Prof Peter Norreys FInstP;

Abstract:

An integrated experiment relevant to fast ignition. A Cu-doped deuterated polymer spherical shell target with an inserted hollow Au cone is imploded by a six-beam 900-J, 1-ns laser. A 10-ps, 70-J laser pulse is focused into the cone at the time of peak compression. The flux of high-energy electrons through the imploded material is determined from the yield of Cu Kα fluorescence by comparison with a Monte Carlo model. The electrons are estimated to carry about 15% of the laser energy. Collisional and Ohmic heating are modeled, and Ohmic effects are shown to be relatively unimportant. An electron spectrometer shows significantly greater reduction of the transmitted electron flux than is calculated in the model. Enhanced scattering by instability-induced magnetic fields is suggested. An extension of this fluor-based technique to measurement of coupling efficiency to the ignition hot spot in future larger-scale fast ignition experiments is outlined. © 2008 American Institute of Physics.

Abstract:

Images of imploding fusion plasmas reveal complex electric and magnetic field structures.

Abstract:

A scheme for artificially collimating fast-electron beams produced in high intensity (>1019Wcm-2) laser-solid interactions is proposed. The scheme uses a laser pulse at the relativistic threshold (1018Wcm-2) that precedes the high intensity pulse to pregenerate a collimating magnetic field. This concept is supported by analytical calculations and numerical calculations performed using a novel hybrid-Vlasov-Fokker-Planck code called LEDA. This scheme may be highly useful for fast ignition inertial confinement fusion. © 2008 The American Physical Society.

Abstract:

Metal foil targets were irradiated with 1μm wavelength (λ) laser pulses of 5 ps duration and focused intensities (I) of up to 4×1019Wcm-2, giving values of both Iλ2 and pulse duration comparable to those required for fast ignition inertial fusion. The divergence of the electrons accelerated into the target was determined from spatially resolved measurements of x-ray Kα emission and from transverse probing of the plasma formed on the back of the foils. Comparison of the divergence with other published data shows that it increases with Iλ2 and is independent of pulse duration. Two-dimensional particle-in-cell simulations reproduce these results, indicating that it is a fundamental property of the laser-plasma interaction. © 2008 The American Physical Society.