Oxford Centre for High Energy Density Science (OxCHEDS)

Abstract:

Neutron time of flight signals have been observed with a high resolution neutron spectrometer using the petawatt arm of the Vulcan laser facility at Rutherford Appleton Laboratory from plastic sandwich targets containing a deuterated layer. The neutron spectra have two elements: a high-energy component generated by beam-fusion reactions and a thermal component around 2.45 MeV. The ion temperatures calculated from the neutron signal width clearly demonstrate a dependence on the front layer thickness and are significantly higher than electron temperatures measured under similar conditions. The ion heating process is intensity dependent and is not observed with laser intensities on target below 1020 W cm-2. The measurements are consistent with an ion instability driven by electron perturbations. © 2005 IOP Publishing Ltd.

Abstract:

We report the observation of monoenergetic electron beams (dE/E < 5%) produced by the interaction of a 12TW, 40fs laser pulse with underdense plasma, in contrast to all previous experiments, which produced energy spreads ∼100%. © 2005 Optical Society of America.

Abstract:

Supernovae launch spherical shocks into the circumstellar medium (CSM). These shocks may interact with both the intergalactic magnetic field (IGM) and local mass accumulations (possibly with their own local magnetic fields). The latter interaction may trigger star formation. The shocks have high Mach numbers and may be radiative. We have created similar shocks in the laboratory by focusing laser pulses onto the tip of a solid pin surrounded by ambient gas; ablated material from the pin rapidly expands and launches a shock through the surrounding gas. The shock may then be allowed to interact with (a) mass accumulations, (b) magnetic fields, or (c) allowed to expand freely. We will present examples of each type of experiment, but mainly discuss a new phenomena observed first in (c); at the edge of the radiatively heated gas ahead of the shock, a second shock forms. The two expanding shocks are simultaneously visible for a time, until the original shock stalls from running into the heated gas. The second shock remains visible and continues to expand. A minimum condition for the formation of the second shock is that the original shock is super-critical, i.e., the temperature distribution ahead of the original shock has an inflexion point. In a non-radiative control experiment the second shock does not form. © 2005 American Institute of Physics.

Abstract:

The melting dynamics of laser excited InSb have been studied with femtosecond x-ray diffraction. These measurements observe the delayed onset of diffusive atomic motion, signaling the appearance of liquidlike dynamics. They also demonstrate that the root-mean-squared displacement in the [111] direction increases faster than in the [110] direction after the first 500 fs. This structural anisotropy indicates that the initially generated fluid differs significantly from the equilibrium liquid. © 2005 The American Physical Society.