1. High gradient IFEL acceleration and deceleration in strongly tapered undulators P. Musumeci, J. Duris, N. Sudar EAAC 2015

2. Outline IFEL accelerator background Rubicon experiment GeV IFEL @ ATF2 Towards IFEL applications (ICS, FEL, recirculation) TESSA – First results from Nocibur experiment at ATF Acknowledgements – Collaborators: A. Murokh, A. Gover, J. B. Rosenzweig, all ATF staff – Funding agencies: DTRA, DNDO, DOE

3. IFEL Interaction 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. In an FEL energy in the e-beam is transferred to a radiation field In an IFEL the electron beam absorbs energy from a radiation field. High power laser

4. What you need to know about IFELs ? IFEL scales ideally well for mid-high energy range (50 MeV – up to few GeV) due to – High power laser wavelengths available (10  m, 1  m, 800 nm) – Mature permanent magnet undulator technology (cm periods) Plane wave or far field accelerator : minimal 3D effects. – Transverse beam dimensions can be mm-size for  m-scale accelerating wavelength! Vacuum-based accelerator – Efficient mechanism to transfer energy from laser to electrons – Simulations show high energy/ high quality beams with gradients ~GeV/m achievable with current technology! – Preservation of e-beam quality/emittance and high capture. Microbunching: longitudinal phase space manipulation at optical scale

5. Rubicon IFEL Helical geometry high gain high gradient IFEL First strongly tapered helical undulator 93 MeV – 1.8 % energy spread Very reproducible (mean energy std < 1.5 %) Input e-beam energy50 Mev Average accelerating gradient100 MV/m Laser wavelength10.3 μm Laser power100-300 GW Laser focal spot size (w)980 μm Laser Rayleigh range25 cm Undulator length54 cm Undulator period4 – 6 cm Magnetic field amplitude5.2 – 7.7 kG No laser Laser on – consecutive shots

6. Most recent Rubicon run UCLA permanent magnet based prebuncher Permanent magnet chicane with adjustable R56 Transverse matching into undulator yields emittance preservation ~30 cm

7. First TW-class laser driven IFEL Strongly tapered Kurchatov undulator for diffraction-dominated interaction Short pulses (sub-ps) interaction 77 MeV – 122 MeV in 22 cm > 200 MV/m gradient ! Sub-ps synchronization and timing LLNL Ti:Sa IFEL accelerator Design Parameters InitialFinal Period1.5 cm5.0 cm Peak K parameter 0.22.8 Neptune Energies 14 MeV52 MeV LLNL Energies 50 MeV200 MeV EOS measurement IFEL signal Laser off Laser on Simulations Courtesy of J. Moody Laser Electric Field 100 fs

8. Demonstrate GeV/m gradients Demonstrate GeV-class energy gain ATF-2 design Flexibility : undulator can be retuned as demonstrated in Rubicon Towards next generation GeV IFEL ParameterValue Laser power 25 TW Laser pulse length> 0.5 ps M2M21.5 Gap 10 mm Input energy90 MeV Output energy500 MeV Energy spread2 % Undulator length75 cm Gradient> 0.5 GeV/m Undulator options  UCLA Halbach-style undulator: scalable and affordable NdFeB magnets Varying period and field strength Entrance/exit periods keep trajectory on axis  Praesodymium-based cryo-undulator  Superconducting helical undulator

9. IFEL accelerator advances Ponderomotive resonant phase determines all properties of accelerator – Area of bucket – Acceptance – Gradient Can we control the output energy spread by ponderomotive phase tapering? Longitudinal emittance compensation regime Beyond limits set by diffraction dominated interaction Waveguide IFEL Enabled by short (ps-scale) CO2 pulses Gradient nearly uniform along the undulator Collaboration with S. Tochitsky From Sung et al. AAC Proceedings

10. Applications Inverse Compton Scattering- based compact gamma-ray source Compact Modelocked soft-X-ray FEL – Take advantage of current increase – Use chicane to realign radiation spikes with e-beam modulation IFEL Recirculate drive laser for IFEL to increase repetition rate and average flux of photons

11. Lessons from Inverse FEL FEL beam-laser energy exchange is usually < 1 MeV/m IFEL demonstrated energy exchange rate ~ 100 MeV/m Design studies indicate possibility of 1 GeV/m Can we run IFEL in reverse? High power laser In an IFEL the electron beam absorbs energy from a radiation field. UCLA results from prebunched RUBICON. J. Duris et al, Nature Comm. 5, 4928, 2014

12. TESSA Inverse IFEL = FEL TESSA (Tapering Enhanced Stimulated Superradiant Amplification) E-beam rapid deceleration  laser amplification Requires seed pulse of high intensity (larger than FEL P SAT ) E-beam can be prebunched, or it can be bunched in the first few undulator periods High efficiency conversion of electron beam energy to coherent radiation opens door to very high average power light sources. Wavelength set by e-beam energy and resonant condition -> wide tunability High average power IR and visible lasers. X-rays. EUV-L applications.

13. NOCIBUR Use RUBICON IFEL set up in reverse at ATF/BNL Potentially demonstrate >40 % (!!!) energy extraction from a relativistic electron beam IFEL uses no boundaries, no structure (dielectric), no medium (plasma) to couple laser to relativistic electrons => No losses ! All hardware base (with some readjustments) and personnel is there; experiment is ongoing!!!

14. TESSA simulations at 13.5 nm Refocusing mirrors to recreate high intensity condition With 3 kA beam achieved 50% efficiency in 15 meters !

15. Conclusion All results point to IFEL as mature and reliable laser-based high gradient accelerator technology Need to push to GeV/m gradient and GeV energy gain Enable laser-driven compact accelerator applications IFEL physics – Beam quality control. Final emittance and energy spread. – Prebunched injection – Guided IFEL – Extreme tapering. Undulator technology driver. – Beam loading? TESSA is a novel concept to achieve over an order of magnitude improvement to FEL efficiency – Nocibur limitation: deceleration experiment (low gain), laser beam is much longer than e- beam, amplification is hard to measure – To demonstrate orders of magnitude amplification (high gain), we plan to use 266 nm laser as a seed, focused to a small spot size – Preliminary studies indicate over 3 orders of magnitude TESSA amplification in 4-5 meters undulator at 266 nm, with energy extraction in the range of 10-30 % depending on the e-beam parameters.

16. TESSA vs. tapered FEL TESSA operates at much stronger field intensity: – stimulated emission vs. spontaneous (order of magnitude higher rate of beam-laser energy exchange) – 100% beam capture despite very aggressive tapering ~ρ SASE seed adiabatic taper TESSA seed