(Color) Left: A typical example of a laser-driven wake field, obtained from a 3D OSIRIS simulation (see Ref. 38). The laser pulse (red/green) travels from right to left, exciting a wake field by pushing away the plasma (blue). The laser pulse has an intensity of and a wavelength of . The plasma density is . The figure shows the laser pulse and trailing wake field after a propagation distance of . The projections on the bottom and side of the simulation box show the plasma density for this wake field; a small bunch of self-trapped plasma electrons can be seen near the back of each wave bucket. Such wake fields can be used to accelerate conventional particles as well as photons. Right: The wake field of a 2D OSIRIS simulation performed using the exact same parameters for laser pulse and plasma. The 2D wake field is very similar to the 2D projections of the 3D wake field pictured on the left. This proves that a 2D model is sufficiently close to a 3D one in this regime, and justifies the use of 2D OSIRIS for most of the simulations discussed in this paper.
(Color) (a) Optical spectra taken at different plasma densities. The blue shift is seen to increase as the density decreases. This is most noticeable when the first blue peak visible in the spectrum is considered. It has respective relative intensities of 0.20, 0.17, 0.11, and 0.06 when listed from the lowest to the highest density. The indicated peak at is the first Stokes peak produced by the Raman forward scattering instability. (b) Frequency spectra taken from one-dimensional photon-kinetic simulations modeled after the experiments. The behavior of the blueshifted plateau is qualitatively well reproduced by the simulations. The larger width and intensity of the simulated spectra compared to the observed ones are most likely attributed to the fact that the simulations are not three-dimensional.
(Color) Phase space plot when the laser pulse has propagated (a) 200, (b) 500, (c) 750, and (d) in a plasma of density . The green dots denote the photon macroparticles at the beginning of the simulation. The red dots represent the photons after various propagation distances. The black line represents the generated wake-field density perturbation.
(Color) Snapshots of the Wigner transform of a laser pulse propagating through a plasma of density . The images correspond to propagation lengths of (from left to right): 0, 340, 450, and . These snapshots were taken from a two-dimensional PIC simulation on OSIRIS. Note the similarities to the photon density plots produced by the photon-kinetic code.
Experimental (left) and photon-kinetic simulation (right) results taken at a lower plasma density , but a higher laser intensity (, or ). The split fundamental peak is clearly visible in both cases, in addition to the “shoulder” on the blue side of the spectrum. All the features of the experimental spectrum are accurately reproduced in and explained by the simulations.
Transmission spectra of intense laser pulses after interaction with a plasma, taken from two-dimensional OSIRIS simulations. The simulation parameters are: , , . The solid curve was obtained using a pre-ionized plasma, while for the dashed curve the plasma was created by the pulse itself. The fact that these curves are identical proves that laser-driven ionization effects do not influence the transmitted spectrum for the high laser intensities used in the experiments.
Article metrics loading...
Full text loading...