Transcript Soft X-ray Self

Soft X-ray Self-Seeding
in LCLS-II
J. Wu
Jan. 13, 2010
2
Schematics of Self-Seeded FEL
Originally proposed at DESY [J. Feldhaus, E.L. Saldin,
J.R. Schneider, E.A. Schneidmiller, M.V. Yurkov, Optics
Communications, V.140, p.341 (1997) .]
– Chicane and gratings in two orthogonal planes x and y
chicane
1st undulator
grazing
mirrors
FEL
slit
SASE FEL
electron
2nd undulator
grating
Seeded FEL
electron
dump
3
Transform Limited Pulses
For a Gaussian photon beam
s w  1/2s t   wFWHMs t  2 ln 2  1.18
– Gaussian pulse, at 1.5 Å, if Ipk= 3 kA, Q = 250 pC,
sz  10 mm, then transform limit is: sw/w0  10-6
– LCLS normal operation bandwidth on order of 10-3
– LCLS electron bunch, double-horn but central part
effectively flat top, for flat top wFWHMs t  1.61
Improve longitudinal coherence, and reduce
the bandwidth improve the spectral
brightness
4
Single Spike vs Self-Seeding
Reaching a single coherent spike?
– LG = 1 m, 20LG= 20 m, for lu= 2 cm, there is ~1000 periods
– Take 1 nm as example, single spike  1 micron
– Low charge might reach this, but bandwidth will be broad
Narrow band, “relatively long” pulse  Self-Seeding.
In the following, we focus on 250-pC case with a
“relatively” long bunch, and look for “narrower”
bandwidth and “good” temporal coherence
For shorter wavelength (< 1 nm), single spike is not
easy to reach, but self-seeding still possible
5
Two-Stage FEL with Monochromator
Seeding the second undulator (vs. single
undulator followed by x-ray optics)
– Power loss in monochromator is recovered in the
second undulator (FEL amplifier)
– Shot-to-shot FEL intensity fluctuation is reduced
due to nonlinear regime of FEL amplifier
– Peak power after first undulator is less than
saturation power  damage to optics is reduced
With the same saturated peak power, but with two-orders of
magnitude bandwidth reduction, the peak brightness is
increased by two-orders of magnitude
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Monochromator
J. Hastings suggested varied line spacing
gratings (to provide focusing) as the
monochromator for the soft x-ray self-seeding
scheme
– Yiping Feng, Michael Rowen, Philip Heimann
(LBL), and Jacek Krzywinski et al. are designing
John Arthur, Uwe Bergmann, Paul Emma,
John Galayda, Claudio Pellegrini, and Jochen
Schneider et al. are giving general advices
Feng-Rowen-Heimann-Krzywinski-Hastings-Wu-et al.
Optics Specs
Performances
Parameter
symbol
value
unit
Energy range
e
200 – 2000
eV
Pulse length
(rms)
t
34 – 12
fs
Pulse energy
E
1.2 - 17
mJ
Peak Power
Pinput
10 - 400
MW
E-beam size
(rms)
s
50 -15
mm
Resolving power
R
> 20000
Throughput
htotal
0.2 – 0.005
%
Output peak
Power
Poutput
10 - 20
kW
Time delay
T
10.8 – 9.6
ps
Feng-Rowen-Heimann-Krzywinski-Hastings-Wu-et al.
Optics Components
Cylindrical horizontal focusing M1
– Focus at reentrant point
Planar pre-mirror M2
– Vary incident angle to grating G
Planar variable-line-spacing grating G
– Focus at exit slit
Exit slit S
Spherical vertical focusing mirror M3
– Re-focus at reentrant point
electron-beam
M3
M1
g
source
point
M2
h
G
re-entrant
point
Feng-Rowen-Heimann-Krzywinski-Hastings-Wu-et al.
Geometry (Dispersion Plane)
Optical components
– Deflecting mirror; Pre-mirror; VLS Grating; Collimation
mirror
M1
ZR
w0
M2
Gv
M3
w0’’
w0’
L1
LM1M2 rM2G
r’G
rM3
r’M3
LRe-entrant
rtotal
L1
LM1M2
rM2G
r’G
rM3
r’M3
LRe-entrant
rtotal
200 eV
13.761030
4.204372
0.036709
5.981053
0.351780
1.993796
1.656204
27.984945
2000 eV
13.761030
3.901582
0.339127
6.021674
0.311159
3.400840
0.249160
27.984572
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Monochromator
Might need more than one monochromators
Efficiency:
– Monochromator efficiency
– Phase space conservation: bandwidth reduced by
one to two-order of magnitudes
– Overall efficiency will be on order of a percent to a
few 10-5 (about 0.2 – 0.005 %)
– Still looking for design to have higher efficiency
• Use blazed profile -- efficiency increases by x10
• Use coating to improve reflectivity
11
LCLS SASE FEL Parameters
S-2-E electron distribution: slice emittance
entering the undulator
Slice Emittance small 
Gain Length Short
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6-nm Case: Electron Bunch
Peak current ~1 kA
Undulator period 5 cm, Betatron function 4 m
For 250 pC case, assuming a step function
current profile, sz ~ 22 mm
Gain length ~ 1.4 m
SASE spikes ~ 70
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LCLS high-brightness electron beam
S-2-E electron distribution: electron current
profile entering the undulator
tail
head
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6-nm SASE FEL Parameters
6-nm FEL power along first undulator
saturation around 28
m with ~5 GW
Present LCLS-II plan uses 40 meter long undulators
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6-nm Case - Requirement on Seed Power
Effective SASE start up power is 200 W.
– In a bandwidth of 2.210-5, there is only 0.5 W
Use small start up seed power 10 kW…
– Monochromator efficiency  10% (at 6 nm)
– Phase space conservation: bandwidth decreases
1 to 2-orders of magnitude (about 70 spikes)
– Take total efficiency 1.010-3 Need 10 MW on
monochromator to seed with 10 kW in 2nd und.
10 MW
10 kW
16
6-nm Seeded FEL Parameters
FEL power along 2nd undulator for seed power
of: 10 MW (black), 100 kW (red), 10 kW (cyan)
Saturation
around 18, 25
and 29 m with
power ~5 GW
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6-nm Seeded FEL Parameters
Temporal profile at ~26 m in 2nd undulator for
seed of 100 kW (black) and 10 kW (red)
~35 mm
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6-nm Seeded FEL Parameters
FEL spectrum at ~26 m in 2nd undulator for
seed of 100 kW (black) and 10 kW (red)
FWHM 3.110-4
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6-nm Case - Transform Limit
Effective pulse duration 35 mm (sz  10 mm)
Transform limited Gaussian pulse 
bandwidth is 1.110-4 FWHM
(For uniform pulse  1.510-4 FWHM)
Here the seeded FEL bandwidth is about twice
the transform limited bandwidth
20
Polarization
The second undulator can be APPLE type
– Linear (black), circular (red), or elliptical
polarization
– Pol. ~ 100%
21
6-nm Seeded FEL: Polarization
Temporal profile in 2nd undulator with seed of
100 kW for planar (black) and circular (red)
~35 mm
Planar at 26 m; Circular at 18 m
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6-nm Seeded FEL : Polarization
FEL spectrum in 2nd undulator with seed of for
planar (black) and circular (red)
FWHM 3.110-4
Planar at 26 m; Circular at 18 m
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6-Å Case: Electron Bunch
Peak current ~3 kA
Undulator period 5 cm, Betatron function 4 m
For 250 pC case, assuming a step function
current profile, sz  7 mm.
Gain length ~ 2.1 m
SASE spikes ~ 160
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LCLS high-brightness electron beam
S-2-E electron distribution: electron current
profile entering the undulator: compress more
tail
head
25
6-Å SASE FEL Parameters
6-Å FEL power along the first undulator
saturation around 32 m
with power ~10 GW
Present LCLS-II plan uses 40 meter long undulators
26
6 Å SASE FEL Properties
6 Å FEL temporal profile at 30 m in the first
undulator: challenge
27
6 Å SASE FEL Properties
6 Å FEL spectrum at 30 m in the first undulator
– Spiky spectrum: challenge
28
6-Å Case - Requirement on Seed Power
Effective SASE start up power is 1.3 kW.
In a bandwidth of 6.610-6, there is only 1.6 W
Use small start up seed power 20 kW…
– Monochromator efficiency ~ 0.2 % (at 6 Å)
– Phase space conservation: bandwidth decreases
1 to 2-orders of magnitude (~ 160 spikes)
– Take total efficiency 5.010-5 Need 400 MW on
monochromator to seed with 20 kW in 2nd und.
400 MW
20 kW
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6-Å Seeded FEL Parameters
Power along 2nd undulator for seed power of
20 kW (black) and 10 kW (red)
Saturation around
35 m with power
on order of 10
GW
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6-Å Seeded FEL Parameters
Temporal profile at ~35 m in the 2nd undulator
for seed of 20 kW (black) and 10 kW (red)
~12 mm
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6-Å Seeded FEL Parameters
FEL spectrum at ~35 m in the 2nd undulator
for seed of 20 kW (black) and 10 kW (red)
FWHM 6.210-5
32
6-Å case — transform limited
Effective pulse duration 12 mm, sz ~ 3.5 mm
Transform limited Gaussian pulse 
bandwidth is 3.210-5 FWHM.
(For uniform pulse  4.410-5 FWHM)
The seeded FEL bandwidth (6.210-5 FWHM)
is less than twice the transform limited
bandwidth
Self-Seeding Summary at 6 nm and 6 Å
Parameter
6 nm
6Å
unit
Emittance
0.5
0.5
mm
Peak Current
1
3
kA
Pulse length rms
35
12
fs
Bandwidth FWHM
31
6.2
10-5
Limited Bandwidth
15
4.4
10-5
Seed Power
10
20
kW
Power on Mono
10
400
MW
Mono Efficiency
10
0.2
%
Sat. Power
5
10
GW
Sat. Length
30
35
m
Brightness Increment
50
150
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Ongoing work
VLS gratings are being studied in more details
looking for larger overall efficiency
Three dimensional overlap of the electron pulse
and the photon pulse
Electron chicane will be studied in more detail
Statistics of the self-seeded FEL performance
Full simulation with monochromator wavefront
propagation
More detailed study on APPLE undulator
possibility as the second undulator to generate
narrow bandwidth FEL with variable polarization