Oxford Centre for High Energy Density Science (OxCHEDS)

Abstract:

Synchrotrons and free-electron lasers are the most powerful sources of X-ray radiation. They constitute invaluable tools for a broad range of research 1 ; however, their dependence on large-scale radiofrequency electron accelerators means that only a few of these sources exist worldwide. Laser-driven plasma-wave ccelerators 2-10 provide markedly increased accelerating fields and hence offer the potential to shrink the size and cost of these X-ray sources to the niversity-laboratory scale. Here, we demonstrate the generation of soft-X-ray undulator radiation with laser-plasma-accelerated electron beams. The well-collimated beams deliver soft-X-ray pulses with an expected pulse duration of ∼ 10 fs (inferred from plasma-accelerator physics). Our source draws on a 30-cm-long undulator and a 1.5-cm-long accelerator delivering stable electron beams with energies of ∼ 210 MeV. The spectrum of the generated undulator radiation typically consists of a main peak centred at a wavelength of ∼ 18 nm (fundamental), a second peak near ∼ 9 nm (second harmonic) and a high-energy cutoff at ∼ 7 nm. Magnetic quadrupole lenses ensure efficient electron-beam transport and demonstrate an enabling technology for reproducible generation of tunable undulator radiation. The source is scalable to shorter wavelengths by increasing the electron energy. Our results open the prospect of tunable, brilliant, ultrashort-pulsed X-ray sources for small-scale laboratories. © 2009 Macmillan Publishers Limited. All rights reserved.

Abstract:

Saturable absorption is a phenomenon readily seen in the optical and infrared wavelengths. It has never been observed in core-electron transitions owing to the short lifetime of the excited states involved and the high intensities of the soft X-rays needed. We report saturable absorption of an L-shell transition in aluminium using record intensities over 10 16 W cm 2 at a photon energy of 92 eV. From a consideration of the relevant timescales, we infer that immediately after the X-rays have passed, the sample is in an exotic state where all of the aluminium atoms have an L-shell hole, and the valence band has approximately a 9 eV temperature, whereas the atoms are still on their crystallographic positions. Subsequently, Auger decay heats the material to the warm dense matter regime, at around 25 eV temperatures. The method is an ideal candidate to study homogeneous warm dense matter, highly relevant to planetary science, astrophysics and inertial confinement fusion. © 2009 Macmillan Publishers Limited. All rights reserved.