X-Ray Scatter Experiments

We carry out a variety of experiments including proton heating of matter, XUV absorption and K-edge spectroscopy. However in recent years our focus has been on X-ray scattering as a diagnostic. Our experiments are aimed at deducing the microscopic structure of matter that is relevant to planetary and fusion physics. We do this by scattering X-rays from the samples created in the laboratory, usually with large pulsed lasers. The angular variation in scatter tells us aboout the relative position of ions in a similar way to X-ray diffraction telling us about crystal structure, but with far less sharply defined features. The figure on the left below, shows a schematic of the set up for an experiment. A cluster of beams is used to drive a high power shock wave with pressures of a couple of million atmospheres into a sample foil. Another cluster of beams creates an intense burst of X-rays. The He-like 1s2-1s2p transition of Ti at 4.75keV dominates the emission in the ~5keV region. The scattering of these X-rays is measured using our wide angle spectrometer which uses a HOPG crystal to achieve spectral resolution with high reflectivity. The data on the right shows how the scatter as a function of angle evolves with time from the start of the shock drive. The comparison between the unscreened one component model (OCP) and a model based on the embedded atom model, shows just how important screening between the ions is. For more details see Garcia Saiz et al, Physical Review Letters 101 (7) 075003, 2008.

Schematic of an experiment carried out at the central claser facility of the Rutherford Appleton Laboratory.
Schematic of an experiment carried out at the central claser facility of the Rutherford Appleton Laboratory.
Selection of X-ray scatter data showing fits of experiment to simulation based on different models for the microscopic structure of the warm dense Al.
Selection of X-ray scatter data showing fits of experiment to simulation based on different models for the microscopic structure of the warm dense Al.

In addition to the work at the central laser facility we access large laser systems in collaboration with colleagues at LULI in Paris and at teh Gekko laser in Osaka. With these lasers we are able to make spectroscopically resolved measurements to investigate the dynamical structure factior of the plasmas [see e.g. B Barbrel et al Phys. Rev. Lett. 102 165004, 2009].

Photos

The control room at the Gekko laser in Osaka
The control room at the Gekko laser in Osaka
Former student D McSherry during an X-ray scatter experiment at the CLF
Former student D McSherry during an X-ray scatter experiment at the CLF