Time evolution of laser-plasma interaction. (I) Near critical interaction (ponderomotive scaling), (II) steepened interface interaction with shelf plasma (shoulder electrons), and (III) direct interaction with dense target (dc-ponderomotive electrons).
(a) Electron density evolution at . Each observation time corresponds to the each three stages described in Fig. 1. Arrows in the figure indicate the pulse front at that particular time. (b) Time evolution of absorption.
Electron phase , time averaged longitudinal electric field , intensity , and electron density profiles in (a) stage (II) at , and (b) stage (III) at . The shaded areas indicate the target region.
(a) Electron energy spectrum observed after 10 ps laser irradiation with and . (b) The electron temperature evolution inside the gold target. (c) The time evolution of charge states.
Electron phase evolution, , near interface of a slab target of , for . (a) , dc-ponderomotive electrons emerging, arrows show electron flow (b) , first jet of extraction (c) , and dc-ponderomotive electrons at later time (d) , second jet of . Electric fields, magnetic field, and electron density profile are shown by colored lines. Arrows point in the direction of rotating blob in phase space. Broken line shows the initial position of vacuum plasma interface. Dashed line shows the position of peak electron density at respective times.
Electron phase evolution, , near the interface of a slab target of , for . The snapshots taken at (a) , (b) , (c) , and (d) show dc-ponderomotive electrons of low energy. Electric fields, magnetic field, and electron density profile are shown by colored lines. Arrows point in the direction of rotating blob in phase space. Broken line shows the initial position of vacuum plasma interface. Dashed line shows the position of peak electron density at respective times.
Electron energy spectrum obtained in various density targets. Arrows indicate the energy of oscillating electrons in the trapped region and energies in brackets are the estimated peak energy of dc-ponderomotive electrons by Eq. (4).
dc-ponderomotive electron model: energy flux and particle flux conservation in the blob (oscillation region).
Longitudinal electron peak momentum in vs dc-ponderomotive acceleration. Analytical scaling for longitudinal calculated in Eq. (4) (dc-ponderomotive) and Eq. (10) () with the observed maximum electron momentum in simulation for various densities.
Total absorption measured in various density targets for linearly polarized laser with (solid circles), circularly polarized laser with (solid triangles), and linearly polarized laser with (solid squares).
Ratio, , of dc-ponderomotive electron energy to the total electron energy for various density increases with target density.
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