Oxford Centre for High Energy Density Science (OxCHEDS)

Abstract:

The first experiments on the National Ignition Facility (NIF) have employed the first four beams to measure propagation and laser backscattering losses in large ignition-size plasmas. Gas-filled targets between 2 mm and 7 mm length have been heated from one side by overlapping the focal spots of the four beams from one quad operated at 351 nm (3ω) with a total intensity of 2×10¹⁵W cm^-2. The targets were filled with 1 atm of CO2 producing of up to 7 mm long homogeneously heated plasmas with densities of ne = 6 × 10²⁰cm^-3 and temperatures of Te = 2 keV. The high energy in a NIF quad of beams of 16kJ, illuminating the target from one direction, creates unique conditions for the study of laser plasma interactions at scale lengths not previously accessible. The propagation through the large-scale plasma was measured with a gated x-ray imager that was filtered for 3.5 keVx rays. These data indicate that the beams interact with the full length of this ignition-scale plasma during the last ∼1 ns of the experiment. During that time, the full aperture measurements of the stimulated Brillouin scattering and stimulated Raman scattering show scattering into the four focusing lenses of 3% for the smallest length (∼2 mm), increasing to 10-12% for ∼7 mm. These results demonstrate the NIF experimental capabilities and further provide a benchmark for three-dimensional modeling of the laser-plasma interactions at ignition-size scale lengths.

Abstract:

Vulcan is established as a world leading user facility for studies of ultra-high intensity laser interactions with matter. The Petawatt(PW) Upgrade project will deliver an order of magnitude increase in laser power to target, delivering interaction intensities of 10 21 Wcm -2. Before commencing operation to users a commissioning study was carried out. The objectives of this investigation were: To perform radiological surveys of the facility in anticipated target configurations and to install additional shielding for high-Z interactions as required To measure the target irradiance and the X-ray spot size To investigate the effects of electromagnetic noise on the chamber and target area equipment To determine the level of debris produced from the target and to devise a suitable operating scheme to protect chamber optics from damage Installation of an optical probe Contrast measurement of the laser pulses This report will review the experimental and radiological data obtained from the thirteen solid target interaction shots fired during the commissioning run in the newly constructed PW interaction facility 1) where doses of 45 mSv @ 1m were obtained. Commissioning of the PW laser and compressor chain is reported in separate articles, as is the commissioning of the large aperture gratings at high laser fluences 2).

Abstract:

High power lasers are nowadays a tool that can be used to determine important parameters in the context of Warm Dense Matter. In this paper, the general issues that have been addressed in the last few years are presented. Recent results concerning water experiments are exposed as a paradigm of laser driven shocks experiments. Finally the first application of proton radiography to the study of shocked material is presented.

Abstract:

The transport of intense relativistic electron beams in solid density plasma was analyzed. Ponderomotive kinetics modeling code (MPK) for the average relativistic laser-plasma interaction for laser absorption in under-dense or below critical density plasma was proposed. The focused peak intensity of the Petawatt laser system was measured as high as 3.1020W / cm 2. It was found that there was significant lateral electron transport, but of low energy electrons, into the solid.

Abstract:

A study of the stimulated Brillouin scattering (SBS) in helium-hydrogen plasmas has been performed using a gas jet at the Janus Laser Facility. We observe three regions of reflectivity by varying the probe intensity from 10¹⁴ to 10¹⁶: a saturated region, a linear region, and a region near the threshold for SBS. In the linear regime we find that adding small amounts of hydrogen to a helium plasma reduces the SBS reflectivity by a factor of 4.