X-ray Free Electron Laser (XFEL)
Part-1: Accelerator
Sept. 14, 2015
Takahiro Inagaki
RIKEN SPring-8 center
XFEL research & development division
AOFSRR Cheiron School 2015, XFEL Part-1

1Outline• Introduction• FEL mechanism• SACLA machine configuration• Present status and outlook• Summary

AOFSRR Cheiron School 2015, XFEL Part-1

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What is X-ray Free Electron Laser (XFEL)?

Synchronous
EM wave
phase

Coherence

No medium, no mirror

Random phase
10 µm

XFEL

Laser

High peak brightness,
Short pulse length,…

Intense,Sharply focused,…

Synchrotron
radiationLamp orsunlight
Visible1 µm

100 nmsoft X-ray
10 nm

1 nm

←Long wavelength
AOFSRR Cheiron School 2015, XFEL Part-1X-ray
1Å

Wavelength

Short wavelength→For microscopic resolution3

Remarkable features of XFELHigh Peak Brilliance (~1033) for physics in extremely intense field, …
Narrow Pulse Width (~10 fs) for ultrafast reaction or interaction, …
High Degree of Coherence (~100 %) for diffraction imaging, …
XFEL(SACLA)
Degree of Coherence（%）•••

100×1031010-15
10-14

110-13
10-12

0.1

10-11

0.01 10-101010

3rd Generation
Light Sources
2nd Generation (SPring-8)

×10-3Light Sources
10201030
AOFSRR Cheiron School 2015, XFEL Part-1

×1094

Outline• Introduction• FEL mechanism• SACLA machine configuration• Present status and outlook• Summary

AOFSRR Cheiron School 2015, XFEL Part-1

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Laser amplification mechanism
Fluorescence light• Visible light laser– Stimulated photon emission in thelaser medium.
– Amplified by the optical cavity.
• X-ray– Almost transmit the materials.
– Low interaction to laser medium.
– No available reflection mirror.

Excited state

Ground stateLaser medium

MirrorLaser
MirrorPump light• Free electron laser (FEL)– Stimulated synchrotron radiation fromthe electron beam (free electron) inthe undulator.
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Micro-bunched electron enhances the radiation power

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How to make micro bunch in the undulator
Electrons emit undulator radiation

Electron’s energies are periodicallymodulated by EM field of the radiation
Accelerated electrons
Electric fieldof the radiation
Decelerated electrons

Photon wavelength:
λx = λu / 2γ2 * (1+K2/2)
Magnet period

Magnetic field strength

Electron beam orbit
in the undulator

Electrons are gathered (micro-bunch)in the wavelength intervals
AdvancedElectron beam energy

Micro-bunched electrons emit
Intense, coherent radiation (laser)
AOFSRR Cheiron School 2015, XFEL Part-1

Delayed8

Various type of Free Electron Lasers• Seeded FEL– External laser gives initial modulation.
Electron– Wavelength range is limited by seed laser. beam
– 61 nm @SCSS/SPring-8 in 2010.
(T. Togashi et al. Opt. Express, 2011)

Undulator

Seed laser• Cavity FELFELOptical cavity– Long electron pulse length (> several µs).
– Wavelength range is limited by mirror.
– Popular for infrared (THz) FELs.

MirrorsFEL• Self Amplified Spontaneous Emission (SASE) FEL– Spontaneous emission works as a seed.
– No limitation for wavelength.
– Intrinsic fluctuation due toinitial spontaneous emission.

Long (~100 m) undulator section

FEL
Spontaneous emission

AOFSRR Cheiron School 2015, XFEL Part-1

9SASE-type XFEL schematicElectrongun
電子銃Undulator

Linear accelerator

Beam dumpTo userexperiments

ElectronBeam

Accelerated &compressed

Spontaneousemission

Microbunching

Coherence
Large

SASE-FELPowersaturation

Coherence
Small
Undulator length
AOFSRR Cheiron School 2015, XFEL Part-1

10SASE-XFEL, basic formulas (for linear undulator)•𝜆𝜆𝑢𝑢
𝐾𝐾2𝜆𝜆𝑥𝑥 = 2 � (1 + )2𝛾𝛾
2Photon wavelength:– Beam energy: 𝛾𝛾 ≡ 𝐸𝐸𝑒𝑒 /(𝑚𝑚𝑒𝑒 𝑐𝑐 2 )– Undulator period: 𝜆𝜆𝑢𝑢••

Typical parameters at SACLAFEL parameter:

𝜌𝜌 ∝

𝜆𝜆2𝑥𝑥
𝛾𝛾𝐼𝐼𝑒𝑒 � 𝜎𝜎 �𝜎𝜎 � 𝑓𝑓(𝐾𝐾)
𝑥𝑥 𝑦𝑦– Beam current: 𝐼𝐼𝑒𝑒– Beam size: 𝜎𝜎𝑥𝑥 , 𝜎𝜎𝑦𝑦
– Undulator parameter: 𝐾𝐾 ≡ (𝑒𝑒𝑒𝑒𝜆𝜆𝑢𝑢 )/(2𝜋𝜋𝑚𝑚𝑒𝑒 𝑐𝑐)

X-ray 0.12 nm
High 8 GeV
Short 18 mm1/3𝜆𝜆

Gain length (1D approx.): 𝐿𝐿𝑔𝑔 = 4𝜋𝜋 𝑢𝑢3𝜌𝜌•

Radiation power growth:•– Energy spread: Δ𝛾𝛾/𝛾𝛾

Saturation power:

Px (z) ∝ exp𝑧𝑧
Δ𝛾𝛾/𝛾𝛾 2(1 − 12𝜌𝜌2 )𝐿𝐿𝑔𝑔

𝑃𝑃𝑠𝑠𝑠𝑠𝑠𝑠 ~ 𝜌𝜌𝑃𝑃𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏 = 𝜌𝜌𝑁𝑁𝑒𝑒 𝐸𝐸𝑒𝑒

AOFSRR Cheiron School 2015, XFEL Part-1

High

typ. 5x10-4

High several~10 kA
Small typ. 30 µm
High 2.1
Shorttyp. 2 m

Small<10-3High

typ. 500 µJ11XFEL requires high-dense, low-emittance electron beam
• Ring type accelerator– Synchrotron radiation causes energy spread and emittance growth.
– Unable to obtain enough quality beam for XFEL.

SR• Linear accelerator– Low emittance (beam size x divergence) electron gun.
– Transverse beam size is focused by the beam acceleration.
– Bunch length compression to several kA.

Acceleration
cavityXFEL

Low emittance Bunch compression
electron gun
chicaneLinear accelerator

Undulator

AOFSRR Cheiron School 2015, XFEL Part-112Outline• Introduction• FEL mechanism• SACLA machine configuration• Present status and outlook• Summary

AOFSRR Cheiron School 2015, XFEL Part-1

13Short wavelength FEL facility in the world （λx < 100 nm）

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Comparison of three XFELs
Facility

LCLS

SACLA

European-XFEL

Place

SLAC, USA

SPring-8, Japan

DESY, Germany

Length2 km

0.7 km

3.5 km

E-gun

RF gunThermionic gun

RF gunAccelerator

Normal cond. (S-band)

Normal cond. (C-band)

Super cond. (L-band)

Undulator

Out-vacuum

In-vacuum

Out-vacuum

Energy14 GeV

8 GeV

17.5 GeVWavelength> 0.15 nm> 0.06 nm> 0.05 nm

Const. cost

75 G-yen

37 G-yen

150 G-yenOperation

2009~

2011~

2017 ?

First XFEL

Compact, low-cost

Long pulse, multi bunch

AOFSRR Cheiron School 2015, XFEL Part-115

Concept of compact XFEL “SACLA”(SPring-8 Angstrom Compact X-ray Free Electron Laser)•••

Short period undulator  Lower beam energy
𝜆𝜆𝑢𝑢 = 18 mm
High gradient accelerator  Short accelerator length
𝐸𝐸𝑎𝑎𝑎𝑎𝑎𝑎 = 35~40 MV/m
Low emittance e-gun (𝜖𝜖𝑁𝑁 < 1π µrad ) +bunch compression Short saturation length
400 m𝜆𝜆𝑢𝑢
𝐾𝐾 2𝜆𝜆𝑥𝑥 = 2 � (1 + )2𝛾𝛾2𝜆𝜆2𝑥𝑥𝜌𝜌 ∝ 𝛾𝛾𝐼𝐼𝑒𝑒 �� 𝑓𝑓(𝐾𝐾)
𝜎𝜎𝑥𝑥 � 𝜎𝜎𝑦𝑦250 m

1/3𝜆𝜆𝐿𝐿𝑔𝑔 = 4𝜋𝜋 𝑢𝑢3𝜌𝜌

XFEL

Low emittancethermionic gun

Bunch compression C-band high gradientchicaneaccelerator

Short periodIn-vacuum undulator
Key technologies
of SACLAAOFSRR Cheiron School 2015, XFEL Part-1

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K. Togawa, PRST-AB 10-020703 (2007)

Low emittance (ε/γ < 1 π mm*mrad) thermionic gun••••Thermionic gun
CeB6 single crystal
Heated to 1500 °C
500 kV high voltage

Stable, long life, maintenance free
Naturally form a flat surface
Intense e-beam (~1 A) from 3 mmφ cathodeLaunch to forward direction against space charge.
CeB6 cathode

Graphite heaters

1 A @1470°C

BeamAnode
Electric field
500 kV/ 50 mm

500 kVAOFSRR Cheiron School 2015, XFEL Part-1

17Thermal electrons
from cathodeElectron accelerator consists of microwave cavities• Microwave cavity works as a LC resonator.
• Supplying microwave power for a certain period (so called “filling time”),
high electrical field is generated between the gap.
• Electron beam is accelerated with this electrical field.
Equivalent circuit

TM010 mode
pillbox cavity
0.77 � 𝜆𝜆Resonant frequency: 𝑓𝑓 = 1/(2𝜋𝜋 𝐿𝐿𝐿𝐿)
22Stored energy:
𝑈𝑈 = 𝐶𝐶𝑉𝑉𝑚𝑚𝑚𝑚𝑚𝑚/2 = 𝐿𝐿𝐼𝐼𝑚𝑚𝑚𝑚𝑚𝑚/2Acceleration voltage:
𝑉𝑉𝑚𝑚𝑚𝑚𝑚𝑚 = 2𝑈𝑈/𝐶𝐶 = 𝐿𝐿/𝐶𝐶 � 𝐼𝐼𝑚𝑚𝑚𝑚𝑚𝑚

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Velocity bunch compression at “buncher” cavities• Electron beam from the gun is accelerated at off-crest phase.
• Due to the velocity difference, electron beam is compressed.
• At the downstream, smaller cavity and higher microwave frequency areused because of the shorter bunch length.
1 ns

1st cavity
238 MHz(T=4 ns)
2nd cavity
476 MHz(T=2 ns)

30 ps
L-band accelerator
1428 MHz(T=0.7 ns)
AOFSRR Cheiron School 2015, XFEL Part-1

19Bunch compression chicane
After the L-band accelerator, the bunch
length is compressed using a chicane (BC1).
In SACLA we use 3 stage compression (BC1,BC2, and BC3) to compress the beam to 50 fs
bunch length.

Microwave atL-band accelerator cavity

Energy chirp
Head: low energy
Tail: high energy

These compression process is quite sensitive
to the initial phase of the L-band microwave.
Accuracy ~ 100 fs (0.05° of 1.4 GHz)

3 ps

30 ps

Bending magnets
Head and tail are overlaid
at the exit

Beam
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SACLA machine configuration
500 keV1 A x 1nsInjector

35 MeV S-band
60 A x 3ps 4 units

Bunch compression

400 MeV
600A x 200fs C-band (42° off-crest) 12 units

S-band accelerator
Crest acceleration

1.4 GeV
3 kAx 50fsC-band(crest)
52 units64 units
C-bandaccelerator

3 GeV

8 GeV
3 kAUndulators
AOFSRR Cheiron School 2015, XFEL Part-1

X-ray Laser
λmin < 0.1 nm21

T. Inagaki, PRST-AB 17-080702 (2014)

C-band (5.7 GHz) high gradient accelerator
World first accelerator to use Cband microwave frequency.
The cavity generate twice highgradient than conventional S-band
cavity, to make the facility compact.

Microwave35 MW

RF pulsecompressor

SACLA

Conventional

Frequency

5.7 GHz(C-band)

2.8 GHz(S-band)

Cavity size

CompactLarge

Power efficiency

BetterWorse

Acceleration gradient35~40 MV/m

15~20 MV/m

Accelerator for 8 GeV

230 m400 ~ 550 m

140 MWe-beam
65 MW

Acceleration gradient
35 MV/mAOFSRR Cheiron School 2015, XFEL Part-1

Each cavity generates
700 kV / 16 mm gap.
Near theoretical limit ofarcing at the pure coppersurface.
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Klystrons (microwave source) and power supplies
Klystron
50 MW microwave source

Supply high power microwave to the accelerator cavity
with high accuracy and stability for stable SASE lasing.

Pulse modulator
350 kV pulse generation
with very high precision.
(∆V/V~0.001%)
Control cabinet
Microwave phase andamplitude control with<100 fs and <0.1%accuracy and stability.
Timing synchronization
optical fiber
Temperature stabilizedwith <0.1 K by a coolingwater.

4 m / unit

AOFSRR Cheiron School 2015, XFEL Part-170 units
in 400 m length23

Beam energy stability at the accelerator end
Green: shot-by-shot data
Red: 1000 shot moving average

E = 7.0 GeV
Q = 0.1 nC
Repetition = 10 pps
Energy feedback: ON

6 hours• Stability: 1.4x10–4 (rms of shot-by-shot data)• Drift: < 1x10–4 (100 s average)

Enough stability for SASE generation.

AOFSRR Cheiron School 2015, XFEL Part-124T. Tanaka, PRST-AB 15-110701 (2012)In-vacuum, variable-gap undulator
Vacuum chamber

Minimum gap~ 3.5 mm

Beam
Permanent(neodymium)magnetIn-vacuum type
Narrow gap
High magnet field

Out-vacuum type
Minimum gap isrestricted by chamber.

Short period 𝜆𝜆𝑢𝑢 = 18 mm (half of SPring-8).
Lower beam energy to generate X-ray.
Short undulator length for SASE saturation.
Permanent magnet arrays
AOFSRR Cheiron School 2015, XFEL Part-125

H. Maesaka, NIM A 696 (2012) p66-74µm level control of electron beam trajectory in the undulator
Electron should sufficiently overlap to SASE
radiation in the undulator.
Required accuracy ~ several µm
Cavity-type beam position monitors (BPMs)
measure beam position with 0.5 µm
accuracy.
Pickup microwave
signalOff-axis beam

TE110 mode
mocrowave
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Outline• Introduction• FEL mechanism• SACLA machine configuration• Present status and outlook• Summary

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History of SACLA
2001
2005
2006 June
2006~20102011 Feb.
Jun.
Oct.

FEL design and development started.
Prototype FEL machine “SCSS” construction.
First lasing at 49 nm.
XFEL facility “SACLA” construction.
Beam commissioning started.
First lasing at 0.12 nm.
SASE power saturationachieved at 0.12 nm.

2012 Mar.
2013 Apr.
Nov.
2015 June

User experiments started.
2-Color SASE released to user experiments.
First experimental symptom of self-seeding observed.
Simultaneous operation of 2 BLs demonstrated.
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Summary of Present XFEL Performance
Pulse Energy*:
Peak Power*:
Pulse duration*:
Intensity Fluctuation*:
Lasing Wavelength:
Bandwidth:
Spatial Coherence:Repetition:
Mean Fault Interval:
Operation hour per year:
Laser availability rate:~0.6 mJ@10keV>60 GW (assuming 10 fs pulse)<10 fs~10% (σ）
0.83 - 2.8 Å (user operation)~0.5%
nearly full
30 Hz （Max.60 Hz）~50 min
6300 hours in total
3600 hours for user operation
93% (2014 user operation)

AOFSRR Cheiron School 2015, XFEL Part-1*It depends on the condition29

SASE gain curve (July 2015, 10 keV)
Beam is kicked out

MeasureSASE intensity

Saturation

Exponential
growth Lg~2mLog scale

Linear scale

Undulator segmentsAlmost fundamental fluctuation of SASE lasing
process (statistical fluctuation of shot noise).
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Laser Intensity vs Wavelength

Narrow gap
High B-field

Wide gap
Low B-field

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SACLA operation status is available in web: http://sacla-status.spring8.or.jp/

Intensity stability (24 hours)

Beam interruption (accelerator fault) due to themicrowave arcing and high voltage arcing.

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T. Hara, Nature comm. 10.1038, 3919 (2013)

2-Color SASE Routinely Available• Wavelength separation of up to 30%• X-ray wavelength region• Precise delay control with an attosecond resolution

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On-going upgrade plans
450 MeV

More user experiments.
Multi-beamline operation
with fast switching magnet.

Monochromatic XFEL
Self-seeding using
diamond crystal.

AOFSRR Cheiron School 2015, XFEL Part-1

EUV ~ Soft X-ray FEL
Dedicated accelerator
has been constructed.
34

Summary• X-ray free electron laser (XFEL)– Only way to generate laser in X-ray wavelength region.
– Extremely high brilliant, ultra-short, and spatially full coherent X-raypulse.
– Following the two XFEL facilities LCLS and SACLA, several XFEL projects
at PAL, PSI, Shanghai, and European-XFEL are under construction.
• SACLA– Compact and low construction cost, but high performance XFEL facility.
– Based on 3 key technologies; a low emittance thermionic gun, highgradient C-band linear accelerators, and short period in-vacuum
undulators.
– Since 2012, SACLA has successfully provided X-ray laser for various
user experiments, without any serious troubles.
AOFSRR Cheiron School 2015, XFEL Part-135

Thank you

AOFSRR Cheiron School 2015, XFEL Part-136