Download - Free Electron Lasers (II)
Free Electron Lasers (II)
Sincrotrone Trieste
Enrico Allaria
Advanced School on Synchrotron and Free Electron Laser Sources and their Multidisciplinary Applications
Outline
-Basic concepts of light-electron interaction in a Free-Electron Laser-Why a free electron laser-How it work
- Different schemes for FEL-FEL aplifier-FEL oscillator-Self Amplified Spontaneous Emission FEL (SASE)-Coherent Harmonic Generation FEL (CHG)
- Application to the FERMI project at Elettra - Recent experimental results on the Elettra storage ring FEL
The FERMI project at Elettra
(on behalf of the FERMI FEL group)
The FERMI project at Elettra
FERMI will be a photon source based on a multi-stage harmonic generation process.It will be one of the first FEL user facilities in the world operating at wavelengths in the
VUV to soft X-ray range, based on the CHG scheme.
The project is making full use of the existing LINAC, previouslyused for the electron filling of the Elettra storage ring.
The FEL design is based on a “start-to-end” approach. This means that one has to keep track of the electron-beam dynamics and preserve its quality from the gun, through the LINAC, up to the end of the
undulators chain.
This design and realization of a facility like FERMI relies on many different expertises: Accelerator and laser physics, electron and light diagnostics, high level engineering, ...
Overview of the ELETTRA laboratory
Outline
•Presentation of the two FERMI FEL’s
•Numerical simulations for FEL 1 and FEL2
•Problem of FEL sensitivity to fluctuation of input parameters
•Discussion and open issues
Harmonic generation: the principle (1/2)
0 1 2 3 4 5 64
2
0
2
4
Phase
Ener
gy
Modulator
1 2 3 4 5 6 70
0.2
0.4
Harmonic number
Bun
chin
g co
effic
ient
0 50 100 150 200 250 300890
895
900
905
910
z (nm)
Ener
gy (M
eV)
Dispersive section
1 2 3 4 5 6 70
0.2
0.4
0.6
Harmonic number
Bun
chin
g co
effic
ient
n
e- beam from the Linac
From Perseo, a numerical code by L. Giannessi
0 0.5 1 1.5 20.01
0.11
10100
1 103
1 104
1 105
1 106
1 107
Input seed set on the right ->
z (m)
Powe
r (W)
ziz
0 5 10 15 20 25 302
1
0
1
2
Phase
Energ
y
Evolution of the harmonic signal
Distance along the undulator
109
200 100 0 100 200
z (um)
Pow
er (
arb.
uni
ts)
Pulse profileP
ower
(ar
b. u
nit
s)t~ 200 fs
z (m) Spectrum profile
89.2 89.3 89.4 89.5 89.6 89.7 89.8 89.9 90
wavelength (nm)
Pow
er S
pect
rum
(a.
u.)
Pow
. Sp
ec. (
arb
. un
its)
~ transform limited
Wavelength (nm)
Radiator
GW output signal
n
Harmonic generation: the principle (2/2)
e- beam from the Linac
Po
we
r (W
)
Injector
FERMI layout
Bunch compress
or
Linear accelerator
Bunch compress
or
Transport line
FEL
FEL undulators
FEL-1
FEL-2
Parameter Medium bunch Long Bunch
Beam energy 1.2 GeV 1.2 GeV
Peak current 0.8 KA 0.5 KA
Uncorrelated energy spread 200 KeV 150 KeV
Normalized Emittance 1.5 mm·mrad 1.5 mm·mrad
Bunch length ~ 0.6 ps ~ 1.4 ps
Two different ondulator lines for two different spectral regions:
•FEL-1 covers the spectral region between 100 and 40 nm •FEL-2 the region between 40 and 10 nm
Both FEL’s are based on the Harmonic Generation scheme and make use of APPLE II type ondulator for producing light with variable polarization
FEL-1 (100 - 40 nm)
Parameter Value
Type Planar
Structure One segment
Period 10 cm
K >5
Length 3.04 m
Parameter Value
R56 ~ 32 m
Length ~ 1 m
Radiator Value
Type Apple
Structure ~ 6 Segments
Period 6.5 cm
K 2.4 - 4
Segment length 2.34 m
Break length 1.06 m
Total length 19.34 m
Modulator RadiatorDispersive section
Total length of FEL-1 ~ 23 m
nInput seed laser(240 360 nm)
Electrons interact with an external laser field in the first ondulator, the energy modulation produced by this interaction is transformed into spatial modulation (bunching) to the laser wavelength and to its harmonics.
Bunched electrons emit coherently into the radiator tuned to the desired harmonic and FEL process is initiated.
A segmented radiator allows to insert electron optics and diagnostics between modules .
4
3
2
1
0
Output power (GW)
20151050 z (m)
FEL at 40 nm FEL at 60 nm
FEL-1 time independent simulationsInput parameters Medium bunch
Beam energy 1.2 GeV
Peak current 0.8 KA
Uncorrelated energy spread
200 KeV
Normalized Emittance
1.5 mm·mrad
Seed power 100 MW
0.6
0.4
0.2
0.0
Bunching
20151050 z (m)
8
6
4
2
Energy spread (
20151050 z (m)
Within such approach we use ideal electron bunches whose parameters are those predicted for the FERMI Linac.Simulations show the possibility to reach saturation and output power of several GW within 6 radiator segments for the considered wavelength range (100-40nm).
2233.0
2232.0
2231.0
2230.0
mean e
nerg
y (
-400 -200 0 200 400time (fs)
Phase space
Medium electron beam at the end of the LINAC
Note the energy “chirp”
Parameter Value
Beam energy 1.2 GeV
Peak current 0.8 KA
Uncorrelated energy spread 150 KeV
Normalized Emittance 1.5 mm·mrad
Bunch length (flat part) ~ 0.6 ps
The analysis of the electron bunch shows the presence of a cubic chirp in the electron-energy profile with some noisy modulation period of the order of 10 m. Similar microbunching modulation has been found also in the current profile.
0.8
0.7
0.6
0.5
0.4
0.3
0.2
energ
y sp
read (
-400 -200 0 200 400time (fs)
2000
1500
1000
500
0
curr
ent (A
)
-400 -200 0 200 400time (fs)
3.0x10-6
2.5
2.0
1.5
1.0
0.5
0.0 Em
itta
nce (
mm
mra
d)
-400 -200 0 200 400time (fs)
800
700
-300 300
FEL1 simulation using medium bunch (40nm)
1.5
1.0
0.5
0.0
FE
L ou
tput
pow
er (
GW
)
-400 -200 0 200 400time (fs)
2233.0
2232.5
2232.0
2231.5
2231.0
2230.5
2230.0
electron energy (
120x106
100
80
60
40
20
0
Out
put s
pect
rum
40.240.140.039.939.8Output wavelenght (nm)
0.10
0.08
0.06
0.04
0.02
0.00
Inpu
t se
edin
g p
ow
er
(GW
)
-400 -200 0 200 400time (fs)
2233.0
2232.5
2232.0
2231.5
2231.0
2230.5
2230.0
ele
ctron e
nerg
y (
Pulse width (rms) = 66 fsPhoton number = 6.5e+13Bandwidth (%) = 0.03Ratio to Fourier limit = 1.9
100fs seed
•The particle file has been simulated with the optimized setup for FEL1 using a seed pulse of 100fs rms centred on the flat part of the bunch. •This produces an output pulse of the order of 1.5 GW and less than 70 fs long.•The cubic chirp and the microbunched structure in electron energy profile are responsible for the increase of the bandwidth with respect to the Fourier limit.
Parameter Value
Type Apple
Structure Segmented
Period 5 cm
Segment length 2.4 m
K 1.1 - 2.8
Break length 1.06 m
Total length 19.7 m
Radiator
n n nm)
Second Stage
Total length FEL-2 ~ 37.5 m
seed laser
“fresh bunch” break
First Stage
Fresh-bunchfirst stage
Fresh-bunchsecond stage
0.10
0.08
0.06
0.04
0.02
0.00
4002000-200-400
FEL-2 (40 - 10 nm)
•The electrons that are used for the first stage are not useful in the second stage because they have a too large energy spread.•A fresh bunch break is used in order to delay the electrons with respect of the photons •In the second modulator the produced 40 nm radiation is superposed to a fresh part of the electron bunch and a new harmonic generation is performed
FEL-2 time independent simulationsBeam parameters Medium bunch
Beam energy 1.2 GeV
Peak current 0.5 KA
Uncorrelated energy spread 150 KeV
Normalized Emittance 1.5 mm·mrad
Bunch length 1.4 ps
•Simulations show the possibility of reaching saturation within the 20 meter of radiator for the 20 nm case with more than 1 GW of output power.•In the 10 nm case saturation is not reached within the six modules however ~ 500MW are obtained.
0.4
0.3
0.2
0.1
0.0
Bunching
20151050 z(m)
10nm 20nm
4.0
3.0
2.0
Energy spread (
20151050 z (m)
10nm 20nm
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Power (GW)
20151050 z(m)
10nm 20nm
0.4
0.3
0.2
0.1
0.0
rad1 bunching at 40 nm
1086420
3.2
2.8
2.4
rad1 energy spread (
1086420
250
200
150
100
50
0
rad1 power at 40 nm (MW)
1086420 z (m)
2289
2288
2287
2286
2285
2284
2283
mea
n en
ergy
(
-800 -400 0 400 800time (fs)
Phase space
Long electron beam at the end of the LINAC
Note the energy “chirp”
Parameter Value
Beam energy 1.2 GeV
Peak current 0.5 KA
Uncorrelated energy spread 100 KeV
Normalized Emittance 1.5 mm·mrad
Bunch length 1.4 ps
1200
1000
800
600
400
200
0
curre
nt (A
)
-800 -400 0 400 800time (fs)
0.8
0.6
0.4
0.2
0.0
ener
gy s
prea
d (
-800 -400 0 400 800time (fs)
•The longitudinal phase space presents a quadratic chirp and residual fast time fluctuations of the mean energy.•The useful part of the bunch is about 1.4 ps long and presents an energy spread which is of the order of 100keV. The current is of the order of 0.5 kA.
FEL2 simulation using the long bunch (10nm)100
80
60
40
20
0
Inpu
t see
ding
pow
er (M
W)
-800 -600 -400 -200 0 200 400 600 800time (fs)
2289
2288
2287
2286
2285
2284
2283
electron energy (
1.2
1.0
0.8
0.6
0.4
0.2
0.0
FE
L o
utp
ut p
ow
er
(GW
)
-800 -600 -400 -200 0 200 400 600 800time (fs)
2289
2288
2287
2286
2285
2284
2283
ele
ctron e
ne
rgy (
30x106
25
20
15
10
5
0
Out
put s
pect
rum
10.0310.0210.0110.009.999.989.97Output wavelenght (nm)
Pulse width (rms) = 170 fsPhoton number = 2.2e+13Bandwidth (%) = 0.04
250fs seed
•The length of the electron bunch allows the use of a seed pulse of 250 fs rms placed on the tail of the bunch.•After the cascade the 10nm coherent emission is produced from the head of the bunch.•Energy chirp and microbunching lead to a broadening of the bandwidth.
Studies of output power sensitivity to input jitter
(e.g., energy, emittance, energy spread, peak current, seed power, beam offset and tilt)
ParameterShot-to-shot
variation (rms)
Emittance 10 %
Peak current 8 %
Mean energy 0.1 %
Energy spread 10 %
Seed power 5 %
e-beam axis offset 100 m
e-beam tilt 10 rad
FEL 1 at 100 nmtime independent simulations
Input mean energy is varied ONLYAll other parameters assumed constant
=> Global output power standard deviation: 9.6%
3.0x109
2.8
2.6
2.4
2.2
2.0
1.8
1.6
1.4
Pow
er (
W)
23562354235223502348234623442342Energy
3 GW
2 GW
E/E = 0.1 %
Jitter on initial conditions
Series of time independent simulation runs have been performed varying imput parameter of the
electron bunch as predicted from Gun and Linac studies.
Simulations show a critical sensitivity to the
electron mean energy responsible of strong
fluctuations of the output power
3.0x109
2.5
2.0
1.5
1.0
Pow
er (
W)
23562354235223502348234623442342Energy
FEL 1 at 100 nm Energy variation projection
Output power global standard deviation: 16.5%
1.4x109
1.2
1.0
0.8
0.6
0.4
0.2
Pow
er (
W)
2352235023482346Energy
FEL 2 at 40 nm Energy variation projection
Output power global standard deviation: 33%
Simultaneous multi-parameter variation
Simultaneous variation of the following parameter has been considered : energy, current, uncorrelated energy spread, transverse emittance, initial transverse position and tilt
Simulation of 100 jittered electron bunches•100 electron bunches have been propagated starting from the Gun through the Linac considering possible noise sources (timing, phase and amplitude jitters)•The study has been performed for the “Medium bunch” Linac configuration used for FEL1•The central part has been considered as the useful one for the FEL process
Quantity Mean Value Std. Dev.
Gamma 2231.89 0.09%
Current (A) 718 6.6%
Incoherent energy spread
0.32987 19.5%
Normalized emittance 1.35 12.4%
Analysis of 100 jittered electron bunches
Start to end simulation confirm predictions for jitters
Quantity Mean Value Std. Dev.
Average pulse width (fs)
73.2
Average photon number
7.1e+13 23%
Average central wavelength (nm)
40.002 0.013%
Average bandwidth 0.033%
Average Fourier factor 2.2 13%
FEL1 results with 100 jittered electron bunchesResults of FEL simulations with the start to end jittered files are in agreement with time independent predictions and confirm the crucial dependence of output power on fluctuations of electron mean energy. On the contrary, the central wavelength shows a very weak dependence on input parameter fluctuations.
The energy spread problem
1.2x109
1.0
0.8
0.6
0.4
0.2
0.0
Out
put p
ower
@ 1
0nm
(W
) 500450400350300250200
Energy spread (keV)
medium bunch long bunch '10MW' '100MW' '300MW'
2
4
6
107
2
4
6
108
2
4
6
109
Ou
tpu
t p
ow
er
@ 1
0n
m (
W)
500450400350300250200Energy spread (keV)
medium bunch long bunch '10MW' '100MW' '300MW'
3.0x109
2.5
2.0
1.5
1.0
0.5
Ou
tpu
t po
we
r @
40
nm
(W
)
500450400350300250200Energy spread (keV)
FEL(40nm) '500MW' '1GW' '3GW'
5
6
7
89
109
2
3
Ou
tpu
t po
we
r @
40
nm
(W
)
500450400350300250200Energy spread (keV)
FEL1(40nm) '500MW' '1GW' '3GW'
FEL1 and FEL2 have been optimized for electron bunches with an incoherent energy spread of 100-200keV. Larger energy spread can compromise the FEL performance.
In the case of FEL1 at 40 nm output power larger than 1GW is still possible for lower than 450keV.
The sensitivity is dramatic in the case of FEL2, that for =300 show only 100MW at 10nm. Performance is better using the medium bunch also for FEL2.
CHG on the Elettra Storage Ring FEL
(on behalf of the SR-FEL group - Sincrotrone Trieste )
CHG scheme on a storage ring
ELETTRA FEL Layout
Diagnostics
Experimentalhutch
Back mirror Front mirror Optical cavity – 32.4 m
Optical klystron – 4.6 m
Nanospetroscopybeamline
First CHG evidence on 29 April 2007
• Seed @ 780nm laser @ 260nm (3rd harmonic)
Ebeam = 0.75 GeV
Ibeam = 0.57 mA (single bunch)
Seed:
λ = 783 nm
pulse length ≈ 100 fs
pulse energy ≈ 2 mJ
rep. rate = 1KHz
CHG characterization
Considering the difference in the number of photon per pulse and taking into account the differencebetween the pulse length of synchrotron radiation (~35 ps) and coherent signal (~120 fs), the ratio between peak powers can be estimated to be of the order of 10^4
This corresponds to what one can expect from a qualitative calculation using the parameter of our setup
Seed 780nm CHG 390nm (2nd) & 260nm (3rd)
Beam instability at 0.75 GeV and timing jitter/driftsprevent the optimization of the experimental
parameters
Seed at 390nm Ebeam = 1.1 Gev
– better stability– from spectra we can estimate CHG gain with respect to
spontaneous emission
Spectral stability
a): Spectrum of the coherent emission at 132 nm (linear polarization). The integration time is 1 ms; the spectrum is obtained after subtraction of the background due to spontaneous emission. b): Spectrum of spontaneous and coherent emission for the case in which the radiator is tuned at 203 nm, i.e., slightly mismatched with respect to the second harmonic of the seed laser (198.5 nm).
Quadratic dependence of CHG on bunch current
Quadratic dependence of the coherent harmonic at 132 nm vs. (normalized) bunch current. Dots represent experimental data; the curve is a t obtained using a quadratic function.
Test experiments
• Nanospectroscopy (Elettra):– PEEM with gated detector
• CESYRA (CIMAINA/UniMi)– TOF (mass spectrometry)
Nanospectroscopy30
25
20
15
10
5
0
inte
nsity
(a.
u.)
86420-2kinetic energy (eV)
25
20
15
10
5
0
Difference (only Seeded)
Seeded + Single bunch (delay = 1050 ns)
Single Bunch (delay = 200 ns)
gap1 = 23.56gap2 = 31.4phase1 = 0phase2 = 32.8energy: 9.1 eV
VB UPS on Ag/W(110)
50
40
30
20
10
0
inte
nsity
(a.
u.)
12108642kinetic energy (eV)
50
40
30
20
10
0
Difference (only seeded)
gap1 = 23.56gap2 = 34.95phase1 = 0phase2 = 33.67energy: 12.16 eV
Seeded + Single Bunch (delay = 1050 ns)
Single Bunch (delay = 200 ns)
3rd harmonicλ = 133 nm
4th harmonicλ = 99.5 nm
Imaging on SiO2 patterned
sample
seeded
spontaneous emission
Comparison between CHG and NHG
• Sketch of the experimental setup used for the investigation of harmonic generation in a FEL. A powerful Ti:Saphire laser interacts with the electron bunch (e) within the modulator and induces a modulation of the electrons' energy. After the conversion of the energy modulation into spatial bunching, which occur into the magnetic chicane (R56), the bunch enters the radiator and start emitting coherently at the resonant wavelength and, eventually, at its harmonics. The produced coherent harmonic radiation passes through a diaphragm (D) and is transported into a diagnostic area, where temporal (PMT) and spectral (CCD) analyses are performed. The position of the diaphragm de¯nes the angle of emission with respect to the undulator's axis considered for the measurement.
CHG and NHG
Comparison between the third harmonic radiation produced in CHG (a) and NHG (b) configurations. In both cases the modulator and the seed laser are in horizontal polarization. The radiator is tuned to the third harmonic in horizontal polarization (a) and to the fundamental in horizontal polarization (b).
CHG and NHG
• Comparison of the harmonic radiation produced in CHG configuration at the second (a,b) and third (c,d) harmonics of the seed wavelength. The radiator is set in horizontal (a,c) or in circular (b,d) polarization, while the modulator and the seed are in horizontal polarization.
CHG and NHG
• Coherent harmonic signals produced at the second harmonic of the seed wavelength in CHG (a,b) and NHG (c,d) configurations. Figures (a,c) refer to a condition where the seed laser, the modulator and the radiator are in planar polarization, while Figs.(b,d) refer to a condition where both the seed and all undulators are set in circular polarization. Data reported in Figs.(a) and (c) refer to the same experimental conditions, and can be used for a relative comparison. The same holds for Figs.(b) and(d).
CHG and NHG
• Measured angular distribution of the second harmonic in the case of CHG (squares) and NHG (dots) with helical undulators. Measurements are well fitted by theoretical curves,which have been obtained by integrating the expected Gaussian profile (dashed line, CHG case) and the profile predicted in [9] (continuous line, NHG case), over an angle of 0.09 mrad.
Perspectives for SR-CHG
• Seed with Ti:Sa 3rd harmonic (260nm)
• CHG down to 87nm (14.3 eV)
• Pump and probe beamline for time resolved experiments
• Compatibility with normal operation mode at 2.0 GeV (non symmetric SR filling)
PeopleSR-FEL Group
G.De Ninno, F.Curbis, E.Allaria, M.Trovò, L.Romanzin, M.Coreno, E.Karantzoulis, C.Spezzani
Laser
M.B.Danailov, A.Demidovich, R.K.Ivanov
Synchronization
P.Sigalotti
A.Carniel, F.Rossi, M.Ferianis,
Experiments
A.Locatelli, O.Mentes, M.A.Nino, R.Sergo, M.Pittana, G.Cautero
P.Piseri, G.A.Bongiorno, M.Amati, O.Nicoletti, L.Ravagnan, P.Milani
Machine
Linac and Elettra operators
