1. Kwang-Je Kim ANL June 24, 2009 Fermilab Colloquium
An X-Ray Free Electron Laser Oscillator (For Record High Spectral Purity and Brightness)
Kwang-Je Kim ANL
June 24, Fermilab Colloquium

2. Synchrotron Radiation
Intense EM radiation when relativistic charged particles changes directions
Crab Nebula (blue)
Electron storage ring
APS (Argonne)

3. Making Electrons Work Together
Electrons randomly distributed radiates independently (APS, ALS,..)
If electrons are regularly spaced with period l, the radiation intensity is enhanced by >1000,000:
Electrons become regularly spaced when interacting with radiation beam in an undulator Free Electron Laser

4. X-Ray FEL: Self-Amplified Spontaneous Emission (SASE)
Saturation
High-single pass gain  Do not need mirrors High transverse coherence Limited temporal coherence (Dw/w ~10-3 )
Exponential Gain Regime
Transverse mode
z = 25 m
z = 37.5 m
Undulator Regime
z = 50 m
z = 90 m
Electron Bunch Micro-Bunching

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

6. Options for X-Ray FELs
SASE: high-gain in single pass  amplify initial noise to intense, quasi-coherent radiation
Seeded high-gain harmonic generation: Available coherent seed radiation  generate harmonics amplify. Currently limited to soft x-rays
X-ray FEL Oscillator: Use high reflectivity of crystals (diamond)

7. X-Ray FEL Oscillator
An x-ray pulse trapped in optical cavity, meets an electron bunch inside undulator  gain
Exponential increase of the intensity, if gain > loss
Steady state when gain = loss at high intensity
XFELO with crystal cavity proposed in 1984 by Collela and Luccio, has been in hybernation, resurrected in 2008
Zig-zag path cavity allows wavelength tuning

8. FEL Performance: SASE (LCLS) and XFELO
Approx. ratio
XFELO/ SASE
Wavelength : l
1.5 Å
1 Å
Electron energy
15 GeV
7 GeV
# Photons/ pulse : Ng
1012
109
~10-3
Pulse length : Dt
160 fs ps
~10
Bandwidth : Dw/w
0.3 x10-3
0.7 x10-7
~10-4
Temporal coherence
[0.5l] / [cDt Dw/w]
partially coherent
5.1x10-3
fully coherent 1
Intensity fluctuation
7% (classical)
~3x10-5(Quantum) ~0
Trans. coherence
[0.5l] / [Dx Df] ~1
Rep rate : frep
120 Hz
1 MHz
~104
Bp= Ng / [Dt Dw/w(Dx Df)2]
1.5x1033
2.4x1033
Baverage= Bp Dt frep
2.9x1022
6.0x1027
~105

9. Brightness of Current and Future Hard X-Ray Sources

10. XFELO Will Revolutionize the Techniques Developed at 3rd Gen Light Sources and Find New Applications in Areas Complementary to SASE ( Less Violent than SASE !)
High resolution spectroscopy
Inelastic x-ray scattering
Moessbauer spectroscopy
103/pulse, 109/sec Moessbauer gs (14.4 keV, 5 nano-eV BW)
X-ray photoemission spectroscopy
Bulk-sensitive Fermi surface study with HX-TR-AR PES
X-ray imaging with near atomic resolution (~1 nm)
Smaller focal spot with the absence of chromatic aberration
Metrology: Moessbauer wavelength standard for atomic scales

11. Diamond Crystal and Mirror Reflectivity at 12 keV

12. 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)
The scheme makes crystal choice simple  Diamond

13. FELO Modeling Analytical (KJK, R.Linberg) GENESIS (S. Reiche)
Gain calculation, super-mode theory for evolution in optical cavity
GENESIS (S. Reiche)
(x,y) asymmetricslow:1 month computing from noise to saturation
Reduced 1-D FEL code (R. Lindberg)
Transverse dependence integrated out assuming Gaussian mode
Fast!
Reasonable agreement with GINGER and GENSIS
GINGER (W. Fawley, R. Lindberg,Y. Shvyd’ko,..)
Implemented the crystal response
(x,y) symmetricmuch faster than GENESIS

14. The Complex Amplitude Reflectivity of Diamond
The w -dependent phase shift is due to the fact that the effective reflecting surface is inside the crystal (extinction depth)
The angular dependence of the reflectivity can be neglected in most cases
One thin (50 m) for 4% out-coupling and one thick (200 m) for full reflection

15. XFELO Modeling (R. Lndberg, W. Fawley, S. Reiche)

16. XFELO Spectrum After 500 Passes (Two Diamond Crystal Cavity, 50 mm and 200mm)

17. Optics Challenges Quality of diamond crystals
Must be perfect! However, in a small volume < 1x1x0.1 mm3
Heat load & shock wave: Can DE< 1 meV maintained?
Absorption: 1 mJ for 1-ps pulse in 100x100 mm2, 1 MHz rep rate
Thermal expansion coefficient of diamond measured at APS recently and found <10-7 for T<70 K!
Crystal stability
Angular <10 nr, position< 3 mm. ( 50 nr stability achieved recently at APS beamline using null-feedback)
Grazing-incidence, curved mirror for focusing
High reflectivity (>95%) and minimal wavefront distortion

18. Quality of Diamond Crystals

19. Experiment at APS Sector 30 March, 2009 S. Staupin, Y. Shvyd’ko, A
Experiment at APS Sector March, S. Staupin, Y. Shvyd’ko, A. Cunsolo

20. Reflectivity Measurement

21. Spectral Width & Reflectivity

22. Heat Load Problem

23. Diamond Thermal Expansion Measurement at APS, Sector 30

24. Angular Stability of Crystals (<10 nr) May be Achieved by Null Detection FB

25. 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 >20 A
Energy spread <
Bunch rep rate > 1 MHz
The ERL injector in high coherence mode (Cornell) satisfies the requirements
The LCLS low charge mode qualities are nearly the same except the rep rate
A novel type of injector for a dedicated XFELO

26. A Novel Injector Design
Current paradigm of injector design  laser driven rf photocathode
Successful for high-intensity, low emittance production
Ill-matched for XFELO requirements of low intensity, ultra-low emittance
Start from a thermionic cathode inside VHF cavity and beam manipulation
Japanese success of pulsed DC gun
The injector can also be configured to produce beams suitable for ultrafast SASE (a few fs)

27. 8 – quadrupole triplet;
9 – 100 MHz rf cavity;
10 – monochromator, 600 MHz;
1 – RF cavity with thermionic cathode, 100 MHz, 1 MV;
2 – focusing solenoid;
3 - RF chopper to form bunch repetition rate (1 MHz to 3 MHz);
4 – quadrupole;
5 – beam dump;
6 – slits;
7 – chicane and slits (6) as an energy filter;
11 –buncher, 300 MHz;
12 – solenoids;
13 – SC linac section, 66 MeV, f=400 MHz.
14 – high-harmonic cavity (1300 MHz);
15 – bunch compressor – I;
16 – SC linac section,546 MeV, f=1300 MHz.
17 – bunch compressor – II;

28. 3D-Simulation with TRACK, Genetic Optimization (P. Ostroumov, I
3D-Simulation with TRACK, Genetic Optimization (P. Ostroumov, I. Mustapha, Ph. Piot)

29. 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.
Q [pC]
1.6
ex [mm]
0.09
ey [mm]
0.08
σt [fs] 6
σDE[keV] 43

30. R&D Path Injector development X-ray cavity
Establish feasibility by constructing front end ( cathode, VHF cavity, chopper, filter)
X-ray cavity
Procedure for securing “perfect” crystals
Only two large companies May need to work with small institutions
Feedback and stability
High-reflectivity grazing incidence mirror
Start CDR and EDR after 3 years

31. XFELO-Farm
Bunch intensity is low SCRF accelerator can be operated in CW
Recirculation can save SCRF linac cost. (Energy recovery is not necessary) Required energy 7-10 GeV
20 MHz injector 20 XFELOs
SCRF Linac cost (full)~ $ 73 M per GeV

32. 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