1. X-Ray Free-Electron Laser Amplifiers and Oscillators for Materials and Fundamental Research Kwang-Je Kim ANL and U. of Chicago ICABU Meeting November 12, 2013 DaeJeon, Korea October 29, 2013

2. Relativity and Synchrotron Radiation KJK ICABU Nov 12 2013 2

3. 3 More efficient x-ray production with an “undulator” Compact undulator with permanent magnets

4. Radiation by one electron in N u period undulator  The e - emits EM wave in the forward direction due to its x-acceleration. Consider the wave fronts from successive undulator periods:  The e - is slower since (1) c > v  c(1-1/2  2 ), and (2) its trajectory is curved. Thus, the EM wave slips ahead of the e - in one undulator period by a distance   wavelength: –  = u (1+K 2 /2)/2  2,   [keV]=12.4/   Å   After travelling N u periods of the undulator, an N u -cycle wave-train is formed: –  z rad = N u   =1/N u KJK ICABU Nov 12 2013 4 u  N u  e K/  E-field direction

5. An N u -cycle wave-train KJK ICABU Nov 12 2013 5 c

6. Undulator radiation from electrons randomly distributed in a “bunch” KJK ICABU Nov 12 2013 6

7. Transverse coherence KJK ICABU Nov 12 2013 7

8. Amplification in the presence of e-beam  When the EM wavelength satisfies the undulator condition, an electron sees the same EM field in the successive period  sustained energy exchange  An e - arriving at A 0 loses energy to the field (ev  E <0). Similarly the e - at distance n   n=1,2,… also loses energy. However, those at   /2 +n) away gain energy.  The electron beam develops energy modulation (period length  ).  Higher energy electrons are faster  density modulation develops  Coherent EM of wavelength  is generated  “Free electron laser” KJK ICABU Nov 12 2013 8  A0A0   A3A3 A2A2 A1A1

9. 9 SASE: Initial undulator radiation is amplified to intense, quasi-coherent radiation Undulator Regime Exponential Gain Regime Saturation Electron Bunch Micro-Bunching Transverse mode z = 25 m z = 37.5 m z = 50 m z = 90 m KJK ICABU Nov 12 2013

10. SASE: Microbunching in each coherent region  # of coherent regions= n lc  # of electrons in one coherent region = N lc =N e /n lc  Radiation intensity = (N lc ) 2 n lc = (N e ) 2 /n lc =N e N lc  X-ray FELs are driven by linear accelerators KJK ICABU Nov 12 2013 10

11. An FEL for x-rays requires high e-beam qualities not achievable from storage rings  photo-cathode gun & a linear acc KJK ICABU Nov 12 2013 11 KEK/JAERI DC gun LBNL 180 MHz RF Photocathode BNL type LCLS S-band RF Photocathode

12. Hard X-Ray FELs in Operation & Under Construction SACLA 2011 8.5 GeV, 60 Hz NC SACLA 2011 8.5 GeV, 60 Hz NC PAL XFEL 2015 10 GeV, 100 Hz NC PAL XFEL 2015 10 GeV, 100 Hz NC SWISS FEL 2017 5.8 GeV, 100 Hz NC KJK ICABU Nov 12 2013 12 LCLS-I, II 2009, 2018 14.5 GeV, 120 Hz NC LCLS-I, II 2009, 2018 14.5 GeV, 120 Hz NC XFEL 2015 17.5 GeV, 3000 x 10 Hz SC XFEL 2015 17.5 GeV, 3000 x 10 Hz SC

13. Various R&D programs are in progress to enhance the performance of high-gain XFEL  SASE is temporally incoherent  fluctuation in spectrum and intensity  Coherent soft x-rays ( < 1 nm) via seeding –Laser HHG, Cascaded HGHG, EEHG, self-seeding  Self-seeding for hard x-rays  Other spectrum enhancing schemes – iSASE, pSASE, two color generation  LCLS-II will incorporate CW capability by a super-conducting linac KJK ICABU Nov 12 2013 13

14. 14 Free Electron Laser Oscillator  A low-gain device with high Q optical cavity  Optical pulse formed over many electron passes  Difficult for x-rays –Electron beam qualities –High-reflectivity normal incidence mirror KJK ICABU Nov 12 2013

15. 15 X-Ray FEL Oscillator (XFEL-O)  An FEL oscillator is feasible in hard x-ray region by using Bragg mirrors –R. Collela and A. Luccio, 1983; KJK, Y. Shvyd’ko, and S. Reiche, 2008  Tuning is possible with a four mirror configuration –R. M.J.Cotterill, (1968) KJK & Y. Shvyd’ko (2009)  Ultra-high spectral resolution ( meV) with storage ring like stability 15

16. KJK ICABU Nov 12 2013 16 Example Parameters  Electron beam: –Energy  6 GeV, Bunch charge ~ 25-50 pC  low intensity, Bunch length (rms)  1 (0.1 ps)  Peak current 20 (100) A, Normalized rms emittance  0.2 (0.3) mm-mr, rms energy spread ~ 2  10 -4, Constant bunch rep rate @ ~1 MHz  Undulator: –L u = 60 (30) m,  u  2.0 cm, K=1.0 – 1.5  Optical cavity: –2- or 4- diamond crystals and focusing mirrors –Total round trip reflectivity > 85 (50) %  XFELO output: – 5 keV      25 keV –Bandwidth:  ~ 1 (5)  10 -7 ; rms pulse length = 500 (80) fs –# photons/pulse ~ 1  10 9 –Rep rate ~ a few MHz(limited by crystal heat load and damage) 8

17. Diamond is the best material. The tolerance on optical element placement (10 nr), and R. & fig. errors for focusing mirrors appear feasible. KJK ICABU Nov 12 2013 17 Null feedback on HRM to 50 nr High heat diffusivity at < 100K Yamauch, JTEC, R~ 99%, fig error< 1  r

18. Damage issue of diamond crystals for XFELO cavity KJK ICABU Nov 12 2013 18  Power density on XFELO crystal –1 kW/mm2  Power density for APS HHL crystal  Power density of focused beam for ESRF experiment in 1994

19. KJK ICABU Nov 12 2013 19 XFELO applications  High resolution spectroscopy –Inelastic x-ray scattering  Mössbauer spectroscopy –10 3 /pulse, 10 9 /sec Moessbauer  s (14.4 keV, 5 neV BW)  X-ray photoemission spectroscopy –Bulk-sensitive Fermi surface study with HX-TR-AR PES  X-ray imaging with nm resolution –Smaller focal spot with the absence of chromatic aberration  picosecond time resolution  A second user WS was held at POSTECH in Feb 2013

20. Nuclear-resonance-stabilized XFELO (B.W. Adams and K.-J. Kim, to be published)  The XFEL-O output pulses are copies of the same circulating intra-cavity pulse  By stabilizing cavity RT time to less than 0.01 /c, the spectrum of XFELO output becomes a comb  The extreme-stabilized XFEL-O will establish an x-ray-based length standard and have applications in fundamental physics such as x-ray Ramsey interferometer to probe quantum gravity, etc. KJK ICABU Nov 12 2013 20

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23. PossibleAccelerator system  Injector for XFELO is available from ERL research  The 17GeV pulsed Euro XFEL can be operated 7GeV CW  A 2-loop, 3 pass system using 25 CEBAF C-100 cryomodule for 2.3 GeV acceleration can fit CEBAF tunnel for multi-XFELO operation KJK ICABU Nov 12 2013 23

24. Legend of evolving bright & coherent x- ray sources KJK ICABU Nov 12 2013 24