Toward a high quality MeV electron source from a wakefield accelerator for ultrafast electron diffraction

Transcription

1 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 UMR 7639 FemtoElec

2 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

3 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

4 Physics Motivation: need for ultrafast electron bunches, sub-100 fs, 10 fs? 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, (2010) RF MeV guns - Musumeci et al., Appl. Phys. Lett. 97, (2010) Synchronization and jitter: resolution > 100 fs Sciaini and Miller, Rep. Prog. Phys. 74, (2011)

5 Why laser wakefield accelerators? Pros Cons/ open questions 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 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?

6 Experimental principle laser Laser 1J, 30 fs, 30 TW <1 Hz W/cm 2 plasma electrons He plasma n e <10 19 cm -3 Gas jet

7 The ponderomotive force in a plasma Ponderomotive force pushes electrons: F F ~ -di laser In a plasma: creates a wakefield Champ E Laser v g ~ c

8 Plasma wakefields λ p 10 µm Pulse Electron density accelerating E z Requirements: I= W/cm 2 Laser pulse resonant with wakefield: cτ λ p /2 focusing E r Extreme accelerating fields: 100 GV/m instead of 10 MV/m Use laser-plasma interaction for making particle accelerators

9 Injecting electrons in the wakefield Fluid electrons trapped electrons Trapped electron: need minimum p z For injection: provide p z momentum or dephase electrons

10 Injection of quality bunches L bunch > λ p =10 µm L bunch < λ p z-ct z-ct δn e Different phases Different electric fields 100 % energy spread δn e Electrons «in phase» Monoenergetic acceleration Femtosecond bunch 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, (2008), Faure et al., PoP 2010 Ionization induced injection McGuffey et al., Phys. Rev. LeE. 104, (2010) ; Pak et al., PRL 104, (2010) Injection by colliding laser pulses E. Esarey et al., PRL 79, 2682 (1997), J. Faure et al., Nature 2006

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

12 Stable tunable monoenergetic beams 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, (2009) δe/e = 1.3% but less stable

13 Femtosecond electron bunches 10 8 electrons in 1.5 fs rms bunch!! 100 MeV O. Lundh et al., Nat. Phys. 2011

14 Scaling to MeV energies a 0 > (ω 0 /ω p ) 2/5 k p R = 2 a 0 τc = 2R/3 Pulse λ p R Laser pulse needs to be Resonant with 3D wakefield R λ p /2 Lu et al., PRSTAB 10, (2007)

15 Electron acceleration with khz, 5 fs, mj laser system Salle Noire laser: duration 5 fs 1-2 mj energy Focused down to 1.5 µm I >10 18 W/cm 2 % rms fluctuations currently upgraded to 5 mj Pulse Chen et al., Laser Physics 21, 198 (2011) PIC simulations: using experimental laser parameters electronic density n e = cm -3 Explore self-injection / ionization induced injection A. Lifschitz & V. Malka, NJP 14, (2012)

16 Results of PIC simulations: energy distribution Lower quality beam: self-injection He plasma, n e = cm -3 fs electron bunch larger δe/e 5 few pc charge/mev (pc) Pulse Higher quality beam: ionization induced injection N 2 plasma, n e = cm -3 sub-fs electron bunch δe/e= 2.5 % 50 fc! E (MeV)

17 Ionization induced injection with 5 fs pulse Pulse I<10 16 W/cm 2 : fluid electrons I>10 18 W/cm 2 : trapped electrons injection Ionization threshold of N 5+ 5 fs pulse: few cycles Local injection Each half cycle: ionizes N 5+ injects a sub-fs bunch The absolute phase (CEP) matters

18 Movie of the process Pulse

19 Fine structure of fs electron bunches Energy angle correlation spatial filtering

20 First experimental results Collab. with Univ. Michigan (A. Thomas, Z. He) What we need: 5 fs, 2-5 mj, khz, n e = cm -3 What we used: 35 fs, Pulse 8 mj, khz, n e = cm -3 Pulse was too long for efficiently exciting a wakefield Injection in density downramp a)! laser! pinhole! solenoid! spectrometer! FOS! CCD! b)! capillary! 100 µm gas jet! jet sample! lens! laser! CCD! 100 capillary! µm gas jet!

21 Injection in density downramp laser is focused at the back of 100 µm gas jet in density downramp 100 µm gas jet pulse λ p 1/ n e Effective slow down Back of the wakefield slows down and can trap slower electrons Electrons are accelerated as they exit the plasma low energy

22 khz electron beam: spatial profile after pinhole focused with solenoid a)! b)! c)! 5 mm! 500 µm! dn/dx (arb. un.) experiment - - GPT d)! x (mm) 280 µm FWHM at focus /- 7% RMS (5 fc) electrons/bunch in focus From GPT, normalized emittance mm.mrad Transverse coherence is 5 nm More stable than previous experiments on bigger lasers

23 khz electron beam: energy distribution dn/de (arb. un.) E (kev) Z. He et al., submitted to APL - spectrum (all electrons) - spectrum at focus - - GPT at focus δe/e=7.5%

24 First diffraction images on 10 nm Al foil tilted solenoid + spatial chirp in ebeam aligned solenoid broader spectrum 111! 200! 5 mm! 220! 311! Some short term improvements: - Filter low energy electrons for removing halo - Improve quantum efficiency: toward single shot detection - Reduce energy spread for thinner rings

25 Conclusions / perspectives 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

26 EXTRAS

27 Why are N 5+ N 6+ electrons trapped?

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29 Salle Jaune Laser 2 m

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