Shaping FEL radiation: from multipulse/multicolor emission to - - PowerPoint PPT Presentation

Shaping FEL radiation: from multipulse/multicolor emission to generation of twisted light

PRIMOŽ REBERNIK RIBIČ

School on Synchrotron and Free-Electron-Laser Based Methods, ICTP , April 2016

• High peak brilliance and full tunability in the spectral region of

interest

• Possibility of controlling pulse duration
• Full transverse and longitudinal coherence (diffraction imaging,

coherent control)

• Variable polarization (circular dichroism, surface science)
• Ultimate feature: the ability to arbitrarily shape the radiation

pulse in the temporal and spatial (longitudinal and transverse) domains

SOME OF THE PROPERTIES USERS EXPECT FROM A LIGHT SOURCE

• In the IR to UV spectral region, the majority of

previously mentioned requirements are met by conventional table-top lasers.

• In the VUV to X-ray spectral domain, different

approaches must be used in order to achieve laser-like properties of light. Seeded FELs are currently the most promising candidates for reaching this goal.

YOU CAN‘T ALWAYS GET WHAT YOU WANT?

• quick recap of bending magnet and undulator

radiation

• basic principles of FEL operation
• self-amplified spontaneous emission (SASE) vs.

seeded FELs

• advanced FEL concepts: longitudinal

(temporal) and transverse (spatial) shaping of FEL pulses

OUTLINE

Ch05_BendMagRad2_April04.ai

Bending Magnet Radiation (continued)

From Heisenberg’s Uncertainty Principle for rms pulse duration and photon energy thus Thus the single-sided rms photon energy width (uncertainty) is A more detailed description of bending magnet radius finds the critical photon energy In practical units the critical photon energy is (5.4b) (5.4c) (5.7a) (5.7b) ฀

• 2

฀

• m/2e2

Professor David Attwood

• Univ. California, Berkeley

Bending Magnet Critical Photon Energy and Undulator Central Radiation Cone, EE290F, 13 Feb 2007

BENDING MAGNET RADIATION

http://photon-science.desy.de

UNDULATOR RADIATION

Resonant wavelength:

λn = λu 2nγ 2 1+ K 2 2 +γ 2θ 2 ! " # $ % &, (only odd harmonics on-axis, i.e., θ = 0)

u

λ = undulator period

γ

= electron energy

B K

u

λ ∝

= undulator parameter

B

= peak undulator field

n = harmonic number

1

/ ! " # $ % & ω ω d dI

( )

1 1 /ω

ω ω π − N

n n

c λ π ω 2 =

Δω ω      

n

≈ 1 nN N = number of undulator periods

figure by Bastian Holst

UNDULATOR RADIATION „EXPLAINED“

λ = L 2γ 2 1+ K 2 2 +γ 2θ 2 ! " # $ % &

Resonant wavelength: Pulse properties: D detector

TIME STRUCTURE OF SYNCHROTRON RADIATION

Time (µs)

6.7

Time (ps)

110

section

FWHM ≈ 30 ps Streak-camera image

Time structure of synchrotron radiation is a replica of that of the electron bunch, and is invariant over the entire spectrum.

DECREASING THE PULSE DURATION

A femtosecond laser is used to imprint an energy modulation

• nto a long electron bunch (femtoslicing).

Drawback: strong reduction of photon flux (by a factor of 1000).

R.#W.#Schoenlien#et#al.,#Science,#2000!

• f

meth- fem- synchrotron in- elec- reso- wiggler. sepa- modulated dispersive storage

• f

synchro-

R E P O R T S

ALS – Berkeley

SYNCHROTRON RADIATION: TYPICAL PERFORMANCE

Peak brilliance: ≈ 1021 ph/s/0.1%BW/mm2/mrad2 (at 10 keV) Pulse duration: tens of picoseconds Natural spectral resolution: ≈ few percent Coherence: good transverse, poor longitudinal Tunability: Full (between IR and X-rays) Shot-to-shot reproducibility: Very good Polarization: Fully adjustable Repetition rate: hundreds of MHz

INCREASING THE BRILLIANCE

brilliance∝ Ibeam εxεy

Limited!by!wake#field# instabili8es#

10!KeV!!! Present!situa4on! Low Emittance Rings Workshop, Crete, 2011

Limited!by! diffrac4on!

Future!upgrades!

angle solid unit area unit flux photon ) brightness (or brilliance × ∝

INCREASING THE BRILLIANCE, TRY NO. 2 Is this a brute force approach? Yes and no...

WHAT IS A FEL ?

• complete microbunching the

emission is fully correlated

• electrons are

accelerated in a high-energy linear accelerator to a speed close to c (speed of light)

• P. Emma et al., Nat. Photonics (2010) 4, 641
• interaction of electrons with previously

emitted waves leads to microbunching partly correlated emission

• electron bunch enters the undulator

(uncorrelated) emission of radiation by individual electrons

FEL GAIN

Exponential optical gain, I(x) = Io exp x LG ! " # $ % &

electrons lose energy and “fall out” of resonance with the wave

λ ≠ L 2γ 2 1+ K 2 2 " # $ % & ' The electron beam and the emitted electromagnetic wave co-propagate in a long undulator. Electrons couple with spontaneous emission, resulting in exponential amplification (gain) of the intensity until saturation is reached.

A QUESTION OF COHERENCE

Incoherent synchrotron emission

brilliance∝ Ibeam

Coherent FEL emission

brilliance∝ I2

beam

B.W.J. McNeil, N. R. Thompson, Nature Photonics, 2010

WHY MORE BRILLIANCE? AREN’T SYNCHROTRONS POWERFUL ENOUGH?

protein nanocrystallography coherent X-ray diffraction imaging (CXDI) non-periodic objects continuous diffraction pattern oversampling phase retrieval image reconstruction

• H. N. Chapman et al., Nature, 2011
• M. M. Seibert et al., Nature, 2011

measurements on photosystem I CXDI of single mimivirus particles

λ = 6.9 Å

SELF-AMPLIFIED SPONTANEOUS EMISSION (SASE) FEL

∑

=

− − =

N j j j j e

z t t z x x z L eK j

1

)] ( [ )] ( [ 1 ) 2 cos( δ δ γ π ! !

Initial emission that is being amplified originates from electron shot-noise:

5 10 15 20 25 30 0.01 0.1 1 10 100 Erad (µJ) z (m)

z x y

SASE SPECTRAL AND TEMPORAL CHARACTERISTICS (FLASH)

W.!Ackermann!et#al.,#Nature,#2007!

13.5 13.6 13.7 13.8 13.9 14.0 Intensity (a.u.) λ (nm)

10 20 30 40 50 2 4 6 8 10 12 Erad = 40 µJ P (GW) t (fs)

Temporal!profile!(simula4on)!! Spectral!profile!!

SASE PULSE ENERGY STABILITY (FLASH)

End!of!exponen4al!growth! Satura4on! W.!Ackermann#et#al.,#Nature,#2007! Probability!distribu4on!for!the!energy!of!FLASH!radia4on!pulses!

1 2 3 4 5 1 2 3 4 5 0.0 0.2 0.4 0.6 0.8 p (Erad) p (Erad) E

E

σ = 72% M = 1.9 σ = 18% 1 2

OVERCOMING SASE LIMITS 1 – SELF SEEDING

−3000 −2000 −1000 1000 2000 3000 1 2 3 4 5 x 10

4

Relative photon energy (meV) Intensity (arb. units)

Avg seeding

• Sim. seeding

Avg SASE

200 400 600 800 1000 0.2 0.4 0.6 0.8 1

Bandwidth (meV) Fraction of energy

Ephoton!=!930!eV!

• improved!central!

wavelength!stability!

• narrower!bandwidth!

(increased!brightness)!

• limited!ability!to!shape!the!

radia4on!pulse!

−1000 −500 500 1000 2 4 6 8 10 12 x 10

Relative photon energy (meV) Intensity (arb. units)

D.!Ratner!et#al.,#PRL,!2015!

OVERCOMING SASE LIMITS 2 – SEEDING BY AN EXTERNAL COHERENT SIGNAL (HIGH GAIN HARMONIC GENERATION - HGHG)

modulator dispersive section (4 bending magnets) radiator FEL radiation properties are governed by the seed laser => PULSE SHAPING!

λseed λseed / n

radiation at

• E. Hemsing et al.,
• Rev. Mod. Phys,

2014

bn~exp[−n2B2 / 2]Jn[−nAB] max : AB ≈1⇒ bn~exp[−n2 / 2A2]

A = energy modulation normalized to the initial energy spread B = (dimensionless) dispersive strength

seed pulse electron bunch

FERMI SEEDED FEL

FEL1:

100 nm – 20 nm

FEL2:

20 nm – 4 nm

10.7 3 2 1 −1 −2 −3 10.75 10.8 10.85 10.9 Vertical dimension (mm) 0.0 0.2 0.4 0.6 0.8 1.0 Spectrum intensity (a.u.) Wavelength (nm)

a

FEL λ (nm) 10.74 10.76 10.78 10.80 10.82 10.84 10.86 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Spectrum intensity (a.u.) g

b

1

a

0.5 FEL intensity (a.u.) 4 2 −2 −4 −5 5 Vertical size (mm) Horizontal size (mm)

Allaria!et#al.,#Nature#Photonics,!2012!and!2013!

Undulator gallery ~100 m Experimental hall ~50 m Linear accelerator ~130 m

Adriatic sea

FERMI SEEDED FEL

SHAPING FEL LIGHT: TWO COLOR FEL SCHEMES

(FOR X-RAY PUMP-X-RAY PROBE EXPERIMENTS)

TWO COLOR FEL SCHEMES

λ1,2 = λu 1+ K1,2

2

2γ 2 λ1,2 = λu 1+ K 2 2γ 2

1,2

λ = λu 1+ K 2 2γ 2

How can we generate two FEL pulses with different wavelengths?

τ##

probe pump split undulator scheme twin-bunch scheme two colors + delay =

TWO COLOR SASE FEL: SPLIT UNDULATOR SCHEME

e- Magnetic chicane Undulator U tuned at K

2 2

Undulator U tuned at K

1 1 st

x-ray 1 color

nd

x-ray 2 color (a) Scheme I (b) Scheme II Single slotted foil

• Advantage: easy to tune
• Drawback: reduced power due shorter

undulator length available for one color (1/20 to 1/5 of one color SASE power)

• Max delay limited by chicane magnets:

typically from 50 fs to hundreds of fs

• Min delay: below 1 fs
• Time delay jitter: 0.1%
• Energy separation of two colors: 0 to

several 10%

A.!A.!Lutman!et#al.,#PRL,#2013! T.!Hara!et#al.,#Nat.#Commun.,#2013!

(a)

Photon Energy [eV] Electron beam Energy jitter [MeV] −20 −10 10 20 −6 −4 −2 2 4 6

(b)

−20 −10 10 20

2.0x10–2 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 Intensity (a.u.) 13 12 11 10 Photon energy (keV) Second colour First colour

a

TWO COLOR SASE FEL: TWIN-BUNCH SCHEME

A.!Marinelli!et#al.,#Nat.#Commun.,#2015!

Beam Direction Laser Undulator Time Linac Chicane 1 Chicane 2 Photon Energy Energy Time Energy ~5 ps ~50 fs

• Advantage: full undulator available for

both colors -> more power

• Maximum energy separation: 1%,

tuned by compression in Chicane 1

• Maximum delay: 100 fs, tuned by

cathode delay and compression in Chicane 2

• Time delay jitter: 5 fs

–5 5 1 2 3 4 5 6 –60 –60 –40 –40 –20 –20 20 20 40 40 60 60 ∆ Ephoton (eV) ∆ Ephoton (eV) ∆ Eelectron (MeV) Intensity (arb.units)

Average Single Shot

a b