Oxford Centre for High Energy Density Science (OxCHEDS)

Abstract:

The energy transport in cone-guided low- Z targets has been studied for laser intensities on target of 2.5× 1020 W cm-2. Extreme ultraviolet (XUV) imaging and transverse optical shadowgraphy of the rear surfaces of slab and cone-slab targets show that the cone geometry strongly influences the observed transport patterns. The XUV intensity showed an average spot size of 65±10 μm for slab targets. The cone slabs showed a reduced spot size of 44±10 μm. The shadowgraphy for the aforementioned shots demonstrate the same behavior. The transverse size of the expansion pattern was 357±32 μm for the slabs and reduced to 210±30 μm. A transport model was constructed which showed that the change in transport pattern is due to suppression of refluxing electrons in the material surrounding the cone. © 2009 The American Physical Society.

Abstract:

We have focused a beam (BL3) of FLASH (Free-electron LASer in Hamburg: lambda = 13.5 nm, pulse length 15 fs, pulse energy 10-40 microJ, 5 Hz) using a fine polished off-axis parabola having a focal length of 270 mm and coated with a Mo/Si multilayer with an initial reflectivity of 67% at 13.5 nm. The OAP was mounted and aligned with a picomotor controlled six-axis gimbal. Beam imprints on poly(methyl methacrylate) - PMMA were used to measure focus and the focused beam was used to create isochoric heating of various slab targets. Results show the focal spot has a diameter of < or =1 microm. Observations were correlated with simulations of best focus to provide further relevant information.

Abstract:

Anomalous resistivity for hot electrons passing through a dense core plasma is studied for fast ignition laser fusion. The hot electrons generated via the ultra-intense laser pulse and guiding cone interactions are measured after they pass through a dense plasma with a density of 50-100 g cm-3 in a radius of 15-25 m. When significant neutron enhancements are achieved by the ultraintense laser pulse injection, the energy reduction of fast electrons is observed. Also, a reduction in the number of electrons with energy up to 15 MeV can be seen. We offer a new physical mechanism for the stopping of electrons, involving electron magnetohydrodynamic shock formation in the inhomogeneous plasma density region. The dissipation in the shock region can explain electron stopping with energies of the order of 15 MeV. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.

Abstract:

A number of experiments have been undertaken at the Rutherford Appleton Laboratory that were designed to investigate the physics of fast electron transport relevant to fast ignition inertial fusion. The laser, operating at a wavelength of 1054 nm, provided pulses of up to 350 J of energy on target in a duration that varied in the range 0.5-5 ps and a focused intensity of up to 1021 W cm-2. A dependence of the divergence of the fast electron beam with intensity on target has been identified for the first time. This dependence is reproduced in two-dimensional particle-in-cell simulations and has been found to be an intrinsic property of the laser-plasma interaction. A number of ideas to control the divergence of the fast electron beam are described. The fractional energy transfer to the fast electron beam has been obtained from calibrated, time-resolved, target rear-surface radiation temperature measurements. It is in the range 15-30%, increasing with incident laser energy on target. The fast electron temperature has been measured to be lower than the ponderomotive potential energy and is well described by Haines' relativistic absorption model. © 2009 IAEA, Vienna.