Diapositive 1

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Controlled injection of electrons in a
laser-plasma accelerator Jérôme Faure
Laboratoire LOA, ENSTA CNRS - École
Polytechnique,91761 Palaiseau cedex, France
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Collaborators

• Laboratoire dOptique Appliquée (LOA), France
• C. Rechatin (experiments)
• Lifshitz (simulations)
• Norlin, Y. Glinec, V. Malka (experiments)

CEA Bruyères-le-châtel E. Lefebvre, X. Davoine
(simulations)
European projects CARE and EuroLeap
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Context electron beam generation in underdense
plasmas
laser

• E fields gt 100 GeV/m
• compact accelerators
• ultrashort bunches
• lt 50 fs
• Monoenergetic bunches
• are necessary !!

electrons
Gas jet
Tajima Dawson 1979
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Control and stability external injection using
another laser pulse
pump
injection
Counter-propagating geometry
Ponderomotive force of beatwave Fp 2a0a1/?0
(a0 et a1 can be weak)y Boost electrons
locally and injects them y INJECTION IS LOCAL IN
FIRST BUCKET y LINEAR PHENOMENA no need of
self-focusing, no self-trapping
D. Umstadter et al, PRL 76, 2073 (1996) E.
Esarey et al, PRL 79, 2682 (1997) Fubiani PRE
70, 016402 (2004)
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Fluid scenario pre-acceleration and injection
are simulateous
lp
Plasma wave
Electron motion
E. Esarey et al, PRL 79, 2682 (1997)
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Experimental set-up
to shadowgraphy diagnostic
electron spectrometer
Probe beam
LANEX
Gas jet
B Field
Injection beam
Pump beam
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(No Transcript)
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From self-injection to external injection
ne1.251019 cm-3
ne7.51018 cm-3
Faure et al. Nature 2006
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Optical injection by colliding pulses leads to
stable monoenergetic beams
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Laser stability
Pump pulse I3.41018 /- 0.21018 W/cm2
(RMS5.5 ) Injection pulse I4.41017 /-
0.271017 W/cm2 (RMS6.1 )
To camera
Dx 5 µm (rms over 20 shots)
90 angle prisme
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Monoenergetic bunch appears when lasers are
overlapped
Aligned
Misaligned (dx80 µm)
-40
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Monoenergetic bunch comes from colliding pulses
polarization test
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Controlling the bunch energy by controlling the
acceleration length

• By changing delay between pulses
• Change collision point
• Change effective acceleration length
• Tune bunch energy

Pump beam
Injection beam
2 mm
Gas jet
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Tunable monoenergetic bunches
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Tunable monoenergetic electrons bunches summary
190 MeV gain in 700 µm E270 GV/m
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r-z hybrid PIC model
Lifschitz et al arXivphysics 0703020

• Particle-in-Cell code in cylindrical coordinates
  (r,z)
• Laser evolution calculated using the envelope
  equation

push particles

• Project over the grid
• charge
• current
• gamma (for envelope equation)

Calculate high frequency Laser field (over each
particle, not over grid) ELa0 cos(z-ct)a1
cos(zct)

• Gather fields
• E
• B
• a0
• a1

Solve Maxwell equations

• Solve Laser envelope equation
• a0(r,z)
• a1(r,z)

Considerable gain in CPU time (10-20 hours CPU
for one case)
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Self-focusing evolution of laser amplitude
a0 varies with z ? electron injection varies with
z Lower a0 ? lower wakefield amplitude ? lower
injected charge
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Agreement experiments / simulations on peak
energy and charge
peak energy
peak charge
Higher charge due to self-focusing pulse
nonlinear evolution
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Agreement experiments / simulations on electron
spectra
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Conclusions / perspectives on current experiments

• SUMMARY G
• Optical injection by colliding pulses
  demonstrated
• Enhances dramatically stability
• Energy is tunable 20-300 MeV
• Good beam quality
• Monoenergetic, collimated beam
• dE/E down to 5 , dE 10-20 MeV, charge 10s pC
• r-z hybrid PIC code reproduces experimental
  results
• ? good understanding of the physics
• PERSPECTIVES Q
• Stable source DEVELOP APPLICATIONS
• Femtochemistry, radiobiology on short time scales
• Femtosecond X-ray source
• CONTINUE TO INVESTIGATE THIS TECHNIQUE
• More tuning knobs energy spread, charge
• Push energy limit (longer jets or capillaries)

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GeV level with stable and tunable beams ?

• Collaboration in EuroLeap framework
• LOA, ENSTA / Ecole Polytechnique, France
• LPTP, Ecole Polytechnique, France (J. Larour, L.
  Arantchouk)
• Oxford University, England (S. Hooker et al)
• LPGP, Orsay, France (B. Cros et al)? capillary
  tubes
• LLR, Ecole Polytechnique, France (A. Specka et
  al)? electron spectrometer

discharge
COLLIDING PULSE INJECTION IN CAPILLARY DISCHARGE
pump
injection
Up to 1 GeV dE10 MeV ? dE/E 1 Charge 10-100
pC if self-focusing lt 10 pC if no
self-focusing
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Our first guiding results 15 mm capillary
15 mm
entrance
Exit (with discharge)
Exit (no discharge)