X-ray Free Electron Laser
XFEL Research & Development Division
RIKEN SPring-8 Center
Hitoshi Tanaka

2011/10/04

Cheiron School 20111

Outline
1. Introduction –What we can and can notobserve using SR
2. What XFEL enables us to observe
3. Overview of XFEL
4. Performance of XFEL
5. Words to Young Researchers

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1. Introduction -What we can andcan not do using SRx

(a)
PA
Bzx

(b)
PA
Bt'x(c)PABt

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1.1. Features of SR

１．White / monochromatic
２．High Brilliance３．Directivity
４．Polarization Control
５．Pulse
６．Quantitativity
７．Wavelength Tunability

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1.2. What SR makes visible
Incident X-rays excite electrons in the matterand cause scattering, absorption, diffraction,
modulation of transmitted X-ray, photo-electrongeneration, luminescence, etc.
By analyzing these data, we can find electronic
state in the matter, crystal structure, local andglobal atomic order, magnetism, dynamical
distribution of chemical component, etc.

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1.3. Variety of SR-Driving Science• Research object: polymers, organic & inorganic
compounds, biopolymers, meteorites and deep
earth substance, catalysts, green energy relating
material, etc.
• Research target: (1) composition, structure andelectric state changes in a reaction process andtheir responses to external parameter changes,(2) correlation of functionality to electric state andmaterial structure, etc, (3) basic physics.

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1.3. Variety of SR-Driving Science con’t

Figure shows two sets of X-ray fluorescence (XRF) 2D
imaging data of arsenic, potassium, and calcium in themarginal region of a fern leaf; (a)30 min. after giving
arsenic to the fern and (b)12 hr. after giving arsenic to thefern.
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1.4. Weak points of SR• Chaotic light（light of lamp）• Long pulse duration（impossible toshorten）• Weak intensity（difficulty in single-shotmeasurement）

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2. What XFEL enables us to observe
Key words• Observation of material property change
in atomic & femto sec. resolution by asingle-shot measurement• Exploitation & observation of new
physics & phenomena by using
extremely high electric-field intensity
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2.1. Features of XFEL

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2.2. New Frontier Opened by XFEL

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2.2. New Frontier Opened by XFEL con’t
ProteinCan we seefunctional
expressions in a
Protein Complex
life phenomenon
in real timeusing XFEL?

CellMembrane

NucleusEndosome

Living Organisms
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3. Overview of XFEL
• Historical Background• Principle of XFEL• World’s Trend• Japanese Strategic Approach

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Cheiron School 2011133.1. Long Way to XFEL
1916 A. Einstein proposed principle of stimulated emission ofradiation
1958 C. H. Townes invented principle of laser.
1960 T. H. Maiman developed the world’s first Ruby Laser.
1970 I. Maday proposed new laser using free electrons as alaser medium (FEL)
1984 R. Bonifachio and his colleagues proposed“Self AmplifiedSpontaneous Emission”(SASE) based FEL
2009 SLAC team achieved the world’s first SASEXFEL at a wavelength of 1.5 Å2011 RIKEN/JASRI team realized the world’s firstcompact XFEL at a shortest wavelength of0.8 ÅRather new technology !!

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3.1. Long Way to XFEL con’t
Obstacles against shortening the wavelength• A energy level corresponding to X-rays is ata deep inside of atoms and quite difficult tocontrol population inversion condition.
• No mirror is available with high reflectivity ata wavelength of X-rays.
XFEL realization needs overcoming theabove two difficulties.
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3.2. Principle of FEL
Conventional laser using an optical cavity
Stimulated emission
Excited state I
Excited state IIPumpingGround stateMirror
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Stimulated emission
High energy state II
WavelengthλStimulated emission

Absorption

Emission

Energy gap
ΔE

WavelengthλWavelengthλLow energy state I

Stimulated emission is an inverse process ofabsorption as shown above.
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Optical cavity system
Total reflection
mirrorLaser
tube

BrewsterwindowSemi‐permeable
mirrorTypical structure of gas laser system

Conventional laser amplifies laser power of the selectedmode by using stimulated emission from a laser medium
during a huge number of round trips in the medium.
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Using free electrons as a laser medium

Undulator FieldExvez
EM Field by Radiation
Exve

Electron OrbitveEx

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Using free electrons as a laser medium con’t
2'
*"
KeB"u
22u
Resonance " = " # v T $1++ % & ,, K =uz2)
2% (22 -m0 c%condition+Electron beam is trapped in an electro-magnetic potential andthis potential generates energy modulation around the stable
fixed point. Then the energy modulation is converted to density!
modulation through the energy dispersion of the undulator.

λ2011/10/04

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!

Using free electrons as a laser medium con’t
Instead of stimulated emission, the density modulated
electrons with an interval of a resonance wavelength
λ , enables laser amplification.
‐ Independent on energy level in atoms and molecules ‐("u % K 22 2
" = 2 '1 ++ # $ *, K + 1 ~ 2
2# &2)

Let’s estimate λ assuming
λu=15mm, K=2,γ=3915@2GeV

λ = 1.5 nm
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Removing optical cavity system
To shorten a laser wavelength down to X-ray
wavelengths, optical cavity-free is an essentialconcept.

Key point
How to realize a sufficiently high gain by a singlepass?

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Highly brilliant electron beam + long undulator
with a large number of periods＜Brilliant electron beam＞
High electron density achieving a high gain andlow angular divergence keeping density
modulation of Å order.

＜Long undulator＞
A larger number of periods realizes asufficiently high gain by a single pass, whichcorresponds to that obtained by the opticalcavity system.
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Self-Amplified Spontaneous Emission; SASE
based FEL
E-beamUndulator

SASE
Micro-bunching

Interaction with a built-up
laser field

Laser Power

Energy modulation by
spontaneousemission

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3.3. World’s Trend
SASE XFELs under operation &construction in 3-sites

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LCLS achieving the world’s first power saturaion ofSASE-based XFEL at a wavelength of 1.5ÅApril 20092011/10/04

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SASE schemeExperimental
Users

E-gun

Linear Accelerator

Long Undulators

From the viewpoint of generating high brilliant electrons,(1) ring accelerators are unavailable ( linear accelerators).
(2) High beam energy also has an advantage as follows;ε = εn/βγ.

electron beam emittance（phase space volime）

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SASE scheme con’t
From the viewpoint of wavelength shortening,High beam energy has an advantage as follows;
Laser wavelength λ("u % K 22 2
" = 2 '1 ++ # $ *, K + 1 ~ 22# &2)

Huge accelerator system!longer than a few km!!

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3.4. Japanese Approach
Future prediction 1
Although variety of XFEL applications areexpected , one facility can provide only afew BLs.
To widely utilize XFEL, it is essential to
make the facility scale compact as muchas we can. World’s trend goes to thisdirection as XFEL usefulness becomes
gradually clear.
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3.4. Japanese Approach con’t
Future prediction 2
SASE is not the goal, because temporal
coherence is poor (but, spatial coherenceis high).
The goal is X-ray laser having both temporaland spatial coherence.

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Development Target

SCSSversus

LCLS/FLASH

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Two-steps approch
Step-1: XFEL by Self-Amplified SpontaneousEmission scheme (SASE-XFEL) Features
Long Undulatore-beam
SASE-FEL

Gain~105

Micro-bunching

Interaction witha growing-up
laser field[×10 11 ] 5
Photon flux
(photons/pls/0.1%b.w.)

Start-up by
SpontaneousEmission

Log(Gain)

Full spatialcoherence••Poortemporalcoherence
Short pulse duration••Multi-mode
High peak••Largetime jitters•power（~10GW)
Less intensity tunability
HF K=1.9432
1011830
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11840
11850
11860
Photon energy (eV)

11870
32

Two-steps approch con’t
Step-2: Intensity-tunable XFEL with a single
mode (Seeded-XFEL)
Features

SASE Advantage+•Temporal coherence•Single mode
•Tunable laser power•Less power fluctuation

Long Undulator + HGHG
e-beamSeeded-FEL
Photon flux
(photons/pls/0.1%b.w.)

Gain~105Log(Gain)

Micro-bunchingby a laser-seeder[×10 11 ] 5
HF K=1.94321011830

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11840
11850
11860
Photon energy (eV)

11870

33

SPring-8 Compact SASE Source (SCSS)
What the SCSS concept isThis concept generates X-ray using a low-energy beam by
efficiently accelerating a low-emittance beam with C-band
system and passing it through short-period in-vacuum IDs.
CeB6 single crystal
cathode thermionic gun

C-band high gradientacceleration system

εn<1πmm mrad

Short-period
in-vacuum undulators(>35MV/m)(λU=18mm, K=2.2)

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BC3 out

Peak current (A)Peak current (A)BC2 out

Beam energy (MeV)

Peak current (A)BC1 out

Beam energy (MeV)

Beam energy (MeV)

Injector out

Peak current (A)Beam energy (MeV)

Compact design for 8-GeV SASE XFELLow emittance injector
with CeB6 E-gun

Laser system forwavelength compression

C-band accel.
systemShort-period
In-vacuum undulators~700 m2011/10/04

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4. Performance of XFEL（step-1)
Comparison with SPring-8 performance
Parameter• Wavelength(fundamental）• Pulse Duration• Repitition• Spatial Coherence• Peak Power• Peak Brilliance• Averaged Brilliance

XFEL
SPring-8>0.06 Å>0.05 Å<100 fs~40 ps< 60 Hz~40 MHz
100%~0.1%
20~30 GW 100~200 W~1034~1024~1022~1021

Def of Brilliance：phs/sec/mrad2/mm2
XFEL/SPring-8 Beamline Technical Design Report Ver. 1.0, June 17 (2008)2011/10/04

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Typical Performance (1.3Å)Parameter• B.W.

[!10 12]

Photon flux
(photons/pls/0.1%b.w.)

XFEL
0.092%• Pulase Duration 30 fs• Source Size
33 um
• Angular Spread 0.73urad• Epulse
0.78 mJ• Phots/pulse
5E11• 2nd/1st1.3E-4• 3rd/1st
2.8E-3

1

HF K=2.2

0.509700

9710
9720
9730
Photon energy (eV)9740

XFEL/SPring-8 Beamline Technical Design Report Ver. 1.0, June 17 (2008)2011/10/04

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5. Words to Young Researchers
Strong Efield

SpatialCoherence

Short Pulse

Temporal
Coherence

Variable Efield

VariablePulse
Duration

No authority, everything will start from now
on. You are a potential hero (heroine)!
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5. Words to Young Researchers con’t
Everyone has a chance to becaome aauthority in this field. Young guys, beambitious!

Blue ocean is
wai-ng for yourchallenge.
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