Oxford Centre for High Energy Density Science (OxCHEDS)

Abstract:

The expansion of shock waves has been studied in mediums with different opacities and heat capacities, varied in systematic ways by mixing xenon with nitrogen keeping the mass density constant. An initial shock is generated through the brief (5 ns) deposition of laser energy (5 J) on the tip of a pin surrounded by the xenon-nitrogen mixture. The initial shock is spherical, radiative, with a high Mach number, and it sends a supersonic radiatively driven heat wave far ahead of itself. The heat wave rapidly slows to a transonic regime and when its Mach number drops to ∼2 with respect to the downstream plasma, the heat wave becomes of the ablative type, driving a second shock ahead of itself to satisfy mass and momentum conservation in the heat wave reference frame. The details of this sequence of events depend, among other things, on the opacity and heat capacity of the surrounding medium. Second shock formation is observed over the entire range from 100% Xe mass fraction to 100% N2. The formation radius of the second shock as a function of Xe mass fraction is consistent with an analytical estimate. © 2006 American Institute of Physics.

Abstract:

Experiments at the LOASIS laboratory of LBNL recently demonstrated production of 100 MeV electron beams with low energy spread and low divergence from laser wakefield acceleration. The radiation pressure of a 10 TW laser pulse guided over 10 diffraction ranges by a plasma density channel was used to drive an intense plasma wave (wakefield), producing acceleration gradients on the order of 100 GV/m in a mm-scale channel. Beam energy has now been increased from 100 to 1000 MeV by using a cm-scale guiding channel at lower density, driven by a 40 TW laser, demonstrating the anticipated scaling to higher beam energies. Particle simulations indicate that the low energy spread beams were produced from self trapped electrons through the interplay of trapping, loading, and dephasing. Other experiments and simulations are also underway to control injection of particles into the wake, and hence improve beam quality and stability further.

Abstract:

Reduction of proton acceleration in the interaction of a high-intensity, piosecond laser with a 50-μm aluminum target was observed when 0.1-6 μm of plastic was deposited on the back surface (opposite side of the laser). The maximum energy and number of energetic protons observed at the back of the target were greatly reduced in comparison to pure aluminum and plastic targets of the same thickness. This is attributed to the effect of the interface between the layers. Modeling of the electron propagation in the targets using a hybrid code showed strong magnetic-field generation at the interface and rapid surface heating of the aluminum layer, which may account for the results. © 2006 American Institute of Physics.