91福利

Exotic States of Matter Created with an Intense XUV FEL

So-called 4th-generation light sources are set to revolutionize several areas of science. These novel radiation sources, based on free-electron-laser (FEL) technology, have spectral brightnesses between six and ten orders of magnitude greater than those both of any synchrotron, and any laser-plasma-based XUV source. Just a few months ago the world’s first true hard X-ray laser (1.5Å) was demonstrated at LCLS at SLAC, and the FLASH laser at Hamburg (which produces photons up to 200-eV in the fundamental, and 600-eV in the 3rd harmonic) has been operational for over two years. Both these sources are full user facilities, operating at high repetition rate (currently 5Hz for FLASH, and upto 120Hz at LCLS). Here, we present the first results of the interaction of such radiation interacting with matter at high intensities. 

We have taken the output of FLASH, operating with a photon energy of 92-eV, and, using a multilayer-coated off-axis-parabola, have focussed the XUV output to an intensity close to 10^17 Wcm^-2 – an intensity regime that, until recently, had remained the province of optical lasers (it is envisaged that with hard X-ray FELs such as LCLS, intensities in excess of 10^19 Wcm^-2 will be produced). At such high intensities, the XUV energy is transferred to the electrons in the aluminium target placed in the focal region on a time-scale that is short compared with that of significant ion motion. In particular, we infer that we can produce a new form of plasma, where - at least for a short time - the ions are still on their crystallographic positions, yet each ion is further ionized beyond its natural state in the metal.

We demonstrate that within the time-scale of the 15-fs FLASH laser pulse we can photoeject an L-electron from every atom within the focal spot, which produces a shift in the L-edge such that the solid aluminum turns transparent to the incident radiation, resulting in the first observation of saturable absorption in the XUV.[1]  We further discuss the XUV emission spectra recorded while the heated material was at solid density, before the ions had time to move, and how it may provide unique information on the electronic structure of warm dense matter. We show that as many atoms are photo-ionized, from the point of view of the fluorescence emission, the system is acting very much like a photo-generated alloy.

It is our view that these type of experiments could be of importance for the ICF community, and that X-ray FELs will be complementary facilities to much high-energy-density plasma research: we will show that such intense XUV and X-ray irradiation is an ideal way of producing warm dense matter (i.e. matter around solid density, but with thermal energies comparable with the coulombic energies). The equation of state of matter under such conditions is still poorly understood, yet it remains an important regime during the implosion process, as will as being of significance for the physics of the planetary interiors of the gas giants.

·      ^* Co-authors of this work include all of the members of the ‘Peak Brightness Collaboration’, which comprise all authors of reference [1].

·      [1] Bob Nagler et al, ‘Turning Aluminium Transparent by Intense Soft X-Ray Photoionization’, Nature Physics, published online 26 July 2009, doi:10.1038/nphys1341.