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.

In our latest work, we have used the LCLS free electron x-ray laser at Stanford to probe Fe that has been laser-shock compressed to well above solid density and temperatures of well over 10,000 degrees. Fe is important as it is a major constituent of the interior of planets like Earth and other Earth-like planets that exist in the universe. The figure below left shows an experimental arrangement where the FEL beam at 7.0keV is passed through asmaple of Fe that has been shocked using two nanosecond duation laser pulses that are focussed to 3x10^13 Wcm^-2, driving shocks of millions of atmospheres pressure. In the middle is some raw data from the detector that detects the scattered photons as a fucntion of angle. Bragg diffraction lines from cold edges of the target are seen as well as a strong feature from the melted Fe sample region. On the right we see a line-out along increasing scatter angle showing the shape of the scatter for the melted Fe: it is this we can compare to simulation.

Schematic of experimental arrangement. The solid red line shows the path of the FEL beam.
Schematic of experimental arrangement. The solid red line shows the path of the FEL beam.
Raw data image taken for scatter of 7keV beam from shocked Fe target at 13.5g/cc and 2eV. The arrow marked A and B show alternative lineouts along increasing Bragg angle.
Raw data image taken for scatter of 7keV beam from shocked Fe target at 13.5g/cc and 2eV. The arrow marked A and B show alternative lineouts along increasing Bragg angle.
Line outs along lines A and B in centre image, corrected for angle, distance to detector, polarisation and filter transmission. The curves can be compared to models of the Fe structure factor that include or don’t include a short term repulsive term coming from the interaction of the bound electron shells.
Line outs along lines A and B in centre image, corrected for angle, distance to detector, polarisation and filter transmission. The curves can be compared to models of the Fe structure factor that include or don’t include a short term repulsive term coming from the interaction of the bound electron shells.

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

View along the FEL beamline from the MEC (Materials and Extreme Conditions) experimental hutch.
View along the FEL beamline from the MEC (Materials and Extreme Conditions) experimental hutch.
The experimental team at the controls of the LCLS FEL experiment!
The experimental team at the controls of the LCLS FEL experiment!