1. Review of Application to SASE-FELs
Sven Reiche
UCLA
ICFA-Workshop - Sardinia 07/02
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2. FEL as Radiation Sources
High-brightness radiation sources need
high-brightness drive beam!
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3. Sven Reiche - ICFA Sardinia
Principle of an FEL
Undulator
Radiation
Field
Beam
Slice
Energy
Gain + -
Long.
Motion
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4. Sven Reiche - ICFA Sardinia
Length Scales in an FEL
Wavelength (tunable):
Gain length:
Total length needed (SASE FEL):
The FEL parameter r depends primarily on the electron beam parameters and should be as large as possible.
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5. Sven Reiche - ICFA Sardinia
The FEL Parameter r
Strong diffraction (small beam size, long wavelength)
Small diffraction (large beam size, short wavelength)
The radiation field ‘information’ is spread out transversely and sees mainly the current of the electron beam.
The radiation field interacts locally with the electron beam. The interaction strength is given by the electron density.
The key parameters are beam current, normalized emittance and the average beta-function.
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6. Sven Reiche - ICFA Sardinia
Energy Spread
The FEL process is a 90º rotation in longitudinal phase space within the rf bucket of width 2r
A larger energy spread reduces the achievable bunching and thus the coherence level of the FEL radiation.
Small spread
Large spread
Condition for lasing
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7. Emittance
Emittance has an effect similar to the energy spread, which is enhanced by stronger focusing.
Strength of focusing optimized for performance
Optimum Beta-Function
Reduced Density
Enhanced Phase Spread
LCLS
Emittance effects are dominant for short wavelength FELs (TESLA, LCLS), demanding very low values for the initial
emittance.
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8. Generating the Electron Beam
For a short wavelength FEL low emittance and high current are required. To achieved that the driving linac has typically two key components:
RF photo electron gun to generate a high-brightness electron beam (en ~ 1- 3 mm mrad, Q ~ nC, st ~ 1- 3 ps).
Bunch compressor to increase the driving peak current. The demands on the energy spread are kind of relax, which allows it to compress the bunch.
In addition the beam line has to guarantee that the degradation of the initial beam parameters stays within acceptable limits.
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9. Sven Reiche - ICFA Sardinia
VISA FEL
Linac Sections
Gun
Transport
20° Dispersive
Section
Matching Line
VISA
Undulator E
71 MeV
sE/E
0.1 % Ip
250 A en
2.3 mm.mrad lu
1.8 cm K
1.24 l
850 nm
Reduced compression
Scraping
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10. Sven Reiche - ICFA Sardinia
VISA Results
Far Field (Measurement)
Saturation
Single spike in spectrum
High mode in far field
Neg. exp. Fluctuation in linear regime, reduced at saturation
Far Field (Simulation)
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11. Sven Reiche - ICFA Sardinia
LEUTL FEL
Design Parameter E
217/457/700 MeV
sE/E
0.1 % Ip
100/300/500 A en
5/3/3 mm.mrad lu
3.3 cm K 1 l
530/120/51 nm
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12. LEUTL Results Saturation down to 130 nm Higher harmonics at 530 nm5 10 15 20 25
0.01
0.1 1
100 . 3 4 6 7
Distance [m]
Intensity [arb. units]
0.1%
sE/E
8.5mm en
200pC Q
0.3 ps st
266 A
Ipeak
530 nm
Saturation down to 130 nm
Higher harmonics at 530 nm
Bunching (CSR)
Single shot spectra
Statistic
Simultaneous Spectra: Fundamental and 2nd Harmonic
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13. Sven Reiche - ICFA Sardinia
TTF FEL (Phase 1) E
MeV
sE/E
0.06 ± 0.02 % Ip
1.3 ± 0.3 kA en
6 ± 2 mm.mrad lu
2.73 cm K
1.2 l nm
Phase 2 of the TTF FEL will operate at 1 GeV, where more superconducting modules are added and the undulator length is extended.
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14. Sven Reiche - ICFA Sardinia
TTF Results
Shortest FEL wavelength so far (saturation)
Single shot power and spectrum (statistic)
First user applications of FEL radiation
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15. Sven Reiche - ICFA Sardinia
LCLS E
14.3/4.5 GeV
sE/E
0.01/0.025 % Ip
3.4 kA en
1.2 mm.mrad lu
3 cm K
3.7 l
1.5/15 Å
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16. Sven Reiche - ICFA Sardinia
TESLA E
GeV
sE/E
0.05 % Ip
5.0 kA en
1.6 mm.mrad lu
6 cm K l Å
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17. Electron Pulse Structure
In single linac -single undulator set-up the FEL properties are primarily varied by varying electron beam parameter (such as beam energy).
A single linac - multiple undulator set-up (TESLA FEL) requires a variable gap to choose all resonant frequencies independently. Pulse train are filled individually.
Tesla
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18. Bunch & Cooperation Length
With high beam energy the longitudinal information exchange (cooperation length Lc = l/4pr) is shorter than the bunch length. Parts of the bunch radiate independently.
Single spike (longitudinal coherence)
Multiple spikes
The beam is best parameterized by the beam properties per slice. The slice length is defined by the cooperation length.
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19. Sliced Beam Parameters
Variations in the slice beam properties are caused by
Space-charge forces in the gun
Emittance compensation for bunch head and tail
Rf-curvature
Wakefields (Linac)
Non-linear effects during compression
CSR in compressor
Undulator wakefields (dynamic effect on FEL)
The non-uniformity of the beam parameters defines the envelope of the spiky profile of the SASE FEL pulse.
Estimate of this effect by start-end simulations
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20. Start-end Simulations
Set of codes, each specialized on a certain aspect of the FEL beam line (gun, linac, compressor and undulator). The generation and transport of an electron bunch is simulated by exchanging the particle distribution to the next code in the chain.
Successful to analyze recent FEL results (VISA, LEUTL and TTF)
VISA - After Linac
Before Undulator
TTF - Undulator Entrance
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21. Sven Reiche - ICFA Sardinia
Example: LCLS
PARMELA - ELEGANT output for 1 nC
Centroid mismatch
Beta mismatch
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22. Sven Reiche - ICFA Sardinia
Example: LCLS (cont’)
Start-end simulations for two different initial bunch charges of 1 nC and 0.2 nC.
Low charge case is modeled after the GTF results and then propagated through the LCLS beam line.
The impact of wakefields is a reduction of the output power by 35 %.
The low charge case performs worse than the high charge case. The reason is the large slice emittance, while the current is smaller by a factor of 3 (The high charge case assumes an ideal performance at the gun).
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23. Sven Reiche - ICFA Sardinia
Example: LCLS (cont’)
FEL Pulse at Undulator Exit
Spikes due to current modulation
Gaps due to wakefields
Current Profile
Undulator Wake Potential
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24. Sven Reiche - ICFA Sardinia
Multi-stage FEL
Two-stage FEL
Electron Bypass
1st Undulator
2nd Undulator
Radiation Transport
Bandwidth & pulse length control: 2nd undulator is seeded by monochromatized FEL pulse of 1st undulator
Multi-stage FEL
High harmonic generation: Cascading amplification of higher harmonics in next undulator. Some schemes require fresh bunches (jitter < 1 ps).
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25. Sven Reiche - ICFA Sardinia
Bunch Length, Revised
For X-ray FELs (LCLS, TESLA) there is a particular interest in short FEL pulses of 10 fs (e.g. pump-probe experiment of fast chemical reaction).
Current schemes reduced the pulse length by pulse compression, by frequency selection of chirped pulses or by contrast level of higher harmonics.
Ultra short electron bunches are currently not in consideration. The expected degradation by coherent effects (CSR in compressor, wakefields) is too strong, but not unsolvable.
Alternative generation of ultra-short bunches besides the compression of an chirped electron beam in bunch magnet chicane might be promising.
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26. Sven Reiche - ICFA Sardinia
Conclusion
The performance of Free-electron Lasers depends strongly on the beam quality (brightness) of the driving electron beam. For analysis the entire system (gun, linac & FEL) has to be considered (start-end simulation).
The emittance is the most crucial parameter, determing the saturation power and length of the FEL. The beam current is enhanced by compression to reduced the overall needed length to saturate.
Short wavelength FEL tends to emphasize sliced and not projected beam parameters. There also most sensitive to any variation along the bunch.
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