1. Ultrashort (few cycles) Pulse Generation in (IR-THz) FELs
Chirped pulse generation in a FEL oscillator using a chirped electron beam and pulse compression
Mode-locking techniques in FELs
-Active mode-locking
- Passive mode-locking
Generation of short electron pulses

2. Assumed system parameters

3. Assumed system parameters (updated)

4. 3 mm - 6 mm NIR FEL & high rep. rate Pulse Stacker Cavity
- Beam Energy: 100 MeV
- Bunch Charge: 80 pC
- Rep rate: 1.3GHz/s.harmonics
- Outcpl.Pls. Energy: 60-80mJ
- Cav. Enhancement:
Pulse width: ~ fs (fwhm)
IL ~ 1x1014 – 3.5x1014W/cm2
reduced duty cycle operation to avoid excessive intracavity thermal loading
cooling of the cavity components

5. Pulse Stacker Cavity options
consider using bunch charges of ~135 pC (JLab) with 650 MHz rep. rate (or less) to increase the NIR pulse intensities
- Intracavity gas cell  effect on cavity Q ?
stretcher
compressor
Phase locking Electronics
Mode matching telescope
NIR-FEL
HHG
II.)
evacuated cavity with very low dispersion optics
Lcav : min. 115,4 mm
Reduce sensitivity of pulse stacking to time jitter at injection

6. 3 mm - 6 mm Short Pulse FEL (cavity detuning)
100fs (fwhm)
Dw/w~ 4%-5%
low time jitter
low peak to peak power deviations
Outcoupled Pulse Enegies: ~ mJ
~ 10 cycle pulses
(HHG drive laser)
~ 6 mm
Dw/w~ 4%-5%
200fs (fwhm)

7. 3 mm - 6 mm HHG drive FEL - update
- Beam Energy: 100 MeV
- Bunch Charge: 80 pC
- Rep rate: 40 MHz
- Outcpl.Pls. Energy: 50-70mJ
- Cav. Enhancement:
Pulse width: ~ fs (fwhm)
IL ~ 1x1014 – 3.5x1014W/cm2
~15% reduced pulse energy/peak power
timing jitter remains < 20 fs
peak power jitter increased by 5%-10%
sz : 100 fs and sE : 0.5 %
is preferable to the option:
sz : 150 fs and sE : 0.35 %
Due to the lower duty cycle reduced intracavity thermal loading for the FEL and the pulse stacker cavities

8. 3 mm - 6 mm HHG drive FEL - update
~40 MHz (?) micropulse rep. rate (~33th subharmonic of 1.3 GHz)
macropulse length should allow a few thousand roundtrips: ~ 100 ms
(for the 'superradiant mode' as well as to reach the steady state in
the external cavity)
average current ~0.32 mA – 1.9 mA (1 – 6 kHz macropulse rep. rate)
according to the spent beam energy distribution (8% -10% full energy spread) ~1/3 of the particles lie within the interval dg/gr ~ ±1% at around the 'synchronous particle' beam energy. (~16% - 17% of the spent beam within the interval dg/gr ~ ±0.5%).

9. 3 mm - 6 mm Short Pulse FEL (cavity detuning)
Considering that e- pulse energy is ~ 8 mJ
FEL extraction efficiency : ~ 1 %
8% -10% spent beam momentum spread (full) generated by the FEL interaction
large energy spread acceptance is required for beam transport/energy recovery
JLab IR Upgrade (acceptance :~12-15 %)

10. Ultrashort Pulse Generation by passive modelocking
Passive modelocking in conventional (atomic) laser :
Kerr Lens modelocking
Semiconductor Saturable Absorber Mirrors (SESAM)
Does FEL have a self (passive) modelocking mechanism ?
(for instance intensity dependent absorber)

11. High Gain FELs and Superradiance
FEL parameter :
Gain length :
Cooperation length :
( Bonifacio et al.,Phy.Rev. Lett. 73, 70 (1994) )
‘short bunch’
Superradiant emission in FEL oscillators :
(Jaroszynski et al.,Phy. Rev. Lett. 78, 1699 (1997) )
( Watanabe, Murphy, Giannessi, PRL 98, (2007) )
High gain (superradiant) FEL (amplifier)
High gain (superradiant) FEL (amplifier)
High gain (superradiant) FEL (amplifier)
High gain (superradiant) FEL (amplifier)
( Watanabe, Murphy, Giannessi, PRL 98, (2007) )
( Watanabe, Murphy, Giannessi, PRL 98, (2007) )

12. High Gain (superradiant) FEL Oscillator operating at cavity synchronization
Synchrotron Osc. Freq.
fs (fwhm)
Dw/w~ 20%
lc ~ 45fs
nearly an order of magnitude higher outcoupled pulse intensity (despite low outcoupling ratios)
FEL efficiency in superradiance mode more than doubled
generated chirped pulses allow further compression (max. ~ 1.5 times)
But :
spent beam momentum spread (full) generated by the FEL interaction exceeds 20 % !

13. cascaded oscillator schemes
High Gain (superradiant) FEL Oscillator operating at cavity synchronization
Further studies:
cascaded oscillator schemes
(problem: large momentum spread for the beam transport/energy recovery)
use of (assistant) SESAM mirrors
- checking the results with other well established codes

14. Layout of a 30-100 MeV ERL Beamline
Beam Dump
Extractor
LINAC Module II
LINAC Module I
Merger
Injector Cavity
Egun
Arc I
Arc II
Straight Section I
FEL I
FEL II
DBA
HHG Experiments
CBS X-Ray source
Pump-probe NIR-MIR-FIR/THz combinations
Coherent (broadband) THz generation

15. Pump-Probe NIR-MIR-FIR/THz combinations