Details of Thomson images shown in panels (a) and (b) of figure 9
Figure 10. Details of Thomson images shown in panels (a) and (b) of figure 9. These data correspond to file 520/14 June 2012. (a) Isometric view of Thomson image showing extension into the dark x-ray zone Z ≥ 0.5 mm illustrated in figure 9(c). The arrow at Z = Z1 1.8 mm specifies the location of the endpoint of the visible Thomson signal that indicates the presence of ionized material. (b) Axial line-out of the Thomson data pictured in panel (a). A discernable extension of the ionization is visible out to Z = Z1 1.8 mm, a distance that represents a penetration of ~1.3 mm into the x-ray dark region shown in figure 9(c) and is estimated to be comparable to the linear absorption length of the Kr cluster medium. This observation agrees with an earlier estimate  of the ability to reach saturation of the absorption in Kr by Kr(L) emission; the clear conclusion was that, with a saturation parameter ħω/σ 400 J cm−2, linear absorption would necessarily govern.
Experimental evidence demonstrating amplification on the Kr26+ 3s→2p transition at λ 7.5 Å (~1652 eV) generated from a (Kr)n cluster medium in a self-trapped plasma channel produced with 248 nm femtosecond pulses is presented. The x-ray beam produced had a spectral width of ~3 eV and a corresponding beam diameter of ~150 µm, properties that were simultaneously determined by a two-dimensional x-ray spectral image formed with an axially placed von Hámos spectrometer and a matching Thomson image of the spatial electron density generated by the x-ray propagation.