Prof Peter Norreys FInstP;

Abstract:

Collisionless electron beam-plasma instabilities are expected to play an important role in fast ignition. Such beams are produced by the short high power ignition laser interacting with long scale length plasmas. Here we present results from a one-dimensional Vlasov-Poisson code used to investigate different electron beam temperatures and background plasma conditions. The simulations demonstrate that the beam-plasma instabilities drive large amplitude electrostatic waves that undergo the parametric decay instability driving backwards propagating electrostatic waves and much lower frequency ion acoustic waves. Saturation of the beam-plasma instability creates a plateau in the electron distribution function consistent with quasi-linear theory. We observe the creation of high energy tails in the electron and ion distribution functions, formed by the trapping of particles in the waves formed during the collapse of the beam. The high energy tails of the ion distribution are found to account for up to one-half of the energy gained by the ion population from the beam collapse. Furthermore, at the highest electron beam temperatures we observe the formation of long-lived coherent phase-space structures. These phase-space structures are a direct consequence of the cascade nature of the parametric instability driving up lower wavenumber modes that have higher phase velocities that can in turn accelerate electrons to energies in excess of the initial beam energy. A quasi-linear treatment also shows similar effects but the simulations are clearly beyond a simple quasi-linear treatment and demonstrate the transfer of energy from an incident beam to the ion population via collisionless effects. The implications of these mechanisms for the fast ignition scheme will be discussed. © 2008 IOP Publishing Ltd.

Abstract:

The currents induced by arbitrarily strong dc electric fields in plasma and the evolution of electron distributions have been studied by Fokker-Planck simulations. We find that the electron distributions evolve distinctly under different fields; especially, the electron distribution is well represented by the sum of a stationary and drifting Maxwellian at the moderate field. A set of hydrodynamiclike equations, similar to Spitzer's but without the weak-field limit, is given for calculating the current. It is more suitable for application in hybrid particle-in-cell simulations and may extend plasma transport theory in models that do not employ a kinetic description of the electrons. © 2008 The American Physical Society.

Abstract:

The heating of plane solid targets by the Vulcan petawatt laser at powers of 0.32-0.73 PW and intensities of up to 4 × 1020W cm -2 has been diagnosed with a temporal resolution of 17 ps and a spatial resolution of 30 μm, by measuring optical emission from the opposite side of the target to the laser with a streak camera. Second harmonic emission was filtered out and the target viewed at an angle to eliminate optical transition radiation. Spatial resolution was obtained by imaging the emission onto a bundle of fibre optics, arranged into a one-dimensional array at the camera entrance. The results show that a region 160 μm in diameter can be heated to a temperature of ∼107 K (kT/e ∼ keV) in solid targets from 10 to 20 μm thick and that this temperature is maintained for at least 20 ps, confirming the utility of PW lasers in the study of high energy density physics. Hybrid code modelling shows that magnetic field generation prevents increased target heating by electron refluxing above a certain target thickness and that the absorption of laser energy into electrons entering the solid target was between 15-30%, and tends to increase with laser energy. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.

Abstract:

The heating of solid targets irradiated by 5×1020Wcm-2, 0.8 ps, 1.05μm wavelength laser light is studied by x-ray spectroscopy of the K-shell emission from thin layers of Ni, Mo, and V. A surface layer is heated to ∼5keV with an axial temperature gradient of 0.6μm scale length. Images of Ni Lyα show the hot region has ≤25μm diameter. These data are consistent with collisional particle-in-cell simulations using preformed plasma density profiles from hydrodynamic modeling which show that the >100Gbar light pressure compresses the preformed plasma and drives a shock into the solid, heating a thin layer. © 2008 The American Physical Society.