resonance condition controlled by energy or current modulation Micro bunches Radiation after one undulator Slippage in chicane Radiation after next undulator slippage in one chicane * Thompson and McNeil, Phys. Rev. Lett., 100, (2008)

23 IFEL driven mode-locked FEL Energy 954 MeV Relative energy spread 0.18 % Temporal Spectra Bunching period Peak current Microbunch length (rms) 800 nm 13 ka 18 nm FEL wavelength 5 nm Undulator period 16 mm Periods per undulator as FWHM mode separation Periods slipped per chicane 144 number of sidebands Total slippage 160 Slippage enhancement 10 Undulator + chicane segments 54 Pulse width controlled with number of periods per undulator Spectral width controlled by number periods per undulator

24 Summary Rubicon helical IFEL experiment at BNL Observed polarization dependence Doubled e-beam energy: >50 MeV gain High gradient ~100 MeV/m Interest in IFEL's renewed for compact light source applications GeV IFEL possible with helical undulator and 20 TW Ti:Sa laser Natural compact driver for mode-locked soft x-ray FEL