Progress in the Study of an X-Ray FEL Oscillator

1. Progress in the Study of an X-Ray FEL Oscillator Kwang-Je Kim ANL & the Univ. of Chicago 31st Int. FEL Conference August 23-28, 2009 BT Convention Center Liverpool, UK

2. LINAC Coherent Light Source LCLS Project start 1999 2011 SCSS SPring-8 Compact SASE Source European XFEL Facility 2014 Era of Hard X-Ray FEL has Arrived (April, 2009) LCLS August,2008 March, 2009 I=500 A I=3000A April 10, 2009 User experiment September, 2009 LCLS, April 2009

3. (Hard) X-Ray FEL Oscillator • An x-ray pulse trapped in optical cavity, meets an electron bunch inside undulator  gain • Zig-zag path cavity allows wavelength tuning • Exponential increase of the intensity, if gain > loss • Steady state when gain approaches loss at high intensity • XFELO with crystal cavity proposed in 1984 by Collela and Luccio, has been in hibernation, resurrected in 2008 (KJK, S. Reiche, Y. Shvyd’ko, PRL 100, 244802 (2008)

4. FEL Performance: SASE (LCLS) and XFELO

5. Brightness of Hard X-Ray Sources

6. FEL Modeling(R. Lindberg, et. al., WEPC40 ) • Analytical • 3D Gain calculation (KJK),1D supermode theory (G. Dattoli, P. Elleaume), initial noise-seed estimate (R. Lindberg) • Simulation • Combine single-pass codes with propagation and reflection in x-ray cavity with correct crystal response • GENESIS: 3D (S. Reiche) slow • GINGER: 2D (W. Fawley, R. Lindberg) • Reduced 1-D code by an approximate treatment of the transverse effect (R. Lindberg & KJK, PRSTAB, 2009; G. Penn, et al.,MOPC43) • Recent work: estimates of harmonics • 3rd harmonic output ~10-3 – 10-4 of fundamental (R. Lindberg)

7. X-Ray OpticsChallenges &Progresses • Quality of diamond crystals • Must be perfect with reflectivity 98-99%,but only in a small volume < 0.5x0.5x0.1 mm3 • Heat load & shock wave on crystal: Can DE< 1 meV maintained? • Absorption: 1 mJ for 1-ps pulse in 100x100 mm2, 1 MHz rep rate • Initial heat load simulation performed (H. Sinn) • Inter-pulse problem can be solved by cooling, T <100K • Intra-pulse problem is probably OK • Crystal stability • Angular stability <10 nrad, position< 3 mm. • Pilot null-detection FB at APS indicates that 50 nrad stability for f< 2Hz is feasible • Grazing-incidence, curved mirror for focusing • High reflectivity (>95%) and minimal wavefront distortion

8. XFELO Ultra-Narrow Bandwidth: Deg ~ 1 meV or Deg /eg ~1×10-7

9. Technology R&D for XFELO • X-ray optics • Injector development • These R&Ds will advance the state-of–the-arts, benefiting general accelerator and x-ray optics technology

10. Tunable X-ray Cavity • Two crystal scheme has a very limited tuning since q must be kept small • A four crystal scheme is tunable R. M.J.Cotterill, APL, 403,133 (1968) KJK & Y. Shvyd’ko, PRSTAB (2009) • Any interesting spectral region can be covered by one chosen crystal material • Simplify the crystal choice Diamond as highest reflectivity & best mechanical and thermal properties

11. Quality of Diamond Crystals • Require high quality (dislocation-free, etc) in a volume (1-2) mm2× (40-100) mm • Need to prove 98-99 % reflectivity APS experiment with C (995), EH=23.8 keV

12. Reflectivity and Spectral Width Measurement at APS Sector 30in good agreement with TheoryMarch, 2009 C(995) EH=23.765 keV S. Stoupin, Y. Shyv’dko & A. Cunsolo

13. Heat Load Problem • As an intracavity x-ray pulse hit a crystal, r-dependent temperature rise dT  crystal expansion  dE/E = bdT (dL/L=b dT/T) dE/E<<10-7? • Heat load simulation by H. Sinn • Due to high thermal-diffusivity, Inter-pulse effect can be made small if T< 100K (high-diffusivity) • Intra-pulse effect dE/E~ 5-10 × 10-7 if the expansion time scale << pulse duration (~ps). If >>,dE/E<<10-7 • Need further study b S. Stoupin and Y. Shvyd’ko, March 2009

14. Null-Detection FB at APS Sector 30(S. Stoupin, F. Lenkszus, Y. Shvy’dko,..) IC3 IC1 HRM IC2 IC0 • The stability of IC3 signal indicates the angular stabilization of the 3rd crystal pair within 50 nrad is achieved for f < 2 Hz PZT IC0 IC1 IC3 IC2 Feedback correction signal

15. Grazing Incidence, Curved Mirror • JTEC • Developing a technique combining elastic emission machining (EEM, slow) and electrolytic in-process dressing (ELID, fast) to fabricate an “arbitrary” surface, such as ellipsoidal, to <nm height error and 0.25 mrad figure error • Such mirrors are sought after by “every body” in SR business • Other ways of focusing • Curved crystal surface, CRL,.. H. Mimura, et. al. RSI 79, 083104, 2008

16. Prototype X-Ray Cavity at an APS Beamline • About 1/5 model of an XFELO cavity • Adjust the distance M1-M2 to control the stability • Adjust the round trip path length to match/mismatch the spacing (46m) between the APS x-ray pulses • Test overall reflectivity, crystal and mirror stabilization, transverse mode profile ~24 m

17. Electron Beam Quality Requirements • The requirements: • Normalized rms emittance < 0.2 mm-mr • Bunch charge < 50 pC • Bunch length (rms)~ 1 ps (transform-limited spectral width << mirrror bandwidth)  Peak current ~10-20 A • Energy spread < 2.10-4 • Bunch rep rate > 1 MHz • The ERL injector in high coherence mode (Cornell) satisfies the requirements if the 750 kV DC voltage can be achieved • The LCLS injector @ low charge mode produces similar e-beams but at lower rep rate • We are developing a novel type of injector

18. Injector Design: A New Paradigm • Current paradigm of injector design  laser driven rf photocathode • Remove elaborate bunching steps • High-peak current from start, large space charge degradation, require emittance compensation • Successful for hi gain FELs • For low intensity & ultra-low emittance for XFELO  thermionic cathode inside low freq. RF cavity • SCSS/Spring-8 success of pulsed DC gun • The injector can also be configured to produce beams suitable for ultrafast (a few fs) SASE (J. Rosenzweig,..)

19. 10 – monochromator, 600 MHz; 11 –velocity buncher, 300 MHz; 12 – solenoids; 13 – SC linac, 66 MeV, f=400 MHz. 14 – high-harmonic cavity (1300 MHz); 15 – bunch compressor – I; 16 – SC linac, 546 MeV, f=1300 MHz. 17 – bunch compressor – II; 1 – RF cavity with thermionic cathode, 100 MHz, 1 MV; 2 – focusing solenoid; 3 - RF chopper to form bunch repetition rate (< 3 MHz); 4 – quadrupole; 5 – beam dump; 6 – slits handling high power; 7 – chicane and slits (6) as an energy filter; 8 – quadrupole triplet; 9 – 100 MHz rf cavity; • Critical R&D items • Small diameter cathode • 1 MV voltage with 100MHz cavity • High power slits

20. Ultrashort, Ultralow Emittance for Ultrafast SASE • Same general configuration • Adjust the slits in the energy filter. • The second bunch compressor is not required • Introduced an additional harmonic cavity operating at 30 GHz with very low voltage (4 kV). • This is a preliminary study & another factor compression by a factor of 2 appears to be feasible.

21. Test Bench for Thermionic E-Gun Assembly with Pulsed DC Voltage Pepper pot plate 300 kV HV isolator Current transformer Movable FC Scintillator - 300 kV pulsed PS Anode AC heater PS Output window Cathode assembly CCD-camera HV platform Triggering unit

22. XFELO-Facility • Bunch intensity is low SCRF accelerator can be operated in CW • Recirculation without energy recovery can save SCRF linac cost. Required energy 6-8 GeV • 20 MHz injector 20 XFELOs

23. Facility Options Recirculation saves linac cost, but the requires elaborate optics. Maybe 1 recirculation? Straight linac will be most versatile, with possibility of both XFELO and ultrafast SASE

24. XFELO Study Group • Modeling/Simulation • R. Lindberg, W. Fawley, S. Reiche • Injector • P. Ostroumov, KJK,B. Mustapha, Ph. Piot, S. Kondrashev, A. Nassiri, G. Waldschmidt, D. Capatina • LBNL on 100 MHz cavity (F. Sannibale, J. Stable, J. Corlett,..) • RIKEN/Spring-8 on cathode assembly ( T. Shintake, K. Togawa,..) • X-ray Optics • Y. Shvyd’ko, S. Stoupin, D. Shu, A. MacRander, L. Assoufid, H. Sinn (DESY) • RIKEN/Spring-8 (T. Ishikawa, ..)