Laser plasma accelerators: Laser plasma accelerators: state-of-the-art and perspective state-of-the-art and perspective Brigitte CROS CNRS-Université Paris Sud 11 Laboratoire de Physique des Gaz et des Plasmas Interaction et Transport de Faisceaux Intenses dans les Plasmas
Outline Outline Motivation Accelerating field in a plasma Plasma wave How to create it Properties for acceleration Evolution of laser-plasma acceleration Milestones On-going studies Conclusion
Limitation of linear accelerators Limitation of linear accelerators 6 10 NLC 5 10 Accélérateurs conventionnels Energy (MeV) Energie (MeV) RF accelerators 4 10 E164 RF technology RF technology 3 10 LBNL limitation limitation RAL E<50 MV/m LOA E<50 MV/m B<10 Tesla 2 B<10 Tesla RAL 10 E162 Synchrotron radiation Synchrotron radiation (e-) UCLA (e-) KEK 1 RAL 10 ILE Test of new concepts: Test of new concepts: accelerators using LULI accelerators using LLNL plasmas 0 plasmas 10 1930 1940 1950 1960 1970 1980 1990 2000 2010 Année Year
Interest of plasma for acceleration Interest of plasma for acceleration Accelerating fields > 100 GV/m n e +dn e E Charge space field and plasma n e = Zn i wave v φ E Relativistic wave: x phase velocity of the order of c λ p 1 / 2 ⎡ ⎤ − n ( cm 3 ) = e dn E ( GV / m ) 30 e ⎢ ⎥ 10 17 ⎣ ⎦ n e
How to create a plasma wave How to create a plasma wave Ion Laser pulse L=c τ Electron Ponderomotive force Oscillation of Oscillation of electrons over λ p electrons over λ p
How to accelerate electrons How to accelerate electrons Wavebreaking Energy gain in the wave
Energy gain of a relativistic electron Energy gain of a relativistic electron in a plasma wave in a plasma wave E p Energy gain t 1 t 3 t 2 v~c Δ W = e E p L a ~ 4mc 2 γ φ 2 v φ ~c L a < L deph = λ p γ φ 2 γ φ = λ p / λ 0 Δ W ~ n e -1 n e 10 17 cm -3 10 19 cm -3 γ φ E p ~ n e1/2 100 10 L a 1 m 1 mm L a ~ n e-3/2 Δ W max 20 GeV 200 MeV
How to create a plasma wave How to create a plasma wave Faisceau e+ ou e- Plasma wakefield Linear, resonant λ p ~ c τ λ p ~ c τ Laser Laser wakefield Linear, resonant Laser beatwave Lasers Linear, resonant ω p ~ Δω ω p ~ Δω Non linear wakefield Laser Self-modulated λ p < c τ λ p < c τ bubble Instability leads to wavebreaking
Example of wakefield Example of wakefield
Characteristics of laser wakefield Characteristics of laser wakefield • Ponderomotive force • Ultra-short pulse duration • « Resonant » mechanism •Phase velocity • Depends on laser intensity
Laser wakefield is a simple and Laser wakefield is a simple and efficient mechanism efficient mechanism Linear or non linear plasma waves can be created Plasma wave creation and electron acceleration can be controlled Large « resonance » Longitudinal and transverse fields amplitude can be tuned independantly Accelerating and focusing length of the order of λ p /4 Injection of electrons : external source or from the plasma itself
Pioneering work and first advances Pioneering work and first advances Original proposal for plasma accelerators PRL Tajima et Dawson 1979 Proof of principal as soon as 1993: UCLA et LULI First peaked spectra in 2004: RAL et LOA
The progress of laser plasma accelerators The progress of laser plasma accelerators follows the evolution of laser systems follows the evolution of laser systems Chirped Pulse Amplification
First demonstration of wakefield and First demonstration of wakefield and beatwave at LULI beatwave at LULI détecteurs d'électrons s pectromètre abs orption des électrons non-accélérés diagnostiques aimant d'injection Optique de focalisation entr ée laser détecteurs OTR feuille de séparation sortie laser 1,5 µm Al LULI: laser beams mesure de tache focale LULI: laser beams injection des électrons LSI: electron beam LSI: electron beam Beatwave: Beatwave: (Van de Graaf) Plasma (Van de Graaf) Plasma 4J, 200ps + 12 J, 90ps 4J, 200ps + 12 J, 90ps λ p ~ 300 µm λ p ~ 300 µm wakefield: wakefield: W = 3 MeV, I = 300 A, 0.4ms W = 3 MeV, I = 300 A, 0.4ms 2.5 J, 400fs 2.5 J, 400fs ~1000 e-/ps ~1000 e-/ps Collaboration LULI, LPGP, LLR, SESI
Acceleration in linear wakefield: Acceleration in linear wakefield: Proof of principle Proof of principle •1998, 400fs, 2J Electrons injected at 3 MeV Electrons injected at 3 MeV •n e = 5 10 16 cm -3 Accelerated to 4.5 MeV Accelerated to 4.5 MeV L laser = λ p in a field of 1 GV/m in a field of 1 GV/m 400 Nombre d'électrons Few electrons 300 No trapping 200 γ e − << γ onde ∼100 100 0 3.2 3.4 3.6 3.8 4 4.2 4.4 Noise produced by scattered Energie [MeV] electrons in the plasma or the spectrometre F.Amiranoff et al. , Phys.Rev.Lett. 81 , 9950 (1998)
Self-modulated wakefield (1995) Self-modulated wakefield (1995) • Laser power P = 25 TW (VULCAN), 0.8 ps, 20 J L laser >> λ p A. Modena et al., Nature 377, 606 (1995)
Maxwellian spectrum in 2002 Maxwellian spectrum in 2002 Gas jet, I = 3x10 18 W/cm 2 , LOA salle Jaune 1J, 30fs L laser > λ p •Electron spectra • n e = 2.5 10 19 cm- 3 (squares) • n e = 6 10 19 cm -3 (dots). • Effective electron temperature 18 MeV exponential fit V. Malka et al., Science 298, 1596 (2002)
Breakthrough in 2004: Breakthrough in 2004: Better quality spectra Better quality spectra L laser ~ λ p Obtained by 3 groups RAL/IC/UK: Mangles et al. High intensity LOA/France: Faure et al. LBNL/USA: C.G.R. Geddes et al. L laser > λ p L laser ~ λ p
Typical experimental set-up Typical experimental set-up using gas jet target using gas jet target L laser ~ λ p ASTRA (Rutherford Appleton Lab) E ~ 350 mJ, Pulse duration ~ 45 fsec Focal spot ~ 25 µm Intensity ~ 2 x 10 18 W/cm 2
Non linear wakefield (Nature 2004) Non linear wakefield (Nature 2004) 500 pC +/-200 pC in the peak at 170 MeV Wavebreaking Wavebreaking Trapping of plasma Trapping of plasma electrons electrons Lot of e- Lot of e- Peaked spectra Peaked spectra Short pulse Short pulse Small emittance Small emittance But difficult to control But difficult to control laser pulse: 1 J, 35 fs, 0.8 µm (30 TW) LOA helium gas jet J. Faure et al., Nature 431, 541 (2004)
Wakefield in a plasma channel (2006) Wakefield in a plasma channel (2006) Plasma channel U. Oxford L = 33 mm, diamètre 190µm Self-focusing, r spot (1/e²) = 25 µm wavebreaking or bubble, Laser LBNL 40fs, 1.6J trapping and guiding 12 TW, n e = 3.5 10 18 cm -3 0.5 GeV, 50pC, dE/E = +/- 5% 40 TW, n e = 4.3 10 18 cm -3 1 GeV, 30 pC, dE/E = +/- 2.5% Leemans et al. Nature Physics 2, 696 ( 2006 )
Summary of experimental results Summary of experimental results Mechanism Labs Energy Gain Acc field Acc length UCLA, LULI, Beatwave 1 à 30 MeV 1 GV/m 1 à 10 mm Canada, ILE Linear laser LULI 1.5 MeV 1 GV/m 2 mm wakefield Non Linear RAL, LULI, 100 à 400 60 à 1000 MeV 1 à 30 mm laser wakefield LOA, LBNL GV/m High accelerating gradients High accelerating gradients Agreement with theory Agreement with theory Broad spectra due to inadequate injectors Broad spectra due to inadequate injectors Guiding and controlled injection to improve Guiding and controlled injection to improve the properties of the accelerated beam the properties of the accelerated beam
Towards a controllable laser plasma Towards a controllable laser plasma accelerator at high energy accelerator at high energy Strongly non linear regime: the bubble Laser compression, ultra-high intensity >10 18 W.cm -2 Seld-injection of electrons High electron density Energy of accelerated e- can be increased by increasing laser energy Linear regime Intermediate intensity < 10 18 W.cm -2 External injection of electrons Low electron density Energy of accelerated e- can be increased by guiding and staging
Non linear wakefield with self-injection Non linear wakefield with self-injection • Compression and self- • Compression and self- focusing of the pulse focusing of the pulse • Expulsion of electrons: • Expulsion of electrons: creation of a bubble (ions) creation of a bubble (ions) • Electrons self-injected at the • Electrons self-injected at the back of the bubble by back of the bubble by accelerating and focusing accelerating and focusing fields fields • Injected electrons modify the • Injected electrons modify the back of the bubble (beam back of the bubble (beam loading) loading) Wei Lu talk, HEEAUP05 – UCLA & IST
Scaling in non-linear regime Scaling in non-linear regime (200 PW laser) For a P = C 0 ⇒ P ∝ n c constant Δ E ∝ P value of P n p c The increase of laser power allows to decrease electron density and maintain self- focusing (to compensate diffraction) • IST, UCLA
Evaluation of non-linear regime Evaluation of non-linear regime Single stage, single laser beam….more simple to set-up Progress is linked to the evolution of laser systems: Current power up to 1PW (100 TW) Efficiency and repetition rate tend to decrease when the power is increased
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