Experimental wavelength spectra of a 180 fs laser pulse after passage through a supersonic gas jet (intensity vs wavelength in nm). Backing pressures of 5, 10, 20, and 40 bars correspond to electron densities of 2, 4, 8, and (second from top to bottom) A “vacuum” spectrum, taken when the gas jet was turned off, has been included for reference (top). An overall shift of the spectrum toward shorter wavelengths can be seen, which depends on the electron density and has been attributed to ionization blueshift. The spectrum also separates into a sequence of narrow peaks about 3–5 nm apart. Since, for the electron densities used, the first (anti)-Stokes peaks will be located at a distance of 30–50 nm from the fundamental laser peak, the emergence of these narrow peaks cannot be explained by stimulated Raman scattering alone, but is a strong indication of the modulation of the laser pulse by its own wakefield.
Simulation results for the modulational instability of a long laser pulse. Depicted are the amplitude of the longitudinal electric field (left) and the width of the laser’s spectrum (right) vs propagation distance in mm, for background electron densities of (black), 4 (blue), 8 (green), and (red). It is found that both quantities, in particular, the wakefield amplitude, grow exponentially once the modulational takes off properly. The growth time has been found to scale with due to the effect of finite pulse length on the development of the modulational instability. For the highest density, it is found that the modulational instability saturates after about 4 mm of propagation.
Simultaneous subpicosecond time framed interferogram (a) and Schlieren image (b) of a radiative blast wave 24 ns after being launched into a medium of 6 nm Ar clusters by a 700 mJ, 750 fs laser pulse. As material is heated and ionized, the resulting free electron refractive index change causes fringes to bend in the image (a). Image (b) highlights refractive index gradients. Here, a thin shelled blast wave has formed, with a strong radiative precursor leading the compression wave.
A 2D section taken perpendicularly through the tomographically reconstructed 3D electron density profile of two colliding, cylindrical, thin shelled blast waves in a cluster gas. The color scale refers to electron density in . Eighteen individual interferograms taken at 5° intervals at in a energy bin were used in the reconstruction. Electron density spikes are seen at the two collision “cusps” where the thin shells intersect and a Mach stem forms.
Layout of experimental setup showing diagnostic view angles relative to the cone-wire axis and the PW laser.
Microphotograph of a typical target: long Cu wire attached to Al cone. (b) image of diameter wire. (c) image of diameter wire. (d) image of diameter wire. View angle is at 28.4° from wire axis. Aluminum cones (outlined) are not visible in these images due to the limited crystal bandwidth.
Top view of the model geometry for opacity correction above the plane of observation. Right side: front and rear side views in the plane of observation.
Experimental and 1D numerically modeled profiles of vs wire length for the diameter wire case. The fitting is between the reduced model profile and the experimental profile, as explained in the text. The computed model profiles were also compared to those from analytical 1D resistive transport model of Bell et al. (Ref. 121).
Physical picture of the Weibel or current filamentation instability. If a small fluctuation of the magnetic field is present in a neutral plasma flow, with opposing streams of electrons, or with a small anisotropy in the distribution function, in the direction transverse to the field perturbation (a), the two streams of electrons will be under opposite forces in the regions of nonzero magnetic field (b), thus leading to the modulation of the transverse velocity of the electrons and the modulation and separation of the current associated with the two streams of electrons, the formation of current filaments, and the amplification of the magnetic field (c). The corresponding evolution of the field in the collision plane/plane transverse to the streams of electrons is also shown on the right.
3D structure of plasma filaments in the linear phase of the electromagnetic beam-plasma instability (after seven e-foldings) due to the warm relativistic beam (isosurfaces of the plasma density (: red, : green); projections of the density along each direction in surfaces of the simulation box (blue-white scale).
(a) Electron pressure at . (b) Deuteron density profile at the same time (Al ions not shown). (c) Quasistatic magnetic fields at 1 ps (just after the laser pulse).
(a) Charge density and electrostatic field at target center at 1.65 ps. Only half the target is shown for symmetry reasons. (b) Ion longitudinal phase plots at the same time with (a). Blue, green, and red colors indicate Al, C, and D, respectively.
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