Particle acceleration by circularly polarized lasers

W-M Wang1,2, Z-M Sheng 1,3, S Kawata2, Y-T Li1, L-M Chen1, J Zhang1,3

1Institute of Physics, Chinese Academy of Sciences, Beijing, China2Utsunomiya University, Utsunomiya, Japan

3Shanghai Jiao Tong University, Shanghai, China

Why do we employ CP lasers

■ Ponderomotive force of a circularly polarized (CP) laser pulse has a much larger acceleration phase than a linearly polarized (LP) laser pulse.

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Ion acceleration by CP lasers

■ 2007-2008, several groups proposed to use a CP laser to accelerate ions of a thin solid foil, which called as RPA or PSA. The CP laser pushes electrons directly and the electrons pull ions.

X Zhang et al, Phys. Plasmas 14, 123108 (2007); X Q Yan et al, Phys. Rev. Lett. 100 ,135003 (2008); A Robinson et al, New J. Phys. 10, 013021 (2008); O Klimo et al, Phys. Rev. ST Accel. Beams 11, 031301 (2008)

Acceleration with an ion-mixed foil

■ A foil is composed with a variety of ions, usually.

Three regimes in RPA

■Regime I: I0 is low. Protons and ions are accelerated together. ( I0<Ii)■Regime II: I0 is moderate. Only protons are accelerated. ( Ip<I0<Ii)■Regime III: I0 is large. Neither is accelerated. ( I0>Ip)

W-M Wang et al., Plasma Sci. Technol. 12, 277 (2010)

PIC

simulations

Efficient proton acceleration in Regime II

■ Proton acceleration of the ion-mixed foil (20% protons) is more efficient than the pure proton foil (100% protons).

Electron acceleration in Regime III

■ We can use a CP laser to accelerate foil electrons. Why shall we do in this way? Two reasons.

Ability: the laser technology has been developing to be more intense and resulting shorter (e.g. ELI lasers), which can drive the ponderomotive field/force acceleration (LPFA)

very efficiently. Necessity: further laser pulses (a few fs, much shorter than λ

p ) are not optimized for laser wakefield acceleration (LWFA). Its acceleration efficiency can be exceeded by LPFA. (0.34a0 MeV in LWFA vs 0.26 a0

2 MeV in LPFA)

■To get high energy electron beams, LPFA is an alternative with the ultrashort ultraintense lasers.

20 0/PF a

A feasible scheme of LPFA for monoenergetic beams

■ Thin source foil. 1) Easy electron trapping by ponderomotive field.

2) Monoenergetic and short electron burst. ■ Prediction can be given by the single electron model. GeV electron beams can produce by 13fs, 1022W/cm2 laser with the acceleration distance 1.8 mm. TeV electron beams can produce by 13fs, 1025W/cm2 laser with the acceleration distance 1.8 m.

W-M Wang et al., Phys. Rev. ST

Accel. Beams 13, 071301 (2010)

Demonstration by 1D PIC simulations

■ Energy up to 800MeV, acceleration time is a few ps or about 1 mm (930 MeV, 6.1 ps and 1.8 mm in the model).

■ At 3.15 ps, energy spread 0.24%, and the burst duration 0.4 um.

I0=1022W/cm2;Sine temporal waveform with the duration 13.3 fs;Wavelength 1 um.

Foil density 100 nc ;Thickness 1nm.

Without blocking thick foil

Change the thickness of the source thin foil

■ Energy from 1-10nm foils approaches the prediction. For the 100nm foil, the energy is higher than the prediction.

■ With the increase of the thickness, the spread grows, and the trapped electrons are decreased.

2D PIC simulation results

■ Energy about 600MeV, the energy spread 0.7%, and the burst duration 0.4um.■ Stable acceleration for 2ps due to the relativistic mass increase rapidly.■ Transverse motion. It will be very slight, when the intensity is large.

I0=1022W/cm2;Sine temporal waveform with the duration 6.7 fs;Spot radius 10 um (Gaussian);Wavelength 1 um.

20nm, 5 nc source foil (=1nm, 100 nc foil)

100nm, 200nc reflecting thick foil at 618.5 um,

Conclusions

■ LPFA can produce highenergy ultrashort monoenergetic electron beams efficiently.

■ GeV-TeV electron beams are produced by the 1022-1025 W/cm2 lasers.

■ Electron acceleration distance in the ultraintense laser regime is large enough to allow one to separate the beam from the laser before the beam is decelerated.

■ Electrons are accelerated to gain large relativistic masses quickly, which keeps the acceleration stable for a long time.

■ For the ultrashort ultraintense lasers, LPFA has the larger acceleration field and can produce higher energy electron beams than LWFA.