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0 Relativistic induced transparency and laser propagation in an overdense plasma Su-Ming Weng Theoretical Quantum Electronics (TQE), Technische Universität Darmstadt, Germany Joint work with Prof. P. Mulser, Prof. H. Ruhl EMMI Workshop “Particle dynamics under extreme matter conditions” Speyer, Germany, 26-29 September 2010

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1 Preface “Open Sesame”, “Ali baba and the forty thieves” One Thousand and One Nights Who open the door for ultrahigh intense laser into an overdense plasma?

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2 Outline Theoretical background Classical eletromagnetic (EM) wave propagation Relativistic induced transparency Numerical simulations Relativistic critical density increase Relativistic laser beam propagation Applications Fast ignition Relativistic plasma shutter Shortening of laser pulses Conclusion

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3 Classical EM wave propagation Maxwell’s Equations For a high-frequency monochromatic laser beam in a plasma Maxwell’s Equations Eq. (1)-(2) current density plasma frequency

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4 Classical EM wave propagation Dispersion relation Group velocity (or propagation velocity) Critical density

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5 Relativistic induced transparency Single particle’s 8-like motion for a ≥ 1 x y a<<1 x-x d y a ≥1a ≥1 T. C. Pesch and H. – J. Kull, Phys. Plasmas 14, 083103 (2007). Dimensionless laser amplitude a:

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6 Relativistic induced transparency If |v| ~ c, Group velocity (relativistic) Relativistic critical density the Lorentz factor averaged from the single particle‘s 8-like motion P. Mulser and D. Bauer, “ High Power Laser-Matter Interaction ”, Springer, 2010.

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7 Particle-in-Cell (PIC) simulation Motion equations (particles) PIC simulation cycle Maxwell’s equations (fields) R. Lichters, et al., LPIC++, (1997).

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8 Determination of critical density Laser and plasma parameters Cycle-averaged propagation appears very regular, and laser is mainly reflected at the critical surface as that in nonrelativistic regime

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9 Critical density VS laser intensity A very smoothed critical density can be achieved after being averaged over 10 cycles. In a normally incident and linearly polarized laser pulse, relativistic critical density increase is in a good agreement with the averaged value from the single particle’s 8-like motion

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10 Effect of plasma density profile For a very steep and highly overdense plasma. For normal incident, if density scale length L>λ L,

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11 Effect of laser polarization For circular polarization, a density ridge prevents the laser from further propagation and restricts the critical density increase

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12 Effect of laser polarization For normal incident, the relativistic critical density increase can be well fitted by

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13 Critical density VS incident angle The highest critical density coincides with the lowest reflectivity in the case of 30 o incidence. This phenomenon is due to the formation of semi- black density cavity and resonance.

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14 Relativistic laser beam propagation (LP) Sakagami and Mima attributed the inhibition of the propagation velocity to the oscillation of the ponderomotive force and hence the oscillation of electron density at the laser front. H. Sakagami, K. Mima, Phys. Rev. E 54, 1870 (1996).

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15 Relativistic laser beam propagation (CP) Inhibition of propagation velocity is not attributed to the oscillation of ponderomotive force. Ponderomotive force for circular polarized laser CP pulse propagates even more slowly than LP pulse.

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16 Relativistic laser beam propagation Laser pulse can not propagate freely into an overdense plasma, only a part of field penetrates into the plasma by skin effect. Relativistic induced transparency is guided by the skin penetration. Propagation velocity depends on both the group velocity and the efficiency of skin penetration. If this field is big enough, it can accelerate the electrons into relativistic motion and hence induce the relativistic critical density increase for further propagation Open Sesame! Open, skin penetration!

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17 Relativistic propagation velocity the different heights of density ridge formed before the front of laser

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18 Application (a): Fast ignition The configuration of fast ignition J. J. Honrubia et al, Nucl. Fusion 46, L25 (2006) n e about 10 5 n c Relativistic induced transparency, together with cone- guiding, channeling and hole boring may make the deeper penetration into an overdense target possible, and make the fast ignition easier.

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19 Application (b): Relativistic plasma shutter A relativistic plasma shutter can remove the pre-pulse and produce a clean ultrahigh intensity pulse This shutter is overdense but relativistic underdense. S. A. Reed et al., Appl. Phys. Lett. 94, 201117 (2009).

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20 Application (c): Shortening of laser pulses A quasi-single-cycle relativistic pulse can be produced by ultrahigh laser-foil interaction L. L. Ji et al., Phys. Rev. Lett. 103, 215005 (2009). Initial thin overdense foil evolves into a thick but rare plasma, which is relativistically transparent.

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21 Conclusion Relativistic induced transpancy makes the penetration of a laser pulse into a overdense plasma possible. For normal incident linearly polarized pulse, the critical density increase follows approximately the theory from a single particle orbit. Due to density profile steepening before the laser front, the critical density increase is much lower than predicted for a circular polarized pulse. Relativistic induced transparency is guided by the skin penetration. The propagation velocity depends upon both, the group velocity and the efficiency of skin penetration. Relativistic induced transparency finds wide applications in fast ignition scheme, relativistic plasma shutter, and shortening of laser pulses. We proposed a method for determining the critical surface and the relativistic critical density increase in PIC simulations.

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22 Vielen Dank! 谢 谢!谢 谢! Thank You!

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