Prof Peter Norreys FInstP;

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.

Abstract:

University research in the UK with high power lasers is carried out at the SERC’s Central Laser Facility with a multiterawatt neodymium glass laser, VULCAN, and a developmental KrF laser, SPRITE. These systems are briefly described together with the design of a new KrF laser to supersede VULCAN. The new laser design, SUPERSPRITE, is based on optical and raman multiplexing which is being developed with the present SPRITE system. The Specification of SUPERSPRITE is For 3.5 kj in 1 ns and a peak power of 300 TW in short pulses. The new technology is seen as highly cost effective in relation to neodymium glass lasers. A Resumé of the development of XUV lasers in the UK in collaboration with laboratories overseas is given. The work is based on laser action through recombination in highly ionized ions and recent progress includes collaborative experiments on the GEKKO XII facility in japan which have demonstrated laser action at the shortest wavelength to date at 45 Å in Mg XII. The physics of energy transport in short pulses is fundamental to the extrapolation of recombination lasers to shorter wavelengths and is being studied from a more basic standpoint using both the VULCAN and SPRITE facilities. Some details of this work are given. © 1990, Cambridge University Press. All rights reserved.

Abstract:

The mass ablation rate has been measured using the 12-beam GEKKO XII glass laser. An X-ray pinhole camera coupled to a streak camera was used to provide both spatial and temporal history of the emission from buried maker layers. The camera was filtered by 0.75 μm of Al to enhance the image of the cool, denser plasma near the ablation front. The targets (diameter approximately 500 μm) were both polymer shells and solid polymer pellets and were overcoated with up to four maker layers sandwiched between CH ablator layers. Laser energies of up to 8 kJ in 1.7 ns were incident on the targets, with absorbed irradiances of up to 4.0 × 1014 W-cm-2. Good agreement has been found between the experiment and a one-dimensional hydrodynamic code in which the heat flow is obtained from the Fokker-Planck equation. The mass data were consistent with the temporal behavior of spectral line emissions observed by a streaked crystal spectrograph. The introduction of random phase plates had no significant influence on the mass ablation rate for imploding targets. The results indicate that a highly uniform drive pressure has been achieved for the GEKKO XII laser.