Prof Peter Norreys FInstP;

Abstract:

A new mechanism for the absorption of energy during the interaction between an ultra-intense laser pulse and a sharp-edged overdense plasma, which we term the zero vector potential (ZVP) mechanism, is presented. The ZVP-mechanism is a nonponderomotive absorption mechanism that should dominate in the interaction of very strong short laser pulses (a0≫1) with overdense plasmas in the case of sharp density gradients. In the ZVP-mechanism the existence of moving zeroes in the vector potential of the relativistic skin layer is crucial to the generation of both fast electron bunches and coherent x-rays. We demonstrate that the laser energy is absorbed from the plasma on the attosecond timescale in the form of electron bunches with unprecedentedly short duration. The numerical simulations are able to validate all qualitative and quantitative aspects of the ZVP-mechanism. © 2011 American Institute of Physics.

Abstract:

Experiments were performed using the Omega EP laser, operating at 740 J of energy in 8 ps (90 TW), which provides extreme conditions relevant to fast ignition studies. A carbon and hydrogen plasma plume was used as the underdense target and the interaction of the laser pulse propagating and channeling through the plasma was imaged using proton radiography. The early time expansion, channel evolution, filamentation, and self-correction of the channel was measured on a single shot via this method. A channel wall modulation was observed and attributed to surface waves. After around 50 ps, the channel had evolved to show bubblelike structures, which may be due to postsoliton remnants. © 2011 American Physical Society.

Abstract:

This paper presents the goals and some of the results of experiments conducted within the Working Package 10 (Fusion Experimental Programme) of the HiPER Project. These experiments concern the study of the physics connected to "Advanced Ignition Schemes", i.e. the Fast Ignition and the Shock Ignition Approaches to Inertial Fusion. Such schemes are aimed at achieving a higher gain, as compared to the classical approach which is used in NIF, as required for future reactors, and making fusion possible with smaller facilities. In particular, a series of experiments related to Fast Ignition were performed at the RAL (UK) and LULI (France) Laboratories and were addressed to study the propagation of fast electrons (created by a short-pulse ultra-high-intensity beam) in compressed matter, created either by cylindrical implosions or by compression of planar targets by (planar) laser-driven shock waves. A more recent experiment was performed at PALS and investigated the laser-plasma coupling in the 1016 W/cm2 intensity regime of interest for Shock Ignition. © 2011 SPIE.