http://loa.ensta.fr/ UMR 7639

Toward a high quality MeV electron source from a wakefield accelerator

for ultrafast electron diffraction

Jérôme FAURE Laboratoire d’Optique Appliquée Ecole Polytechnique Palaiseau, France

FemtoElec

Collaborators

Laser wakefield experiments : C. Rechatin, Y. Glinec, A. Norlin, O. Lundh, J. Lim, V. Malka (LOA team) A. Ben Ismail, A. Specka, H. Videau (LLR, Palaiseau, France)

Simulations / theory A. Lifschitz (LOA, France) E. Lefebvre, X. Davoine (CEA-DAM, France) A. Pukhov (Univ. Dusseldorf, Germany) L. Silva & J. Vieira, R. Fonseca (GolP, Lisbon, Portugal)

Electron source for UED: 5mJ, 5 fs laser pulse: R. Lopez-Martens and his LOA team B. Beaurepaire (LOA) Z. He, A. Thomas, K. Krushelnick, (University of Michigan)

FemtoElec

Outline

•  Motivation for using laser wakefield accelerators for UED

•  Principles of laser wakefield accelerators

•  State-of-the art: 100 MeV – GeV with few fs duration

•  For UED experiments: scaling to few MeV femtosecond bunches with kHz laser systems

•  First experimental results with 100 keV electrons at kHz repetition rate

Motivation: need for ultrafast electron bunches, sub-100 fs, 10 fs ?

Physics •  Structural phase transition and electron-phonon coupling •  Many phonon modes with sub-100 fs period (graphite, graphene) •  Being able to see the onset of a phase transition

Technology •  Photocathode + static field: > 300 fs & < fC charge

–  Space charge limit –  Velocity dispersion

•  Photocathode + RF field: sub-100 fs & pC charge –  RF compression of 100 keV electrons - Van Oudheusden et al., PRL 105, 264801 (2010) –  RF MeV guns - Musumeci et al., Appl. Phys. Lett. 97, 063502 (2010) –  Synchronization and jitter: resolution > 100 fs

Sciaini and Miller, Rep. Prog. Phys. 74, 096101 (2011)

Pros

•  100 keV/µm accelerating gradient –  MeV electron bunches –  Decreases space charge

•  Few femtosecond duration •  Accelerating structure is generated

by the laser pulse –  Perfect synchronization –  No jitter in pump-probe experiment

Why laser wakefield accelerators ?

Cons/ open questions

•  Large energy spread: 1-10% –  Chirped bunches, stretches in time

•  Stability sufficient for UED ? •  Scaling of current system ?

–  From 100 MeV to MeV –  From Hz to kHz –  From J laser to mJ laser

•  Beam quality / coherence sufficient for UED ?

Gas jet

laser

electrons

Experimental principle

Laser 1J, 30 fs, 30 TW

F ~ -dIlaser

F

Champ E

Laser vg ~ c

•  Ponderomotive force pushes electrons:

•  In a plasma: creates a wakefield

The ponderomotive force in a plasma

Plasma wakefields

Extreme accelerating fields: 100 GV/m instead of 10 MV/m

Ez

Er

Electron density Pulse

Use laser-plasma interaction for making particle accelerators

accelerating

focusing

Requirements: I=1018-1019 W/cm2 Laser pulse resonant with wakefield: cτ≈λp/2

λp≈10 µm

Injecting electrons in the wakefield

Fluid electrons trapped electrons

Trapped electron: need minimum pz

For injection: provide pz momentum or dephase electrons

•  Wavebreaking (self-injection) •  Faure et al., Nature 2004, Mangles et al. Nature 2004, Geddes et al., Nature 2004

•  Injection in a density transition •  Bulanov  et  al,  PRE  58  R5257  (1998),  Geddes  et  al,  PRL  100,  215004  (2008),  Faure  et  al.,  PoP  2010  

•  Ionization induced injection •  McGuffey  et  al.,  Phys.  Rev.  LeE.    104,  025004  (2010)  ;  Pak  et  al.,  PRL  104,  025003  (2010)

•  Injection by colliding laser pulses •  E. Esarey et al., PRL 79, 2682 (1997), J. Faure et al., Nature 2006

Injection of quality bunches

z-ct δne

Lbunch > λp=10 µm

Different phases Different electric fields 100 % energy spread

z-ct δne

Lbunch < λp

Electrons « in phase » Monoenergetic acceleration Femtosecond bunch

The first laser beam creates the accelerating structure, the second one is used to heat electrons (provide extra pz)

Statistics (30 shots):

E = 206 +/- 11 MeV

charge = 13+/- 4 pC

dE = 14 +/- 3 MeV

dE/E = 6%

J. Faure et al, Nature 444, 737 (2006) C. Rechatin et al, Phys. Rev. Lett 102, 164801 (2009)

Stable tunable monoenergetic beams

δE/E = 1.3% but less stable

O. Lundh et al., Nat. Phys. 2011

108 electrons in 1.5 fs rms bunch !! 100 MeV

Femtosecond electron bunches

Scaling to MeV energies

€

a0 > (ω0 /ω p )2 /5

€

k pR = 2 a0

€

τc = 2R / 3

Laser pulse needs to be Resonant with 3D wakefield R≈λp/2

Pulse

R

λp

Lu et al., PRSTAB 10, 0613001 (2007)

Electron acceleration with kHz, 5 fs, mJ laser system

Pulse Salle Noire laser: duration 5 fs 1-2 mJ energy Focused down to 1.5 µm I >1018 W/cm2 % rms fluctuations currently upgraded to 5 mJ

PIC simulations: using experimental laser parameters electronic density ne=6-10×1019 cm-3 Explore self-injection / ionization induced injection

Chen et al., Laser Physics 21, 198 (2011)

A. Lifschitz & V. Malka, NJP 14, 053045 (2012)

Results of PIC simulations: energy distribution

Pulse

0 5 10 150

1

2

3

4

5

E (MeV)

char

ge/M

eV (p

C)

! 100

Higher quality beam: ionization induced injection N2 plasma, ne=7×1019 cm-3

sub-fs electron bunch δE/E= 2.5 %

50 fC

Lower quality beam: self-injection He plasma, ne= 1020 cm-3 fs electron bunch larger δE/E few pC

Ionization induced injection with 5 fs pulse

Pulse

I1018 W/cm2 : trapped electrons injection

Ionization threshold of N5+

Local injection

5 fs pulse: few cycles

Each half cycle: •  ionizes N5+ •  injects a sub-fs bunch The absolute phase (CEP) matters

Movie of the process

Pulse

Fine structure of fs electron bunches

Energy angle correlation

spatial filtering

First experimental results Collab. with Univ. Michigan (A. Thomas, Z. He)

Pulse What we need: 5 fs, 2-5 mJ, kHz, ne=5-10×1019 cm-3 What we used: 35 fs, 8 mJ, kHz, ne= 1019 cm-3 Pulse was too long for efficiently exciting a wakefield Injection in density downramp

sample!

CCD!

capillary!gas jet!

solenoid!pinhole!

CCD!

laser!

laser!

spectrometer!

lens!

a)!

b)!

FOS!

capillary!gas jet!

100 µm gas jet

100 µm gas jet

Injection in density downramp

pulse

1 2

3

1 2 3

Effective slow down

laser is focused at the back of 100 µm gas jet in density downramp

100 µm gas jet

Back of the wakefield slows down and can trap slower electrons Electrons are accelerated as they exit the plasma low energy

€

λ p ∝1/ ne

kHz electron beam: spatial profile

280 µm FWHM at focus 3 ×104 +/- 7% RMS (5 fC) electrons/bunch in focus From GPT, normalized emittance 2×10-2 mm.mrad  Transverse coherence is 5 nm More stable than previous experiments on bigger lasers

50 60 70 80 90 100 110 120 130 1400

0.2

0.4

0.6

0.8

1

E (keV)

dN/d

E (a

rb. u

n.)

1 0.5 0 0.5 10

0.2

0.4

0.6

0.8

1

x (mm)

dN/d

x (a

rb. u

n.)5 mm! 500 µm!

a)! b)! c)!

d)!

focused with solenoid after pinhole

-  experiment -  - GPT

kHz electron beam: energy distribution

50 60 70 80 90 100 110 120 130 1400

0.2

0.4

0.6

0.8

1

E (keV)

dN/d

E (a

rb. u

n.)

1 0.5 0 0.5 10

0.2

0.4

0.6

0.8

1

x (mm)

dN/d

x (a

rb. u

n.)5 mm! 500 µm!

a)! b)! c)!

d)!

- spectrum (all electrons) - spectrum at focus -  - GPT at focus

δE/E=7.5%

Z. He et al., submitted to APL

First diffraction images on 10 nm Al foil

5 mm!

111! 200!

220! 311!

a)! b)!

Some short term improvements: -  Filter low energy electrons for removing halo -  Improve quantum efficiency: toward single shot detection -  Reduce energy spread for thinner rings

tilted solenoid + spatial chirp in ebeam

aligned solenoid broader spectrum

•  Laser wakefield accelerators with J lasers and controlled injection –  Relativistic electrons 100 MeV, δE/E≈1-10 % –  Femtosecond bunches –  Stability of parameters 5 % level

•  Scalability to MeV electrons with mJ, 5 fs laser pulse –  PIC simulations: 1-10 MeV, fs electrons with δE/E≈2 %

•  First proof-of-principle experiment with mJ, kHz laser –  100 keV energy level –  Better stability –  Transverse coherence for diffraction patterns

•  Perspectives for 2013: –  upgrade our 5 fs, kHz laser system –  perform experiments with 5 fs pulses: MeV bunches, MeV UED –  Postdoc opportunities for 2013

Conclusions / perspectives

EXTRAS

Why are N5+N6+ electrons trapped ?

2 m

Salle Jaune Laser