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P. Musumeci UCLA Department of Physics and Astronomy

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1 P. Musumeci UCLA Department of Physics and Astronomy
Inverse Free Electron Laser accelerators for 5th generation light sources P. Musumeci UCLA Department of Physics and Astronomy Catalina Island, Oct 2nd, 2010

2 Outline Inverse Free Electron Lasers
Outlook of Past and Future IFEL experiments Design of a compact laser accelerator suitable as injector for an advanced light source Control of beam longitudinal phase space at optical scale: The Linear pre-buncher. An example of IFEL-driven FEL Conclusions

3 IFEL history Palmer. Journal of Applied Physics. 43, 3014, Interaction of relativistic particles and free electromagnetic waves in presence of a static helical magnet. Courant, Pellegrini, Zakowicz. Physical Review A, 32, 2813, 1985 High energy Inverse Free Electron laser Accelerator. IFEL for many years has been considered a form of pay-back to the High Energy Physics community for the fundamental contribution from HEP-driven accelerator research to the development of Free-Electron Lasers.

4 IFEL Interaction In an IFEL the electron beam absorbs energy from a radiation field. In an FEL energy in the e-beam is transferred to a radiation field High power laser Undulator magnetic field to couple high power radiation with relativistic electrons Significant energy exchange between the particles and the wave happens when the resonance condition is satisfied.

5 Why you don’t want to hear anymore about IFELs
Complicate experiment. Difficult requirements on laser and magnet technology. Synchrotron losses at high energy. NOT feasible for HEP multi-TeV machines. Gradient is energy dependent. Ion linac-like dynamic. Dwarfed by successes of laser/plasma and beam/plasma schemes.

6 Why IFELs (again…)? Injector for advanced light sources (ICS or FELs)
IFEL scales ideally well for mid-high energy range (50 MeV – up to few GeV) due to high power laser wavelengths available (10 um, 1 um, 800 nm) permanent magnet undulator technology (cm periods) Simulations show high energy/ high quality beams with gradients >500 MeV/m achievable with current technology! 70 MeV/m gradient already demonstrated at UCLA 70 % trapping already demonstrated at BNL. Preservation of injected e-beam quality/emittance. (Essentially 1D acceleration) Microbunching: still the preferred interaction for longitudinal phase space manipulation at optical scale. Efficient mechanism to transfer energy from laser to electrons Anybody interested in a compact 1-2 GeV injector? Laser-plasma accelerators. Main competitors. But…. Need > TW laser power to accelerate beams to 1 GeV. Strongly non-linear injection mechanism. Controlled injection ? Beam quality ?? Injector + (phase-locking) microbuncher for other kinds of advanced accelerators Injector for advanced light sources (ICS or FELs)

7 STELLA2 experiment 80 % of electrons accelerated, energy spread less than 0.5 % FWHM ~30 l = 10.6 mm, gain up to 17 % of initial beam energy W. Kimura et al. First demonstration of high trapping efficiency and narrow energy spread in a laser accelerator, PRL, 92, (2004)

8 Diffraction dominated IFEL @ UCLA
IFEL Advanced Accelerator at the Neptune Laboratory 0.5 TW 10.6 m laser Strongly tapered Kurchatov undulator Highest recorded IFEL acceleration 15 MeV beam accelerated to over 35 MeV in 25 cm Relative energy gain 150 % Accelerating gradient ~70 MeV/m ! Observation of higher harmonic IFEL interaction P. Musumeci et al.,High energy gain of trapped electrons in a tapered diffraction-dominated IFEL PRL, 94, (2005)

9 Inverse Free Electron Laser: lessons learned
Even though radiation guiding would help, significant gain can be obtained controlling the diffraction effects Strong tapering of both period and field is possible. Prebunching helps beam quality. There is no laser wavelength preference intrinsic in the IFEL equations NIR lasers advantages Commercial high power sources available Table-top-sized laser systems. Mitigated diffraction effects

10 Current IFEL projects Most of them UCLA-centric
Microbunching experiment at Neptune (7th harmonic) Helical bunching experiment at Neptune (again harmonic coupling, interesting beam modes) Permanent magnet helical undulator development. Praseodymium based cryogenic undulator. Prebunching at 800 nm at SLAC High repetition rate IFEL experiment at LLNL High gradient helical IFEL experiment at BNL Proposal for experiment at SPARC-LIFE(Italy)



13 Short laser pulse IFEL 100 fs Gradient profile of undulator nm light requires > 3TW laser (4-5 TW preferred) Laser system is CPA, flashlamp pumped, Ti:Sapphire 100 fs fiber oscillator >500 mJ, <120 fs, 10 Hz 100 mJ UV arm for photo-cathode Undulator has 19 periods; requires ~50 fs slippage of on-resonance particles Significant laser intensity variation over interaction length! Laser Electric Field 3D simulation of IFEL. Captured bunch is ~ 100 fsec. Short laser pulse results in tail in energy distribution.

14 Current Status: Laser and experimental layout are under construction
50 cm UCLA undulator Chicane couples in IFEL drive laser and allows compression of blow-out mode electron bunch. Spectrometer and diagnostic beamline Quad triplets match into undulator Laser entrance port; not shown is vacuum transport line from compressor 50 MeV beam from LLNL photo-gun/linac

15 Radiabeam Ucla BNL-IFEL COllaboratioN: RUBICON
The experiment main goal is to achieve energy gain and gradient significantly larger than what possible with conventional RF accelerators to propose IFEL as a viable technology for mid-high energy range accelerators. This can be achieved using the existing ATF e-beam and high power CO2 laser system TOGETHER WITH Helical geometry. Permanent magnet double tapered undulator. Table 1. Parameters for BNL high gradient high energy gain IFEL experiment

16 Helical interaction vs. Electron transverse velocity is never zero.
Interaction with circularly polarized laser is always ON Factor ~2.3 extra gradient for same electric field. Planar Helical vs.

17 Optimized undulator tapering design
Use regular NdFeB magnets. Br = 1.22 T Take into account not ideal laser transverse profile M2 = 1.5 Provide large enough gap (15 mm) to minimize laser losses >98 % transmission to allow for recirculating schemes. Mechanical design finalized Magnets ordered Machining started Particle trajectory

18 RUBICON to demonstrate IFEL Recirculation
IFEL does not need to wait for any plasma recombination time-scale. Laser power can be recirculated to increase average power and wall-plug efficiency !!! A 22-m reamplification loop will carry 6 pulses (12 ns apart), to achieve RUBICON goal of pulse train IFEL acceleration. IFEL undulator IP F1 F2 F3 2*F3 Amplifier: 100 cm x 2 passes ZnSe window Beam loading and phase front evolution Next step in IFEL simulations !!!

19 IFEL efficiency Beam loading or pump depletion effects for high accelerated beam charge ( 1 1GeV = 1 J of energy ). Modified Genesis version + script to take into account varying period. Simulate radiation (and particle) IFEL dynamics with GENESIS 1.3 Power along the undulator Power profile along the bunch for max current Energy extraction very efficient (> 80%) adjusting tapering to compensate for peak power variation along the undulator.

20 1 GeV IFEL design: If successful, these experiments (LLNL+ BNL) will pave the way for Application of IFEL scheme as 5th generation light source driver Compact-size accelerator ESASE (Zholents, PRL 92, , 2004) benefits intrinsic Exponential gain length reduction due to peak current increase. Absolute timing synchronization with external laser optical phase at attosecond level. Control of FEL radiation pulse envelope. Need control of output energy spread !!! Valid competitor for first Advanced Accelerator driven/ 5th generation light radiation source. See FEL 2006, Berlin.

21 Useful scalings for IFEL accelerator
Assuming no guiding and a single stage helical undulator The ideal relationship between the Rayleigh range and the total undulator length is A tight focus increases the intensity, but only in one spot. A large zr maximizes the gradient over the entire undulator length The final energy (assuming a constant K and a constant resonant phase) will be given by In order to have the final energy 1 GeV (gf2 = 106) with a 1 um laser, zr = 20 cm and K ~ 4 The laser power P needs to be 10 TW or higher

22 Praesodymium based cryogenic undulator

23 Cryogenic undulator + 10 TW laser power “green-field” design
Helical undulator to maximize energy exchange (interaction always ON). Fully permanent magnet design (no iron poles) Keep magnetic field amplitude well under the Halbach limit for 6 mm gap to ensure technical feasibility. 1 GeV goal with minimum laser power to get to the soft-x-ray region.

24 IFEL longitudinal phase space

25 Tapering optimization
The undulator period and magnetic field amplitude are changed trying to control the resonant phase of acceleration and the longitudinal phase space parameters. Compromise between stability (low resonant phase) and gradient (high resonant pahse). Varying phase along the undulator. Improvements in gradient, energy spread and peak current ! From KMR, IEEE. J. Quantum Electronics, 1981 For yr -> p/2 -> 0

26 Hamiltonian of IFEL interaction
In the longitudinal phase space (for small variation around the design energy), the Hamiltonian of the system looks like a physical pendulum This phase space flow explain why Inverse Free Electron Laser is a strong longitudinal lens. From IFEL thesis, 2004 Lasers 2001

27 Longitudinal bunching and aberrations
Harmonic potential: limited by initial energy spread Cos-like potential: limited by non linerarities Lasers 2001

28 Higher Harmonic IFEL Higher harmonic interaction has been first observed in UCLA experiment, and then studied in SLAC experiment. More recently the efficiency of the interaction has also been shown in the Neptune 7th harmonic IFEL experiment. Even harmonic interaction is also strong when there is an angle between electron and laser beams. New concept: Combine first harmonics to “linearize” the IFEL buncher.

29 The >90 % bunching factor-buncher
Seed with harmonics of Ti:Sa laser Need to control relative phase and amplitude (phase retardation plates) Non linear harmonic generation crystals IR laser pulse Laser energy (in 100 fs) to bunch 120 MeV beam 800 nm 100 uJ 400 nm 130 uJ 266 nm 85 uJ 200 nm 50 uJ Electric field waveform Angle for even harmonic-coupling

30 Linear “perfect” IFEL pre-Buncher
By using a multiple-harmonic buncher one could approximate harmonic oscillator and linearize the potential. S. Pottorf and X.J. Wang, “Harmonic Inverse Free Electron Laser Micro -buncher”, BNL –68013 (2000). Not worth for “coherent radiation production” since bunching factor is already Significant improvements for injection into advanced accelerator. Particle tracking simulations show >99 % capture and below 0.1 % energy spread !!! Captured fraction 99.5 % Energy spread 0.04 %

31 FEL radiation from IFEL accelerator
Sending the IFEL beam into an undulator FEL l = 3 nm (water window) Slippage dominated regime. Start-to-end simulations Current peak FWHM 80 nm or 250 as 1.7 GeV energy spike distance 800 nm From 20TW IFEL design

32 Slippage Slippage in the undulator Slippage in a gain length
Different FEL dynamics (weak superradiance) when Lb~ Lc

33 2006 proposed solution: Insert slippage sections between undulators
Larger energy (because of longer period SPARC-like undulators). Smaller gain. Between undulator sections we insert a magnetic delay section for the electron beam to realign current and radiation spikes. The slippage section effectively is a positive R56 region that helps the conversion between energy modulation and bunching. Optical Klystron Need to seed for longitudinal coherence Radiation e- Undulator sections

34 Use low charge@injection approach
Use 0.1 mm-mrad from low-charge operation of RF photoinjector. Assuming emittance is preserved through IFEL. Usually get a factor of 10 enhancement from ESASE mechanism p/2 resonant phase + perfect linear pre-bunching give an extra improvement in compression. We obtain 5 kA – 0.1 mm-mrad at 1 GeV.

35 IFEL-driven soft-x-ray FEL
Efficiency can be increased by laser recirculation. Option to HHG seed FEL GW-level peak 3 nm. Intrinsic synchronization of microbunch structure with optical phase. Strawman design 10 TW laser system 5J -500 fs RF Linac Linear Prebuncher Strongly tapered undulator RF photogun FEL 45 MeV 1 GeV 4.5 m 0.5 m 1.5 m 3.5 m Cryogenic short-period FEL undulator < 10 meters !!!

36 Conclusions Laser accelerators have made tremendous progress and will soon be competitive with more conventional machines. IFEL accelerator among these offers control of the beam properties. Radiabeam-UCLA-BNL will show high gradient helical IFEL acceleration. UCLA-LLNL IFEL will show high rep-rate, good beam quality. If successful, these experiments will pave the way towards IFEL-based compact soft-x ray radiation source. Ultrashort probe beams will come from a synergy between laser and accelerator worlds.

37 Acknowledgements Collaborators: S. Anderson, LLNL
I. Pogorelsky, V. Yakimenko, BNL A. Murokh, A. Tremaine, Radiabeam Technologies F. O’Shea, E. Hemsink, G. Andonian, R. Li, M. Westfall, J. B. Rosenzweig, UCLA Funding agencies: DTRA DOE-HEP / DOE-BES University of California Office of the President

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