1. High efficiency electron acceleration and deceleration in strongly tapered undulators Joseph Duris SLAC RHIC/AGS User's Meeting Thursday, June 9, 2016

2. Outline Inverse free electron laser (IFEL) acceleration RUBICON helical IFEL experiment at ATF – Main experimental work of my dissertation Current and future IFEL projects at ATF – NOCIBUR decelerator – IFEL driven Compton scattering

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 intensity laser

4. Why are we interested in IFELs? IFEL well suited for mid-high energy ranges (50 MeV – up to few GeV) – High power lasers 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 wavelengths. Vacuum-based accelerator – Efficient mechanism to transfer energy from laser to electrons – Simulations show high energy, high quality beams with large gradient ~GeV/m achievable with current technology! – Preservation of e-beam quality/emittance and high capture. Stable energy output: static undulator field sets resonant energy. Potential for compact GeV-class accelerators for light sources

5. Rubicon IFEL experiment 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 range30 cm Undulator length54 cm Undulator period4 – 6 cm Magnetic field amplitude5.2 – 7.7 kG Helical geometry high gain high gradient IFEL First strongly tapered helical undulator Two different tapers used – Demonstrate control of the final beam properties by undulator tuning

6. High quality output beams 93 MeV – 1.8 % energy spread Very reproducible (mean energy std < 1.5 %) despite 30% rms laser power fluctuations Laser intensity 5 orders of magnitude lower than LWFA No laser Laser on – consecutive shots Emittance measurement o Quad scan on energy dispersed beam o Emittance growth (from 2  m -> 3  m) is due to mismatching in the undulator

7. Prebunched Rubicon IFEL UCLA permanent magnet based prebuncher Permanent magnet chicane with adjustable R 56 Achieved > 55% capture IFEL interaction preserves emittance ~30 cm 93 MeV peak 2% rms energy spread 90 pC >90 MeV Unaccelerated emittance 2.3 um Accelerated emittance 2.4 um GPT Energy modulator Chicane

8. Present and future IFEL projects Experiments point to IFEL as mature and reliable laser-based high gradient accelerator technology capable of producing useful beams for compact light sources The UCLA-BNL IFEL collaboration is still going strong! Ongoing experiment using the IFEL beams to produce x-rays via inverse Compton scattering Recirculation project underway to improve net IFEL efficiency with goal of achieving high rep-rate Compton light source Recently completed the NOCIBUR IFEL decelerator experiment, extracting 30% of the energy from the electron beam as 10 um radiation – Coupled with a high average power e-beam and optical cavity, could produce a high average power laser Future possible IFEL projects at ATF2 Push to approach GeV/m gradients and GeV energy gain with upgraded CO 2 laser High-gain UV light source: amplify 266 nm laser by 3 orders of magnitude in 4 m IFEL driven mode-locked FEL taking advantage of the IFEL microbunching

9. Acknowledgements The Rubicon IFEL and related experiments could not have happened without Pietro Musumeci (advisor), Yusuke Sakai, Nick Sudar, Evan Threkheld, Ivan Gadjev, Oliver Williams, Jamie Rosenzweig (UCLA) Igor Pogorelsky, Mikhail Polyanskiy, Mikhail Fedurin, Marcus Babzien, Karl Kusche, Christina Swinson, Paul Jacob, Robert Malone, Marc Montemagno, George Stenby (ATF) Vitaly Yakimenko and Ren Kai Li (SLAC) Alex Murohk of Radiabeam Technologies Funding agencies: DOE, DTRA, DNDO Also thanks to the Agostino Marinelli, the LCLS group, and BNL for helping me to attend this meeting