Over the past couple of years we have been involved in experiments using the new FLASH facility at DESY in Hamburg. This is a free electron laser operating in the XUV regime (from ~32 to 6nm). The pulses are only about 20fs (1fs=10-15 s) duration and contain ~20 microjoules per pulse. We have used multi-layer optics to focus the beam down to the micron scale and thus have been able to focus to over 1016 Wcm-2 (AJ Nelson et al Optics Express 17 p18271 2009) This affords a unique opportunity to study the interaction of XUV radiation at high intenrsity with solid matter and our group has been closely involved alonside a range of collaborators from Oxford, LLNL in the USA, Prague, Warsaw and elsewhere.
In one of the experiments, with our colleagues at Oxford, we measured the transmission of the XUV beam through a thin foil of Al at high intensity with 13.6nm (92eV) photons. The Al atoms are able to absorb a single photon by photo-ionisation from the L-shell. The shift in binding due to ionisation means no further photons can be absorbed. This is because the timescale for Auger decay back to the L-shell is longer than the pulse duration. This means that the foil now becomes 'transparent' to the rest of the beam energy and we see an intensity dependent saturable absorption for the first time in the XUV regime [B Nagler et al Nature Physics 5 p693 2009]. A new 'exotic' state of matter is momentarily created where the Al is ionised but the lattice retains its crystal structure until the ions have time to move on a ~10-12s timescale.
In a parallel experiment [T Dzelzainis et al High Energy Density Physics 6 p109 2010] we make emission spectroscopy (see figures below) measurements of the Al and indeed saw a saturated temeprature from the Al4+ lines emitted late in time as the plasma created expands.
Views of the FLASH experimental hall
| View from the user room || |
| View from the gallery || |
| Some of the collaboration. || || || |
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| Tom Dzelzainis (left) works on the chamber for FLASH BL3 experiments with Colleagues from Oxford. || || || || |