The geometry of the von Hámos spectrometer that recorded the axial Kr(L) spectrum is illustrated
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Figure 3. The geometry of the von Hámos spectrometer that recorded the axial Kr(L) spectrum is illustrated. As described in the text, axial translation along the Z-axis controllably positions the focal point of the instrument so that the spectrum of the amplified Kr(L) beam can be faithfully recorded as a strong signal on the CCD in the plane of the instrument without significant interference from the two regions of isotropic emission that are associated with the formation and termination of the self-trapped plasma channel. (a) Focal point of spectrometer positioned within the isotropic emission zone at the exit of the plasma channel. (b) Focal point of spectrometer positioned outside of both isotropically radiating regions that are accordingly designated as off-axis, defocused zones. With this geometrical configuration, only the directed Kr(L) 7.5 Å beam can produce a strong signal on the plane of the detector, thereby selectively recording the spectrum of the amplified Kr(L) transition on the CCD.
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.