Oxford Centre for High Energy Density Science (OxCHEDS)

Abstract:

We have developed a PW (0.5 ps/500 J) laser system to demonstrate fast heating of imploded core plasmas using a hollow cone shell target. Significant enhancement of thermal neutron yield has been realized with PW-laser heating, confirming that the high heating efficiency is maintained as the short-pulse laser power is substantially increased to a value nearly equivalent to the ignition condition. It appears that the efficient heating is realized by the guiding of the PW laser pulse energy within the hollow cone and by self-organized relativistic electron transport. Based on the experimental results, we are developing a 10 kJ-PW laser system to study the fast heating physics of high-density plasmas at an ignition-equivalent temperature.

Abstract:

High Mach number shockwaves were launched in laboratory plasmas to simulate supernova shockwave propagation. The experiments were carried out at inertial fusion facilities using large lasers. Spherical shocks were created by focusing laser pulses onto the tip of a solid pin surrounded by ambient gas. Ablated material from the pin would rapidly expand and launch a shock through the surrounding gas. Planar shocks were created by ablating material from one end of a cylindrical shocktube. Laser pulses were typically 1 ns in duration with ablative energies ranging from <1 J to >4 kJ. Shocks were propagated through various plasmas, and observed at spatial scales of up to 5 cm using optical and x-ray cameras. Interferometry techniques were used to deduce densities, and emission spectroscopy data were obtained to infer electron temperatures. Experimental results confirm that spherical shocks are Taylor-Sedov, and that radiative shocks stall sooner than non-radiative shocks. Unexpected results include the birth of a second shock ahead of the original, stalling shock, at the edge of the radiatively preheated region. We have begun experiments to simulate the interaction between shocks and interstellar material (ISM), and the subsequent turbulent mixing. Comparisons between experimental data and numerical simulations of shock evolution, stall, second shock birth, and interstellar material (ISM) interaction will be presented.

Abstract:

Tens of MeV proton beams have been generated by interactions of the VULCAN petawatt laser with foil targets and used to induce nuclear reactions in zinc and boron samples. The numbers of 11C, 66Ga, 67Ga, 68Ga, 61Cu, 62Zn, 63Zn, and 69mZn nuclei have been measured and used to determine the proton energy spectrum. It is known that (p,n) reactions provide an important method for producing neutron sources and in the present experiment up to ∼109 neutrons sr-1 have been generated via 11B(p,n)11C reactions. Using experimentally determined proton energy spectra, the production of neutrons via (p,n) reactions in various targets has been simulated, to quantify neutron pulse intensities and energy spectra. It has been shown that as high as 4 × 109 neutrons sr-1 per laser pulse can be generated via 7Li(p,n) 7B reactions using the present VULCAN petawatt laser-pulse conditions. © 2004 American Institute of Physics.

Abstract:

We present the observation of a nonlocal heat wave by measuring spatially and temporally resolved electron temperature profiles in a laser produced nitrogen plasma. Absolutely calibrated measurements have been performed by Rayleigh scattering and by resolving the ion-acoustic wave spectra across the plasma volume with Thomson scattering. We find that the experimental electron temperature profiles disagree with flux-limited models, but are consistent with transport models that account for the nonlocal effects in heat conduction by fest electrons.

Abstract:

Petawatt (PW) lasers are unique tools to study plasmas under extreme conditions. There are many applications for these plasmas that potentially have an impact on a wide range of scientific disciplines. A number of these are highlighted here in this review including: fast ignition of fusion targets; high brightness x-ray harmonic generation from oscillating plasma surfaces and the production of super-strong magnetic fields. This is a rich field of investigation, and space prevents a detailed discussion of some of these fascinating topics, including electron and ion acceleration processes that were highlighted at the London conference. Fortunately, they are presented elsewhere in other invited papers in this special issue.