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1P. Musumeci UCLA Department of Physics and Astronomy Inverse Free Electron Laser accelerators for 5th generation light sourcesP. MusumeciUCLA Department of Physics and AstronomyCatalina Island, Oct 2nd, 2010
2Outline Inverse Free Electron Lasers Outlook of Past and Future IFEL experimentsDesign of a compact laser accelerator suitable as injector for an advanced light sourceControl of beam longitudinal phase space at optical scale: The Linear pre-buncher.An example of IFEL-driven FELConclusions
3IFEL historyPalmer. 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.
4IFEL InteractionIn an IFEL the electron beam absorbs energy from a radiation field.In an FEL energy in the e-beam is transferred to a radiation fieldHigh power laserUndulator magnetic field to couple high power radiation with relativistic electronsSignificant energy exchange between the particles and the wave happens when the resonance condition is satisfied.
5Why 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.
6Why 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 tohigh 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 UCLA70 % 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 electronsAnybody 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 acceleratorsInjector for advanced light sources (ICS or FELs)
7STELLA2 experiment80 % of electrons accelerated,energy spread less than 0.5 % FWHM~30 l = 10.6 mm,gain up to 17 % of initial beam energyW. Kimura et al. First demonstration of high trapping efficiency and narrow energy spread in a laser accelerator,PRL, 92, (2004)
8Diffraction dominated IFEL @ UCLA IFEL Advanced Accelerator at the Neptune Laboratory0.5 TW 10.6 m laserStrongly tapered Kurchatov undulatorHighest recorded IFEL acceleration15 MeV beam accelerated to over 35 MeV in 25 cmRelative energy gain 150 %Accelerating gradient ~70 MeV/m !Observation of higher harmonic IFEL interactionP. Musumeci et al.,High energy gain of trapped electrons in a tapered diffraction-dominated IFEL PRL, 94, (2005)
9Inverse Free Electron Laser: lessons learned Even though radiation guiding would help, significant gain can be obtained controlling the diffraction effectsStrong tapering of both period and field is possible.Prebunching helps beam quality.There is no laser wavelength preference intrinsic in the IFEL equationsNIR lasers advantagesCommercial high power sources availableTable-top-sized laser systems.Mitigated diffraction effects
10Current 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 SLACHigh repetition rate IFEL experimentat LLNLHigh gradient helical IFELexperiment at BNLProposal for experiment at SPARC-LIFE(Italy)
13Short laser pulse IFEL100 fsGradient profile of undulator nm light requires > 3TW laser (4-5 TW preferred)Laser system is CPA, flashlamp pumped, Ti:Sapphire100 fs fiber oscillator>500 mJ, <120 fs, 10 Hz100 mJ UV arm for photo-cathodeUndulator has 19 periods; requires ~50 fs slippage of on-resonance particlesSignificant laser intensity variation over interaction length!Laser Electric Field3D simulation of IFEL.Captured bunch is ~ 100 fsec.Short laser pulse results in tail in energy distribution.
14Current Status: Laser and experimental layout are under construction 50 cm UCLA undulatorChicane couples in IFEL drive laser and allows compression of blow-out mode electron bunch.Spectrometer and diagnostic beamlineQuad triplets match into undulatorLaser entrance port; not shown is vacuum transport line from compressor50 MeV beam from LLNL photo-gun/linac
15Radiabeam 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 systemTOGETHER WITHHelical geometry.Permanent magnet double tapered undulator.Table 1. Parameters for BNL high gradient high energy gain IFEL experiment
16Helical interaction vs. Electron transverse velocity is never zero. Interaction withcircularly polarized laseris always ONFactor ~2.3 extra gradient for same electric field.PlanarHelicalvs.
17Optimized undulator tapering design Use regular NdFeB magnets. Br = 1.22 TTake into account not ideal laser transverse profile M2 = 1.5Provide large enough gap (15 mm) to minimize laser losses>98 % transmission to allow for recirculating schemes.Mechanical design finalizedMagnets orderedMachining startedParticle trajectory
18RUBICON 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 undulatorIPF1F2F32*F3Amplifier:100 cm x 2 passesZnSewindowBeam loading and phase front evolutionNext step in IFEL simulations !!!
19IFEL efficiencyBeam 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 1.3Power along the undulatorPower profile along the bunch for max currentEnergy extraction very efficient (> 80%) adjusting tapering to compensate for peak power variation along the undulator.
201 GeV IFEL design:If successful, these experiments (LLNL+ BNL) will pave the way forApplication of IFEL schemeas 5th generation light source driverCompact-size acceleratorESASE (Zholents, PRL 92, , 2004) benefits intrinsicExponential 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.
21Useful scalings for IFEL accelerator Assuming no guiding and a single stage helical undulatorThe ideal relationship between the Rayleigh range and the total undulator length isA tight focus increases the intensity, but only in one spot.A large zr maximizes the gradient over the entire undulator lengthThe final energy (assuming a constant K and a constant resonant phase)will be given byIn order to have the final energy 1 GeV (gf2 = 106) with a 1 um laser, zr = 20 cm and K ~ 4The laser power P needs to be 10 TW or higher
23Cryogenic 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.
25Tapering 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, 1981For yr -> p/2-> 0
26Hamiltonian of IFEL interaction In the longitudinal phase space (for small variation around the design energy), the Hamiltonian of the system looks like a physical pendulumThis phase space flow explain why Inverse Free Electron Laser is a strong longitudinal lens.From IFEL thesis, 2004Lasers 2001
27Longitudinal bunching and aberrations Harmonic potential: limited by initial energy spreadCos-like potential: limited by non lineraritiesLasers 2001
28Higher Harmonic IFELHigher 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.
29The >90 % bunching factor-buncher Seed with harmonics of Ti:Sa laserNeed to control relative phase and amplitude (phase retardation plates)Non linear harmonic generation crystalsIR laser pulseLaser energy (in 100 fs) to bunch 120 MeV beam800 nm100 uJ400 nm130 uJ266 nm85 uJ200 nm50 uJElectric field waveformAngle for even harmonic-coupling
30Linear “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 alreadySignificant improvements for injection into advanced accelerator.Particle tracking simulations show >99 % capture and below 0.1 % energy spread !!!Captured fraction99.5 %Energy spread0.04 %
31FEL radiation from IFEL accelerator Sending the IFEL beam into an undulatorFEL l = 3 nm (water window)Slippage dominated regime.Start-to-end simulationsCurrent peak FWHM 80 nm or 250 as1.7 GeV energyspike distance 800 nmFrom 20TW IFEL design
32Slippage Slippage in the undulator Slippage in a gain length Different FEL dynamics (weak superradiance) when Lb~ Lc
332006 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 KlystronNeed to seed for longitudinal coherenceRadiatione-Undulator sections
34Use 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 mechanismp/2 resonant phase + perfect linear pre-bunching give an extra improvement in compression.We obtain 5 kA – 0.1 mm-mrad at 1 GeV.
35IFEL-driven soft-x-ray FEL Efficiency can be increased by laser recirculation.Option to HHG seed FELGW-level peak 3 nm.Intrinsic synchronization of microbunch structure with optical phase.Strawman design10 TW laser system5J -500 fsRF LinacLinear PrebuncherStrongly tapered undulatorRF photogunFEL45 MeV1 GeV4.5 m0.5 m1.5 m3.5 mCryogenic short-periodFEL undulator< 10 meters !!!
36ConclusionsLaser 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.
37Acknowledgements Collaborators: S. Anderson, LLNL I. Pogorelsky, V. Yakimenko, BNLA. Murokh, A. Tremaine, Radiabeam TechnologiesF. O’Shea, E. Hemsink, G. Andonian, R. Li, M. Westfall, J. B. Rosenzweig, UCLAFunding agencies:DTRADOE-HEP / DOE-BESUniversity of California Office of the President