1. Design Challenges in PEP-X: An Option for Soft X-Ray FEL R. Hettel for Y. Cai SLAC National Accelerator Laboratory 3 rd Low Emittance Ring Workshop July 8-10, Oxford, England

2. Rep rate 100 MHz 100 Hz 10 6 10 12 Photons per pulse DLSR FEL - - - - Motivation for enhanced DLSR performance FELs becoming more ring-like: higher rep rate, reduced photons/pulse Can rings become more FEL-like: lower rep rate, increased ph/pulse?

3. Acknowledgements FEL & Beam Physics Department: Karl Bane, Yuantao Ding, Zhirong Huang, Alexander Novokhatski, Lanfa Wang SSRL: Xiaobiao Huang, James Safranek, John Schmerge Accelerator Design Department: Yuri Nosochkov, Min-Huey Wang Advance Computing Department: Cho-Kuen Ng, Liling Xiao SLAC Senior Management: Jerome Hastings, Robert Hettel, Norbert Holtkamp, Chi-Chang Kao

4. 4 PEP-X DLSR (“PEP-Hex”) – Diffraction limited ring for 1.5 Å (  = /4  = 12 pm) – Good beam lifetime 3 hours – Good injection with 10 mm acceptance – Achievable machine tolerances 20 microns – Off-axis injection Y. Cai, K. Bane, R. Hettel, Y. Nosochkov, M-H. Wang, M. Borland, Phys. Rev. ST Accel. Beams 15, 054002 (2012) E = 4.5 GeV I = 200 mA  x,y = 12/12 pm-rad 54 7BA cellsoff-axis injection

5. [Note: most recent version of PEP-X is 6- GeV PEP-X (“PEP-Xtra”)] Convert PEP-II hexagonal tunnel to circular geometry 7BA lattice (72 cells), 5-m straight sections 48 ea 5-m straights available for IDs; 17600 m 2 hall needed for 16 beam lines; east and west arcs not practical for beam lines 120-m straights available for injection and bypass options Cost of larger ring offset by reduced cost for experimental halls E = 6 GeV I = 200 mA  x,y = 5/5 pm-rad 72 7BA cellsoff-axis injection

6. 6 PEP-X achromats  ~  3 ~ C -3  = dipole bend angle C = ring circumference  = 29 pm at 4.5 GeV (no damping wigglers) Cell phase advances:  x =(2+1/8) x 360 0,  y =(1+1/8) x 360 0. Good dynamic aperture reached by cancelation of 3 rd - and 4 th - order lattice resonances (Developed from MAX-IV design)

7. Legend: 0.2km/2GeV: ALS-II, 52 pm 0.8km/3GeV: NSLS-III, 30 pm 1.1km/6GeV: APS-II, 80 pm 2.2km/6GeV: PEP-X, 5 pm 6.2km/9GeV: tauUSR, 3 pm Brightness and Coherence of Future Rings Current US Rings Overseas Projects & Plans US Projects DLSR Designs Competitive pressure drives optimization Upgrades & greenfield facilities possible 2-3 orders of magnitude improved brightness over existing rings DLSR Science complements FELs, offering Similar high transverse coherence Long pulses with high repetition rate High average / low peak power High stability High capacity Current US Rings Overseas Projects & Plans US Projects M. Borland for BESAC Meeting, July 2013

8. FEL lasing in a long switched bypass Electron bunch is recycled for 3 damping times in ring after lasing in the bypass Bunch switched into FEL bypass (10-100 kHz) Pellegrini et al., 1992

9. Reduce bunch length from 10 ps to 1 ps without reducing bunch current Calculation of microwave instability threshold An illustration using 4.5-GeV PEP-X nominal parameters: f rf = 476 MHz, V rf =8.3 MV, f rev = 136.312 kHz,  z =3 mm, I b =0.067 mA.

10. ParameterPEP-X (USR)PEP-X (FEL) Beam energy [GeV]4.5 Circumference [m]2200 Current [mA]20010* Betatron tune (H/V)113.23/65.14 Momentum compaction4.96x10 -5 Emittance [pm-rad] (H/V)12/12160/1.6 Bunch length [mm]30.3 Energy spread1.55x10 -3 Energy loss per turn [MeV]2.95 RF voltage [MV]8.3282.0 RF frequency [MHz]4761428** Damping time [ms]18 Length of ID straight [m]5Use also long straights (> 100 meter) Beta at ID center (H/V) [m]4.9/0.8 * Limited by SRF HOMs** 2- or 3-frequency RF to provide long and short bunches is possible

11. Transverse Gradient Undulator (TGU) Generate a linear x-dependence of the undulator fields: For a large energy-spread beam, disperse the beam according to its energy: Choose dispersion and transverse gradient: low gain FEL : T. Smith et. al., J. Appl. Phys. 50, 4580 (1979). N. Kroll et. al., IEEE Journal of Quan. Electro. QE-17, 1496 (1981). high gain FEL applying to laser-plasma driven accelerator : Z. Huang, Y. Ding and C. Schroeder, Phys. Rev. Lett. 109, 204801 (2012). Betatron beam size << dispersed beam size  rotate TGU to take advantage of very small vertically coupled emittance in DLSR

12. IBS growth for 1ps bunch Flat beam option. Vertical emittance is 1% of the horizontal one. Energy Spread Horizontal Emittance

13. Electron beam and radiation size FEL radiation far-field pattern, (Full transverse coherence!) Dispersion dominated size Emittance size Almost round beam Energy spread along the undulator  E /0.511 [MeV] Simulation using a modified Genesis  E /E=5.5x10 -5

14. Radiation power and spectrum Saturation is reached with > 200 MW FEL power For a bunch with  z =1ps, FEL pulse energy is estimated about 0.2 mJ (~2x10 12 ph/pulse) @100m

15. PEP-X(FEL) at 1.5nm ParameterPEP-X(FEL)LCLS (150pC case) Undulator u = 3cm, K=3.7 (TGU) u = 3cm, K=3.5 Radiation wavelength1.5 nm Pulse energy0.2 mJ (1.6x10 12 photons)2 mJ (1.6x10 13 photons) Peak power200 MW20 GW Pulse length1 ps50-150 fs Saturation length90 m40 m Repetition raten b x 100 Hz120 Hz Electron norm. emittance1.45(x)/0.0145 (y) mm mrad0.5 mm mrad Electron peak current300 A1000-3000 A Electron energy4.5 GeV4.3 GeV Electron energy spread1.55x10 -3 3.1x10 -4 Note that n b is number of bunches. Bunches are recycled after three damping times in PEP-X.

16. SCRF in 12 GeV CEBAF Upgrade We assume 20 MV/m for accelerating gradient. Three modules are necessary to reach 1ps bunch length. 8 cavities in a cryomodule produce 108 MV 7-cell SCRF Performance of SCRF (1497 MHz at 2 K)

17. Measured dipole HOMs in JLAB SRF cavity

18. Multi-bunch instability driven by 24 JLab SCRF cavities  =76µs  =7µs  =18µs Beam current: 200mA Bunch Number: 3492 Beam filling pattern: uniform Beam Energy: 4.5GeV Horizontal Vertical 200 bunches with 20mA beam current seems stable based on the current SCRF technology. That provides us 20 kHz repetition rate for the FEL.

19. R&D plan 1ps bunch – Improve design of a 1.5 GHz SCRF cavity similar to those built for CEBAF upgrade – Build a prototype to demonstrate its performance in terms of accelerating gradient and sufficient damping of HOM – Install the prototype in SPEAR3 to further quantify its performance with electron beam Study how to drive an x-oscillator – TGU in low-gain region with 20A peak current – 10,000 bunches to reach 1MHz repetition rate – Increase beam energy to 6 GeV (PEP-Xtra) and further reduce emitttance (5/5 pm) or use harmonic lasing

20. DLSRs are capable to drive SASE FEL in soft x-ray region to saturation within 100 meter using 1ps bunch in transverse gradient undulators. Based on the CEBAF upgrade, three cryomodules with 300 MV accelerating gradient are sufficient to reduce bunch length to 1ps and retain stability of 200 bunches with 20 mA beam current. TGU can be applied to accommodate large energy spread in storage rings. It is necessary to rotate the undulator 90 0, taking advantage of a very small vertical emittance in the ring. A feasible SASE FEL at radiation wavelength of 1.5 nm – Achieves full transverse coherence – Provides 0.2-mJ energy or 2x10 12 photons per pulse – Has pulse length of 1 ps – Reaches a repetition rate of 10 kHz DLSR-based hard X-ray XFELOs might be possible (under study) Conclusion