R. L. Weber

Space and time resolved measurements of the heating of solids to ten million kelvin by a petawatt laser

The heating of plane solid targets by the Vulcan petawatt laser at powers of 0.32–0.73 PW and intensities of up to 4×1020 W cm−2 has been diagnosed with a temporal resolution of 17 ps and a spatial resolution of 30 μm, by measuring optical emission from the opposite side of the target to the laser with a streak camera. Second harmonic emission was filtered out and the target viewed at an angle to eliminate optical transition radiation. Spatial resolution was obtained by imaging the emission onto a bundle of fibre optics, arranged into a one-dimensional array at the camera entrance. The results show that a region 160 μm in diameter can be heated to a temperature of ~107 K (kT/e~ keV) in solid targets from 10 to 20 μm thick and that this temperature is maintained for at least 20 ps, confirming the utility of PW lasers in the study of high energy density physics. Hybrid code modelling shows that magnetic field generation prevents increased target heating by electron refluxing above a certain target thickness and that the absorption of laser energy into electrons entering the solid target was between 15–30%, and tends to increase with laser energy.

Creation of hot dense matter in short-pulse laser-plasma interaction with tamped titanium foils

Dense titanium plasma has been heated to an electron temperature up to 1300eV with a 100TW, high intensity short-pulse laser. The experiments were conducted using Ti foils (5μm thick) sandwiched between layers of either aluminum (1 or 2μm thick) or plastic (2μm thick) to prevent the effects of prepulse. Targets of two different sizes, i.e., 250×250μm2 and 1×1mm2 were used. Spectral measurements of the Ti inner-shell emission, in the region between 4and5keV, were taken from the front-side (i.e., the laser illuminated side) of the target. The data show large shifts in the Kα emission from open-shell ions, suggesting bulk heating of the sample at near solid density, which was largest for reduced mass targets. Comparison with collisional radiative and 2D radiation hydrodynamics codes indicates a peak temperature of Te,peak=1300eV of solid titanium plasma in ∼0.2μm thin layer. Higher bulk temperature (Te,bulk=100eV) for aluminum tamped compared to CH tamped targets (Te,bulk=40eV) was observed. A possible expla...

Development of Time Resolved X-ray Spectroscopy in High Intensity Laser-Plasma Interactions

This article discusses the design of a novel time resolved von Hamos Bragg spectrometer to provide spectra in the region around the titanium K-α and He-α lines. The instrument consists of a highly oriented pyrolitic graphite mosaic crystal coupled to a picosecond x-ray streak camera. Measurements of the time dependent behavior from Ti foils illuminated with intense laser pulses can be used to improve the understanding of recombination dynamics, electron transport, and phase transitions in strongly coupled dense plasma. This is important for the modeling of the compression phase in inertial confinement fusion research and the study of astrophysical environments.

Heating of solid target in electron refluxing dominated regime with ultra-intense laser

Propagation of electron beams generated in laser-plasma interactions is strongly influenced by self-induced electrostatic fields at target-vacuum interfaces, resulting the refluxing of electrons. We confirmed the refluxing and propagation of electrons with three different kinds of target configurations; thin-wide foil, thin-narrow foil, and long-wire geometry. Enhancement of target heating, effective guiding and collimation of high density MeV electrons were observed.

Wide angle crystal spectrometer for angularly and spectrally resolved x-ray scattering experiments

A novel wide angle spectrometer has been implemented with a highly oriented pyrolytic graphite crystal coupled to an image plate. This spectrometer has allowed us to look at the energy resolved spectrum of scattered x rays from a dense plasma over a wide range of angles (approximately 30 degrees ) in a single shot. Using this spectrometer we were able to observe the temporal evolution of the angular scatter cross section from a laser shocked foil. A spectrometer of this type may also be useful in investigations of x-ray line transfer from laser-plasmas experiments.

On point designs for high gain fast ignition

Fast ignition research has reached the stage where point designs are becoming crucial to the identification of key issues and the development of projects to demonstrate high gain fast ignition. The status of point designs for cone coupled electron fast ignition and some of the issues they highlight are discussed.