Oxford Centre for High Energy Density Science (OxCHEDS)

Abstract:

The interaction of high intensity laser pulses with underdense plasma is investigated experimentally using a range of laser parameters and energetic electron production mechanisms are compared. It is clear that the physics of these interactions changes significantly depending not only on the interaction intensity but also on the laser pulse length. For high intensity laser interactions in the picosecond pulse duration regime the production of energetic electrons is highly correlated with the production of plasma waves. However as intensities are increased the peak electron acceleration increases beyond that which can be produced from single stage plasma wave acceleration and direct laser acceleration mechanisms must be invoked. If, alternatively, the pulse length is reduced such that it approaches the plasma period of a relativistic electron plasma wave, high power interactions can be shown to enable the generation of quasimonoenergetic beams of relativistic electrons. © 2005 American Institute of Physics.

Abstract:

The motion of atoms on interatomic potential energy surfaces is fundamental to the dynamics of liquids and solids. An accelerator-based source of femtosecond x-ray pulses allowed us to follow directly atomic displacements on an optically modified energy landscape, leading eventually to the transition from crystalline solid to disordered liquid. We show that, to first order in time, the dynamics are inertial, and we place constraints on the shape and curvature of the transition-state potential energy surface. Our measurements point toward analogies between this nonequilibrium phase transition and the short-time dynamics intrinsic to equilibrium liquids.