• F.S. Tsung, C. Joshi, W.B. Mori
  UCLA, Los Angeles, California
• S.F. Martins
  Instituto Superior Tecnico, Lisbon

Funding: Fundacao Calouste Gulbenkian and Fundacao para a Ciencia e Tecnologia under grants SFRH/BD/35749/2007.

The possibility of using a CO2 laser (10 micron wavelength) to drive a plasma density compression and achieve effective ion acceleration in gaseous targets (density>~ 10¹9cm^-3) is explored. A parameter scan is performed with a set of particle in cell simulations in OSIRIS*, both in 2D and 3D, for various laser intensities, linear/circular polarization pulses, and plasma densities. Results show that, to generate the shock compression, plasma density must be increased above the critical value to account for the relativistic motion of the electrons. Under these conditions, 2-5MeV ions are observed with moderate intensity (a0=3) laser pulses. Finally, configurations to generate a shock structure are suggested, that will more efficiently accelerate the particles. This scenario is also of particular relevance to fast-ignition, inertial confinement fusion, and implications to those regimes can be obtained from numerical simulations by using the appropriate density normalization.

*R. A. Fonseca et al, LNCS 2329, III-342, Springer-Verlag, (2002)

• J.E. Ralph, F. Fang, C. Joshi, W. Lu, K.A. Marsh, W.B. Mori, A.E. Pak, F.S. Tsung
  UCLA, Los Angeles, California

Funding: This work was supported by The Department of Energy Grant No.DEFG02-92ER40727.

The self-guiding of relativistically intense but ultra-short laser pulses has been experimentally investigated as a function of laser power, plasma density and plasma length in the so-called "blowout" regime. Although etching of the short laser pulse due to diffraction and local pump depletion erodes the the head of the laser pulse, an intense portion of the pulse is guided over tens of Rayleigh lengths, as observed by imaging the exit of the plasma. Spectrally-resolved images of the laser pulse at the exit of the plasma show evidence for photon acceleration as well as deceleration (pump depletion)in a well defined narrow guided region. This is indicative of the self-guided pulse residing in the wake excited in the plasma. Energy outside the guided region was found to be minimized when the initial conditions at the plasma entrance were closest to the theoretical matching conditions for guiding in the blowout regime. The maximum extent of the guided length is shown to be consistent with the nonlinear pump depletion length predicted by theory.