Oxford Centre for High Energy Density Science (OxCHEDS)

Abstract:

Recent progress in developing KrF laser pumped Raman lasers has shown their potential as X-ray laser drivers. The combination of short wavelength (268 nm) with low beam divergence and high beam intensity gives the possibility of very high focused intensity in very narrow line foci. An 8 cm × 8 cm beam combining Raman amplifier in the Sprite system has generated 0.4 TW, 12 ps pulses with 20 μrad beam divergence. A 10 μm wide line focus has been produced and plasmas from a NaF target have been studied at irradiances up to 3 × 1014 W cm-2. Numerical simulations suggest that the Sprite system should be able to produce high recombination gain in F IX at 8.1 nm and that larger scale KrF pumped Raman lasers which are being considered for future UK and European facilities would be ideal drivers for Mg XII at 4.5 nm.

Abstract:

In order to investigate the production of plasma with simultaneous high density and high degree of ionisation and to study laser-plasma energy transport, experiments have been undertaken to measure the plasma electron densities formed in a thin layer of aluminium buried below an overlay of plastic when the plastic overlay is irradiated by focussed lasers of wavelength (and pulse length) .53μm (20ps); .35μm (20ps) and .27 μm (50ps). For the .53μm and .35μm wavelengths, the shorter pulse length (20ps) results in higher time-averaged electron densities (up to 6 × 1022 cm-3) than for the .27μm wavelength experiments with 50 ps pulse length, but the production of hydrogen-like aluminium relative to helium-like is less with the shorter pulse-length.

Abstract:

A series of laser fusion implosion experiments of plastic hollow shell targets was performed by using the Gekko XII glass laser in order to achieve the required fuel areal density for ignition. Introducing random phase plates to improve illumination uniformity, high-density compression of more than 600 times deuterium liquid density has been achieved. The implosion dynamics and symmetry were observed with a spatially resolved x-ray streak camera and an x-ray multiframing camera. The three-dimensional emission profile of the laser-heated plasma was reconstructed from the x-ray images by use of computed tomography and was compared with the laser illumination profiles. The areal density of the imploded core was measured by the neutron activation of a silicon tracer, the secondary reaction method, and the knock-on proton method. Although the measured density and areal density were consistent with those from 1-D hydrodynamic simulation, experimental neutron yields were significantly lower than those predicted by the simulation for convergence ratios larger than 20. This suggests that better implosion uniformity is required to create a hot spark.