1. Particle acceleration by circularly polarized lasers W-M Wang 1,2, Z-M Sheng 1,3, S Kawata 2, Y-T Li 1, L-M Chen 1, J Zhang 1,3 1 Institute of Physics, Chinese Academy of Sciences, Beijing, China 2 Utsunomiya University, Utsunomiya, Japan 3 Shanghai Jiao Tong University, Shanghai, China

2. 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.

3. 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)

4. Acceleration with an ion-mixed foil ■ A foil is composed with a variety of ions, usually.

5. Three regimes in RPA ■ Regime I: I 0 is low. Protons and ions are accelerated together. ( I 0 <I i ) ■ Regime II: I 0 is moderate. Only protons are accelerated. ( I p <I 0 <I i ) ■ Regime III: I 0 is large. Neither is accelerated. ( I 0 >I p ) W-M Wang et al., Plasma Sci. Technol. 12, 277 (2010) PIC simulations

6. 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).

7. 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. (0.34a 0 MeV in LWFA vs 0.26 a 0 2 MeV in LPFA) ■ To get high energy electron beams, LPFA is an alternative with the ultrashort ultraintense lasers.

8. 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, 10 22 W/cm 2 laser with the acceleration distance 1.8 mm. TeV electron beams can produce by 13fs, 10 25 W/cm 2 laser with the acceleration distance 1.8 m. W-M Wang et al., Phys. Rev. ST Accel. Beams 13, 071301 (2010)

9. 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. I 0 =10 22 W/cm 2 ; Sine temporal waveform with the duration 13.3 fs; Wavelength 1 um. Foil density 100 n c ; Thickness 1nm. Without blocking thick foil

10. 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.

11. 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. I 0 =10 22 W/cm 2 ; 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,

12. Conclusions ■ LPFA can produce highenergy ultrashort monoenergetic electron beams efficiently. ■ GeV-TeV electron beams are produced by the 10 22 -10 25 W/cm 2 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.