1. Areas of interest Mid-IR FELs Mid-IR FELs THz FELs THz FELs
phase-matched HHG for the generation of attosecond range X-ray (100 eV-multiple keVs) pulses with high average brightness
FEL schemes to realize (low jitter, multi-color) pump-probe experiments
mid-IR Enhancement Cavity to boost up pulse energies
Photonic crystal mirrors with tunable reflectivity for high power THz FELs (broad band outcouplers with nearly 100% efficiency)
realization of THz pulse stacker cavity
tunable Filters and Modulators for intracavity applications

2. Generation of coherent X-Ray pulses by HHG
(A.Foehlisch///Murnane&Kapteyn): FELs as drivers for HHG based coherent X-Ray sources ?
requirements imposed on drive lasers (Popmintchev et al.) :
Phase-matched HHG in keV region photons needs:
preferably few cycle (CEP stabilized) to ~10 cycle drive laser pulses in NIR/MIR ,
intensities in the range of 1-5x1014 W/cm2 ,
noble gas filled hollow waveguide apertures: ~100mm-200mm, (He) gas pressure: tens of atm)
OPCPA’s
NIR sub-10 fs with 70 mJ energy at 100kHz.
NIR sub-10 fs multi-kHz, multi-mJ
Mid-IR (~3mm) sub-100 fs with a few micro-Joule energy at 100kHz
3.9 mm sub-100 fs with ~9 mJ at 20Hz

3. HHG - Predictions & Measurements
Popmintchev et al.,
PNAS 106, (2009)
Curves normalized to phase-matched λ0=0.8µm
@ l= 6µm, 10 MHz rep. rate (He)
estimated Photon flux : ~ ph/sec (1.0%BW)
@ l= 3.9µm, 1 kHz rep. rate (35 atm. He)
Photon flux : ~108 ph/sec (1.0%BW)
(based on experiments)
Phase matched 6cycle, 20 Hz
Popmintchev et al.,
OSA/ CLEO 2011

4. Outline of the project:
short term: carrying out the HHG experiments on an existing FEL facility that meets the requirements set on the mid-IR drive laser, verifying the theory throughout the mid-IR
(particularly at around 6 mm-7mm) (JLab, FHI-FEL, …?)
long term: mid-IR ERL-FELs should be able to perform better than atomic lasers in terms of :
tunability (throughout the nir/mid IR and beyond)
high rep rate (MHz) in generating mJ(s) of ultrafast pulses with high average power
Ongoing simulation work is mainly focused on the latter :
(system requirements imposed on a compact ERL)

5. Ultrashort Pulse Generation in (Mid IR) FELs
Chirped pulse generation in a FEL oscillator using a chirped electron beam and pulse compression (JLab)
Mode-locking techniques in FELs
-Active mode-locking (multiple OK sections used in a
cavity)
- Passive mode-locking
(single spike, high gain superradiant FEL osc.)
Generation of short electron pulses (JLab)

6. Suggested (3-6mm) MIR FEL & Pulse Stacker Cavities
stretcher
compressor
PLE
dielectric mirror
NIR/MIR FELO
mode matching telescope
I.)
II.)
Mode-locked
NIR Laser
high-Q enhancement cavity (EC) smoothes out power and timing jitter of the injected pulses inherent to FEL interaction.
allows ~fs level synchronization of the cavity dumped mid-IR pulse with the mode-locked switch laser.
Depending on the recombination time of the fast switch, sequence of micropulses with several ns separation can be ejected from the EC !

7. FSU-NHMFL NIR/MIR/FIR (&broadband THz) FEL Proposal
E ~ 60 MeV (NIR/MIR)
E ~ 13 MeV (FIR)
135 pC pulses
sz ~ 0.5 – 4 ps
10.7 MHz (21.4 MHz FIR) X
FIR
NIR
inclusion of a HHG based coherent X-Ray source ?
MIR/FIR
Parameter NIR FEL MIR FEL FIR FEL
Wavelength (μm) to to > to 1100
Wawenum (cm−1) to < 70 to to 100

8. system parameters JLab IR FEL BERLinPro (not uptodate) Beam parameters
FEL (1.6mm)
Units
Beam Energy
115
MeV
Bunch charge
110 (135) pC
s_z rms
150 fs
Peak current
~300 A
s_e rms
(uncorrelated)
0.1%
(correlated)
0.5%
nor. trans. Emit. 8
mrad
rep. rate
~75
MHz
Trim Quads reading
Coherent OTR interferometer autocorrelation
scans for bunch length measurements

9. Enhancement Cavity @ JLab
Brewster W.
vacuum vessel
Opt. Switch mount
Folded cavity
FEL
Input Coupler
High Reflector
Q ~ 40 (Finesse ~ 300 ) enhancement :~90
Q~ 50
enhancement :~
estimated JLab ~ 100
T. Stanford IR-FEL achieved enhancement of ~ using an external pls stacker cavity (1996)

10. FEL Osc. sensitivity to temporal jitter
Dt/t = dL/L + df/f
e- bunch
Dt : timing jitter
L : cavity length
dL: cavity length detuning
f : bunch rep. frequency (perfectly synchronized to L)
: cavity roundtrip time ( 2L/c)
Bunch time arrival variation effectively has the same effect
as cavity length detuning.
effect of the timing jitter on the FEL performance
In slippage dominated short pulse FEL oscillators cavity detuning is necessary to optimize the temporal overlap between optical and e- pulses (lethargy effect).Timing jitter induces fluctuations on the operational cavity detuning.

11. FEL Osc. sensitivity to temporal jitter
Jitter 5 fs rms
Jitter 10 fs rms
w/o initial Jitter
A case study / simulations
Peak power fluctuations ~4-5% rms
Pulse width fluctuations limited to a few %
timing jitter ~ ±20 fs (optical pulse)

12. 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
Pump-probe NIR-MIR-FIR/THz combinations
(Coherent (broadband) THz generation)
(CBS X-Ray source)

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